Improving EQ

Improving EQ

[Pip Cet via Emacs development discussions.]

I looked at the "new" code generated for our EQ macro, and decided that
a fix was in order.  I'm therefore sending a first proposal to explain
what I think should be done, and some numbers.

This patch:
* moves the "slow path" of EQ into a NO_INLINE function
* exits early if the arguments to EQ are actually BASE_EQ
* returns quickly (after a single memory access which cannot be avoided
until we fix our tagging scheme to distinguish exotic objects from
ordinary ones) when symbols_with_pos_enabled isn't true.

The effect on the code size of the stripped emacs binary is small, but
significant: 8906336 bytes instead of 8955488 bytes on this machine.
(The effect on the code size of the emacs binary with debugging
information is much larger, reducing it from 32182000 bytes to 31125832
bytes on this system.)  There is no effect on the size of the .pdmp
file, which is expected.

What's missing here is a benchmark, but unless there's a really nasty
surprise when that happens, I'm quite confident that we can improve the
code here.

The proposed code doesn't use __builtin_expect anymore.

I've deliberately written slow_eq so it returns the same value as EQ,
even if the slow code path is disabled.

Pip

commit 2c807f7320bcb9654e0f148d64c92053b1a47b42 (HEAD -> faster-eq)
Author: Pip Cet <pipcet@protonmail.com>
Date:   Wed Dec 11 22:31:07 2024 +0000

    Change EQ to move slow code path into a separate function
    
    * src/data.c (slow_eq): New function.
    * src/lisp.h (EQ): Call it.

diff --git a/src/data.c b/src/data.c
index 66cf34c1e60..5ee383d2f48 100644
--- a/src/data.c
+++ b/src/data.c
@@ -162,6 +162,15 @@ circular_list (Lisp_Object list)
 \f
 /* Data type predicates.  */
 
+/* NO_INLINE to avoid excessive code growth when LTO is in use.  */
+NO_INLINE bool slow_eq (Lisp_Object x, Lisp_Object y)
+{
+  return BASE_EQ ((symbols_with_pos_enabled && SYMBOL_WITH_POS_P (x) ?
+		   XSYMBOL_WITH_POS_SYM (x) : x),
+		  (symbols_with_pos_enabled && SYMBOL_WITH_POS_P (y) ?
+		   XSYMBOL_WITH_POS_SYM (y) : y));
+}
+
 DEFUN ("eq", Feq, Seq, 2, 2, 0,
        doc: /* Return t if the two args are the same Lisp object.  */
        attributes: const)
diff --git a/src/lisp.h b/src/lisp.h
index 832a1755c04..64d4835a499 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -618,6 +618,7 @@ #define ENUM_BF(TYPE) enum TYPE
 extern Lisp_Object default_value (Lisp_Object symbol);
 extern void defalias (Lisp_Object symbol, Lisp_Object definition);
 extern char *fixnum_to_string (EMACS_INT number, char *buffer, char *end);
+extern bool slow_eq (Lisp_Object x, Lisp_Object y);
 
 
 /* Defined in emacs.c.  */
@@ -1353,10 +1354,12 @@ make_fixed_natnum (EMACS_INT n)
 INLINE bool
 EQ (Lisp_Object x, Lisp_Object y)
 {
-  return BASE_EQ ((__builtin_expect (symbols_with_pos_enabled, false)
-		   && SYMBOL_WITH_POS_P (x) ? XSYMBOL_WITH_POS_SYM (x) : x),
-		  (__builtin_expect (symbols_with_pos_enabled, false)
-		   && SYMBOL_WITH_POS_P (y) ? XSYMBOL_WITH_POS_SYM (y) : y));
+  if (BASE_EQ (x, y))
+    return true;
+  else if (!symbols_with_pos_enabled)
+    return false;
+  else
+    return slow_eq (x, y);
 }
 
 INLINE intmax_t

Re: Improving EQ

[Eli Zaretskii]

> Date: Wed, 11 Dec 2024 22:37:04 +0000
> From:  Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> 
> What's missing here is a benchmark, but unless there's a really nasty
> surprise when that happens, I'm quite confident that we can improve the
> code here.

The usual easy benchmark is to byte-compile all the *.el files in the
source tree.  That is, remove all the *.elc files, then say "make" and
time that.

There was also some Emacs benchmark suite that someone posted, but I
cannot find it now, maybe someone else will.

Adding Mattias and Paul to this discussion.

Re: Improving EQ

[Andrea Corallo]

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Wed, 11 Dec 2024 22:37:04 +0000
>> From:  Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
>> 
>> What's missing here is a benchmark, but unless there's a really nasty
>> surprise when that happens, I'm quite confident that we can improve the
>> code here.
>
> The usual easy benchmark is to byte-compile all the *.el files in the
> source tree.  That is, remove all the *.elc files, then say "make" and
> time that.

Agree, considering that tests the non zero 'symbols_with_pos_enabled'
case.

> There was also some Emacs benchmark suite that someone posted, but I
> cannot find it now, maybe someone else will.

<https://elpa.gnu.org/packages/elisp-benchmarks.html>

Re: Improving EQ

[Pip Cet via Emacs development discussions.]

"Eli Zaretskii" <eliz@gnu.org> writes:

>> Date: Wed, 11 Dec 2024 22:37:04 +0000
>> From:  Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
>>
>> What's missing here is a benchmark, but unless there's a really nasty
>> surprise when that happens, I'm quite confident that we can improve the
>> code here.
>
> The usual easy benchmark is to byte-compile all the *.el files in the
> source tree.  That is, remove all the *.elc files, then say "make" and
> time that.

Considering the point of the optimization was to make compilation (when
symbols_with_pos_enabled is true) slower, but speed up non-compilation
use cases, I think that may be the opposite of what we want :-)

Furthermore, the master branch doesn't currently build after deleting
all the *.elc files, because recompilation exceeds max-lisp-eval-depth
in that scenario (together with the known purespace issue, this pretty
much means "make bootstrap" is the only way I can rebuild an emacs tree
right now. It'd be great if Someone could look into this, but I've
failed to understand the native-compilation code (and been told off for
trying to) too often for that Someone to be me. Plus, of course, I fully
understand that native compilation currently has wrong code generation
bugs which obviously have to take priority over build issues...)

> There was also some Emacs benchmark suite that someone posted, but I
> cannot find it now, maybe someone else will.

https://elpa.gnu.org/packages/elisp-benchmarks.html ? It'd be great if
we could agree on a benchmark, and even better if there were a way to
reliably run it from emacs -Q :-)

In fact, I would suggest to move a reduced benchmark suite to the emacs
repo itself, and run it using "make benchmark".

Also, just to let everyone know, I'm planning to make the "exotic"
property (this object must or can use the slow_eq path) part (probably
the LSB) of the tag rather than accessing it via a global variable and
the PVEC type. This should reduce code size further, should speed up
things, and has some other advantages which I'll go into when I have
working code.

Pip

Re: Improving EQ

[Eli Zaretskii]

> Date: Thu, 12 Dec 2024 08:36:50 +0000
> From: Pip Cet <pipcet@protonmail.com>
> Cc: Mattias Engdegård <mattiase@acm.org>, Paul Eggert <eggert@cs.ucla.edu>, emacs-devel@gnu.org
> 
> "Eli Zaretskii" <eliz@gnu.org> writes:
> 
> > The usual easy benchmark is to byte-compile all the *.el files in the
> > source tree.  That is, remove all the *.elc files, then say "make" and
> > time that.
> 
> Considering the point of the optimization was to make compilation (when
> symbols_with_pos_enabled is true) slower, but speed up non-compilation
> use cases, I think that may be the opposite of what we want :-)

That's fine, because knowing where this slows us down and by how much
is also important.

> Furthermore, the master branch doesn't currently build after deleting
> all the *.elc files, because recompilation exceeds max-lisp-eval-depth
> in that scenario (together with the known purespace issue, this pretty
> much means "make bootstrap" is the only way I can rebuild an emacs tree
> right now. It'd be great if Someone could look into this, but I've
> failed to understand the native-compilation code (and been told off for
> trying to) too often for that Someone to be me. Plus, of course, I fully
> understand that native compilation currently has wrong code generation
> bugs which obviously have to take priority over build issues...)

If this is with native-compilation, how about trying without?

Also, enlarging max-lisp-eval-depth (assuming you don't somehow hit
infinite recursion) locally should be easy: just add that to the
relevant Makefiles.

> https://elpa.gnu.org/packages/elisp-benchmarks.html ? It'd be great if
> we could agree on a benchmark, and even better if there were a way to
> reliably run it from emacs -Q :-)

Our benchmark facilities are very rudimentary, so agreement is not an
issue: we just use whatever is available.

> In fact, I would suggest to move a reduced benchmark suite to the emacs
> repo itself, and run it using "make benchmark".

Working on a better benchmark is very useful, but maybe we should try
solving one problem at a time?

> Also, just to let everyone know, I'm planning to make the "exotic"
> property (this object must or can use the slow_eq path) part (probably
> the LSB) of the tag rather than accessing it via a global variable and
> the PVEC type. This should reduce code size further, should speed up
> things, and has some other advantages which I'll go into when I have
> working code.

Whenever you change something in the tags, please remember to update
.gdbinit, otherwise we lose debugging support.

Re: Improving EQ

[Visuwesh]

[வியாழன் டிசம்பர் 12, 2024] Pip Cet via "Emacs development discussions." wrote:

>> There was also some Emacs benchmark suite that someone posted, but I
>> cannot find it now, maybe someone else will.
>
> https://elpa.gnu.org/packages/elisp-benchmarks.html ? It'd be great if
> we could agree on a benchmark, and even better if there were a way to
> reliably run it from emacs -Q :-)

Will the command package-isolate help in this scenario?

Re: Improving EQ

[Andrea Corallo]

Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
writes:

> "Eli Zaretskii" <eliz@gnu.org> writes:
>
>>> Date: Wed, 11 Dec 2024 22:37:04 +0000
>>> From:  Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
>>>
>>> What's missing here is a benchmark, but unless there's a really nasty
>>> surprise when that happens, I'm quite confident that we can improve the
>>> code here.
>>
>> The usual easy benchmark is to byte-compile all the *.el files in the
>> source tree.  That is, remove all the *.elc files, then say "make" and
>> time that.
>
> Considering the point of the optimization was to make compilation (when
> symbols_with_pos_enabled is true) slower, but speed up non-compilation
> use cases, I think that may be the opposite of what we want :-)

Glad you finally agree on the goal of the optimization.

> Furthermore, the master branch doesn't currently build after deleting
> all the *.elc files, because recompilation exceeds max-lisp-eval-depth
> in that scenario (together with the known purespace issue, this pretty
> much means "make bootstrap" is the only way I can rebuild an emacs tree
> right now. It'd be great if Someone could look into this, but I've
> failed to understand the native-compilation code (and been told off for
> trying to) too often for that Someone to be me. Plus, of course, I fully
> understand that native compilation currently has wrong code generation
> bugs which obviously have to take priority over build issues...)
>
>> There was also some Emacs benchmark suite that someone posted, but I
>> cannot find it now, maybe someone else will.
>
> https://elpa.gnu.org/packages/elisp-benchmarks.html ? It'd be great if
> we could agree on a benchmark, and even better if there were a way to
> reliably run it from emacs -Q :-)

What is not reliable in the elisp-benchmarks invocation suggested in the
instructions in it?

> In fact, I would suggest to move a reduced benchmark suite to the emacs
> repo itself, and run it using "make benchmark".

That would be nice.

Re: Improving EQ

[Pip Cet via Emacs development discussions.]

"Andrea Corallo" <acorallo@gnu.org> writes:

> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> writes:
>
>> "Eli Zaretskii" <eliz@gnu.org> writes:
>>
>>>> Date: Wed, 11 Dec 2024 22:37:04 +0000
>>>> From:  Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
>>>>
>>>> What's missing here is a benchmark, but unless there's a really nasty
>>>> surprise when that happens, I'm quite confident that we can improve the
>>>> code here.
>>>
>>> The usual easy benchmark is to byte-compile all the *.el files in the
>>> source tree.  That is, remove all the *.elc files, then say "make" and
>>> time that.
>>
>> Considering the point of the optimization was to make compilation (when
>> symbols_with_pos_enabled is true) slower, but speed up non-compilation
>> use cases, I think that may be the opposite of what we want :-)
>
> Glad you finally agree on the goal of the optimization.

I find that statement to be offensive, because you know it to be
factually incorrect; but even if it weren't, gloating like that is
extremely poor form for a maintainer.

Pip

Re: Improving EQ

[Eli Zaretskii]

> Date: Thu, 12 Dec 2024 17:46:55 +0000
> From: Pip Cet <pipcet@protonmail.com>
> Cc: "Pip Cet via \"Emacs development discussions.\"" <emacs-devel@gnu.org>, Eli Zaretskii <eliz@gnu.org>, Mattias Engdegård <mattiase@acm.org>, Paul Eggert <eggert@cs.ucla.edu>
> 
> "Andrea Corallo" <acorallo@gnu.org> writes:
> 
> >> Considering the point of the optimization was to make compilation (when
> >> symbols_with_pos_enabled is true) slower, but speed up non-compilation
> >> use cases, I think that may be the opposite of what we want :-)
> >
> > Glad you finally agree on the goal of the optimization.
> 
> I find that statement to be offensive, because you know it to be
> factually incorrect; but even if it weren't, gloating like that is
> extremely poor form for a maintainer.

I'm quite sure Andrea meant it as tongue-in-cheek, nowhere near
gloating.  Please keep in mind that for Andrea and others here,
English is not their first language, and their choice of words could
reflect that.

New "make benchmark" target

[Stefan Kangas]

Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
writes:

> https://elpa.gnu.org/packages/elisp-benchmarks.html ? It'd be great if
> we could agree on a benchmark, and even better if there were a way to
> reliably run it from emacs -Q :-)
>
> In fact, I would suggest to move a reduced benchmark suite to the emacs
> repo itself, and run it using "make benchmark".

SGTM, but why a reduced suite and not just the whole thing?

Re: New "make benchmark" target

[Andrea Corallo]

Stefan Kangas <stefankangas@gmail.com> writes:

> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> writes:
>
>> https://elpa.gnu.org/packages/elisp-benchmarks.html ? It'd be great if
>> we could agree on a benchmark, and even better if there were a way to
>> reliably run it from emacs -Q :-)
>>
>> In fact, I would suggest to move a reduced benchmark suite to the emacs
>> repo itself, and run it using "make benchmark".
>
> SGTM, but why a reduced suite and not just the whole thing?

My fear is that if we start going into the rabbit hole of which
benchmark of elisp-benchmarks should or should not be included, we will
never agree and as a consequence succeed.  So I guess I'd favor as well
including all elisp-benchmarks.

  Andrea

Re: New "make benchmark" target

[Pip Cet via Emacs development discussions.]

"Andrea Corallo" <acorallo@gnu.org> writes:

> Stefan Kangas <stefankangas@gmail.com> writes:
>
>> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
>> writes:
>>
>>> https://elpa.gnu.org/packages/elisp-benchmarks.html ? It'd be great if
>>> we could agree on a benchmark, and even better if there were a way to
>>> reliably run it from emacs -Q :-)
>>>
>>> In fact, I would suggest to move a reduced benchmark suite to the emacs
>>> repo itself, and run it using "make benchmark".
>>
>> SGTM, but why a reduced suite and not just the whole thing?
>
> My fear is that if we start going into the rabbit hole of which
> benchmark of elisp-benchmarks should or should not be included, we will
> never agree and as a consequence succeed.  So I guess I'd favor as well
> including all elisp-benchmarks.

I agree a full benchmark suite would be even better, I don't recall why
I typed "reduced" there. So let's do that?

Pip

Re: New "make benchmark" target

[Stefan Kangas]

Pip Cet <pipcet@protonmail.com> writes:

> I agree a full benchmark suite would be even better, I don't recall why
> I typed "reduced" there. So let's do that?

Please go ahead, thanks.

Re: New "make benchmark" target

[Andrea Corallo]

Stefan Kangas <stefankangas@gmail.com> writes:

> Pip Cet <pipcet@protonmail.com> writes:
>
>> I agree a full benchmark suite would be even better, I don't recall why
>> I typed "reduced" there. So let's do that?
>
> Please go ahead, thanks.

Asking as elisp-benchmark author/maintainer, the way to move an external
package in core is to copy the files and keep them manually in sync or
there are other ways?

Re: New "make benchmark" target

[Stefan Kangas]

Andrea Corallo <acorallo@gnu.org> writes:

> Asking as elisp-benchmark author/maintainer, the way to move an external
> package in core is to copy the files and keep them manually in sync or
> there are other ways?

We can make it into a :core package, which means that you copy the file
to emacs.git, and when you update the "Version" header on Emacs master,
the GNU ELPA scripts make a release based on that commit.

Stefan Monnier (in CC) will know if there are any other adjustments that
are needed, but one thing that needs doing is a change to the GNU ELPA
`elpa-packages` file, something like this:

- (elisp-benchmarks	:url nil)
+ (elisp-benchmarks	:core ("lisp/emacs-lisp/elisp-benchmarks.el"))

Re: New "make benchmark" target

[Stefan Monnier]

>> Asking as elisp-benchmark author/maintainer, the way to move an external
>> package in core is to copy the files and keep them manually in sync or
>> there are other ways?

Well, there's the all too famous "bundled ELPA packages" feature we
never finished, of course (see `git branch -a | grep elpa` for
various approaches Philip Lord proposed and that we never integrated 🙁).
[ Stepping down before installing such a feature is my biggest regret
  w.r.t my time as Emacs maintainer.  ]

> - (elisp-benchmarks	:url nil)
> + (elisp-benchmarks	:core ("lisp/emacs-lisp/elisp-benchmarks.el"))

I suspect the above would need to be completed with more files/dirs in
the list.


        Stefan

Re: New "make benchmark" target

[Pip Cet via Emacs development discussions.]

"Stefan Kangas" <stefankangas@gmail.com> writes:

> Pip Cet <pipcet@protonmail.com> writes:
>
>> I agree a full benchmark suite would be even better, I don't recall why
>> I typed "reduced" there. So let's do that?
>
> Please go ahead, thanks.

Here's my proposal:

Expand ERT to handle the benchmark case. Copy ALL benchmarks from
elisp-benchmarks, but use the ERT framework instead of
elisp-benchmarks.el.  Keep things minimal for now, try it out, and add
complexity only if we get the impression this would make a useful
addition; otherwise, revert the changes and go back to using
elisp-benchmarks.el.

This is what would work:
1. add a :benchmark tag to ert tests
2. create a new directory test/benchmark/ for benchmarks
3. modify test/Makefile not to run benchmark tests by default
4. add make targets to run the benchmark tests only

I think the mechanism used by elisp-benchmarks.el to select tests is
very ad-hoc and less powerful than the ert tagging mechanism. It also
doesn't work for me: executing

(progn (elisp-benchmarks-run "" t 1)
       (elisp-benchmarks-run "bubble" t 1))

means all tests are run twice, but I intended to run all tests once, as
a warm up, then run only the bubble test again.  However, I suspect this
is merely a bug which can easily be fixed (maybe it's intentional,
though?).  I'm also seeing problems with a "Window size not as
stipulated by the benchmark" error message, but I'll have to investigate
that one...

The mathematical part of elisp-benchmarks.el is questionable: it's built
around the idea of using an arithmetic average of several test runs
(which is equivalent to a single test run with more iterations); I
believe a median/percentile-based approach to selecting a "typical" run
would yield more useful numbers.  So I'm not proposing to reuse the
avg-based code.

I tried to resist the temptation of making ert.el overly general; for
example, instead of defining a new defstruct to determine HOW tests are
run, I merely added a benchmark flag. Maybe we can revisit this if the
approach is adopted and the need for more detailed benchmark
specifications (inhibit GC? warmup? iteration counts? interact with
"perf"?) becomes apparent.  However, I did fail and give in to the
temptation to allow an inhibit-gc mode specifier, which should probably
be removed again...

The main problems I see are that "make benchmark" starts emacs instances
for all files in test/, which takes a lot of time (but that's a general
ERT problem that should be fixed for pass-or-fail testing, too).

A minor problem is how to copy the elisp-benchmark tests and keep them
in sync.  This would very much depend on how much work Andrea is willing
to do.

Finally, elisp-benchmarks has a very useful feature, somewhat hidden,
that this code lacks: while calculating the arithmetic average of
several benchmark runs isn't useful, calculating the standard deviation
from that average is, because it gives us an indication of how scattered
the results are; scattered test results (i.e. high numbers reported in
the "tot avg err" column) are a sufficient, but not a necessary,
condition for knowing when to discard the test results because something
unexpected happened (most likely system load issues or CPU clock games).

Benchmarking is, of course, very hard. I understand Paul Eggert is using
an ancient machine for benchmarking because it avoids many of the issues
that have arisen in the past decade. With a modern machine, we have a
heterogeneous set of CPU cores (energy/performance), each of which can
be configured in different ways (energy-performance preference for each
core) in addition to running at a variable and unpredictable clock speed
(cpufreq/boost); CPU caches are now large enough to persist across
benchmark runs, and the system memory clock is also variable.

This is a very rough start which would allow us to detect many, but not
all, unexpected catastrophic performance reductions due to code changes.

Finally, if Someone is willing to work on this, we should look into
finding a set of benchmarks representative of "typical" Emacs usage so
we can use PGO when building Emacs. While I'd prefer doing neither of
the two, playing with PGO is a much more promising and maintainable
approach than adding __builtin_expect noise to our C source code.

Pip

From 4217a5b8f990760775709392b24e0205041cfed3 Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@protonmail.com>
Date: Sat, 14 Dec 2024 10:45:42 +0000
Subject: [PATCH 1/3] Add a benchmark from elisp-benchmarks

DO NOT MERGE

FIXME BEFORE MERGING: Should we add a link to
https://git.savannah.gnu.org/gitweb/?p=emacs/elpa.git;a=history;\
f=benchmarks/bubble.el;h=d7101b1b99b60a3bd6945909d1f0125215f4ce1c;\
hb=refs/heads/externals/elisp-benchmarks
here?  Losing git history because we copy a file from elpa to emacs
seems suboptimal...

* test/benchmark/bubble.el: New file.
---
 test/benchmark/bubble.el | 52 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 test/benchmark/bubble.el

diff --git a/test/benchmark/bubble.el b/test/benchmark/bubble.el
new file mode 100644
index 00000000000..0c38cdbce39
--- /dev/null
+++ b/test/benchmark/bubble.el
@@ -0,0 +1,52 @@
+;; -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; From:
+;; https://www.emacswiki.org/emacs/EmacsLispBenchmark
+
+(require 'ert)
+
+(require 'cl-lib)
+
+(defvar elb-bubble-len 1000)
+(defvar elb-bubble-list (mapcar #'random (make-list elb-bubble-len
+						    most-positive-fixnum)))
+
+(defun elb-bubble (list)
+  (let ((i (length list)))
+    (while (> i 1)
+      (let ((b list))
+        (while (cdr b)
+          (when (< (cadr b) (car b))
+            (setcar b (prog1 (cadr b)
+                        (setcdr b (cons (car b) (cddr b))))))
+          (setq b (cdr b))))
+      (setq i (1- i)))
+    list))
+
+(defun elb-bubble-entry ()
+  (cl-loop repeat 100
+	   for l = (copy-sequence elb-bubble-list)
+	   do (elb-bubble l)))
+
+(ert-deftest benchmark-bubble ()
+  :tags '(:benchmark)
+  (elb-bubble-entry))
-- 
2.47.0

From df31e19452dff0fe804af2fd3c73f4cee84b6d16 Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@protonmail.com>
Date: Sat, 14 Dec 2024 10:56:19 +0000
Subject: [PATCH 2/3] Expand the ERT framework to allow for benchmarks

* lisp/emacs-lisp/ert.el (ert-test-result, ert--test-execution-info):
Expand structs to include "benchmark" field.
(ert--run-benchmark-test-internal): New function.
(ert-run-test, ert-run-or-rerun-test):
(ert-run-tests-batch-and-exit):
(ert-run-tests): Add benchmark argument.
(ert-run-tests-batch): Include GC information when running benchmarks.
(ert-summarize-tests-batch-and-exit): Handle 1.0e+INF.
---
 lisp/emacs-lisp/ert.el | 91 +++++++++++++++++++++++++++++++++---------
 1 file changed, 73 insertions(+), 18 deletions(-)

diff --git a/lisp/emacs-lisp/ert.el b/lisp/emacs-lisp/ert.el
index 97aa233f6e2..76365ed8152 100644
--- a/lisp/emacs-lisp/ert.el
+++ b/lisp/emacs-lisp/ert.el
@@ -698,6 +698,7 @@ ert-test-result
   (messages nil)
   (should-forms nil)
   (duration 0)
+  (benchmark nil)
   )
 (cl-defstruct (ert-test-passed (:include ert-test-result)))
 (cl-defstruct (ert-test-result-with-condition (:include ert-test-result))
@@ -723,7 +724,8 @@ ert--test-execution-info
   ;; execution of the current test.  We store it to avoid being
   ;; affected by any new bindings the test itself may establish.  (I
   ;; don't remember whether this feature is important.)
-  ert-debug-on-error)
+  ert-debug-on-error
+  benchmark)
 
 (defun ert--run-test-debugger (info condition debugfun)
   "Error handler used during the test run.
@@ -801,6 +803,39 @@ ert--run-test-internal
         (make-ert-test-passed))
   nil)
 
+(defun ert--run-benchmark-test-internal (test-execution-info benchmark)
+  (setf (ert--test-execution-info-ert-debug-on-error test-execution-info)
+        ert-debug-on-error)
+  (catch 'ert--pass
+    ;; For now, each test gets its own temp buffer and its own
+    ;; window excursion, just to be safe.  If this turns out to be
+    ;; too expensive, we can remove it.
+    (with-temp-buffer
+      (save-window-excursion
+        (let ((lexical-binding t) ;;FIXME: Why?
+              (ert--infos '())
+              time)
+          (letrec ((debugfun (lambda (err)
+                               (ert--run-test-debugger test-execution-info
+                                                       err debugfun))))
+            (handler-bind (((error quit) debugfun))
+              (garbage-collect)
+              (let ((gc-cons-threshold (if (eq benchmark 'inhibit-gc)
+                                           most-positive-fixnum
+                                         gc-cons-threshold)))
+                (setq time (benchmark-run nil
+                             (funcall (ert-test-body (ert--test-execution-info-test
+                                                      test-execution-info))))))
+              (and (eq benchmark 'inhibit-gc)
+                   (not (= (cadr time) 0))
+                   (warn "failed to inhibit gc; time %S" time))
+              (setf (ert--test-execution-info-benchmark test-execution-info)
+                    time))))))
+    (ert-pass))
+  (setf (ert--test-execution-info-result test-execution-info)
+        (make-ert-test-passed))
+  nil)
+
 (defun ert--force-message-log-buffer-truncation ()
   "Immediately truncate *Messages* buffer according to `message-log-max'.
 
@@ -832,7 +867,7 @@ ert--running-tests
 
 The elements are of type `ert-test'.")
 
-(defun ert-run-test (ert-test)
+(defun ert-run-test (ert-test &optional benchmark)
   "Run ERT-TEST.
 
 Returns the result and stores it in ERT-TEST's `most-recent-result' slot."
@@ -855,8 +890,12 @@ ert-run-test
                          (push form-description should-form-accu)))
                       (message-log-max t)
                       (ert--running-tests (cons ert-test ert--running-tests)))
-                  (ert--run-test-internal info))
+                  (if benchmark
+                      (ert--run-benchmark-test-internal info benchmark)
+                    (ert--run-test-internal info)))
               (let ((result (ert--test-execution-info-result info)))
+                (setf (ert-test-result-benchmark result)
+                      (ert--test-execution-info-benchmark info))
                 (setf (ert-test-result-messages result)
                       (with-current-buffer (messages-buffer)
                         (buffer-substring begin-marker (point-max))))
@@ -1206,7 +1245,7 @@ ert--make-stats
                      :test-start-times (make-vector (length tests) nil)
                      :test-end-times (make-vector (length tests) nil))))
 
-(defun ert-run-or-rerun-test (stats test listener)
+(defun ert-run-or-rerun-test (stats test listener &optional benchmark)
   ;; checkdoc-order: nil
   "Run the single test TEST and record the result using STATS and LISTENER."
   (let ((ert--current-run-stats stats)
@@ -1221,19 +1260,26 @@ ert-run-or-rerun-test
     (setf (ert-test-most-recent-result test) nil)
     (setf (aref (ert--stats-test-start-times stats) pos) (current-time))
     (unwind-protect
-        (ert-run-test test)
+        (ert-run-test test benchmark)
       (setf (aref (ert--stats-test-end-times stats) pos) (current-time))
       (let ((result (ert-test-most-recent-result test)))
-        (setf (ert-test-result-duration result)
-              (float-time
-               (time-subtract
-                (aref (ert--stats-test-end-times stats) pos)
-                (aref (ert--stats-test-start-times stats) pos))))
+        (cond ((ert-test-result-benchmark result)
+               (setf (ert-test-result-duration result)
+                     (if (memq benchmark '(no-gc inhibit-gc))
+                         (- (car (ert-test-result-benchmark result))
+                            (caddr (ert-test-result-benchmark result)))
+                       (car (ert-test-result-benchmark result)))))
+              (t
+               (setf (ert-test-result-duration result)
+                (float-time
+                 (time-subtract
+                  (aref (ert--stats-test-end-times stats) pos)
+                  (aref (ert--stats-test-start-times stats) pos))))))
         (ert--stats-set-test-and-result stats pos test result)
         (funcall listener 'test-ended stats test result))
       (setf (ert--stats-current-test stats) nil))))
 
-(defun ert-run-tests (selector listener &optional interactively)
+(defun ert-run-tests (selector listener &optional interactively benchmark)
   "Run the tests specified by SELECTOR, sending progress updates to LISTENER."
   (let* ((tests (ert-select-tests selector t))
          (stats (ert--make-stats tests selector)))
@@ -1245,7 +1291,7 @@ ert-run-tests
             (force-mode-line-update)
             (unwind-protect
 		(cl-loop for test in tests do
-			 (ert-run-or-rerun-test stats test listener)
+			 (ert-run-or-rerun-test stats test listener benchmark)
 			 (when (and interactively
 				    (ert-test-quit-p
 				     (ert-test-most-recent-result test))
@@ -1367,7 +1413,7 @@ ert-batch-backtrace-right-margin
   "The maximum line length for printing backtraces in `ert-run-tests-batch'.")
 
 ;;;###autoload
-(defun ert-run-tests-batch (&optional selector)
+(defun ert-run-tests-batch (&optional selector benchmark)
   "Run the tests specified by SELECTOR, printing results to the terminal.
 
 SELECTOR selects which tests to run as described in `ert-select-tests' when
@@ -1493,7 +1539,7 @@ ert-run-tests-batch
             (let* ((max (prin1-to-string (length (ert--stats-tests stats))))
                    (format-string (concat "%9s  %"
                                           (prin1-to-string (length max))
-                                          "s/" max "  %S (%f sec)%s")))
+                                          "s/" max "  %S (%f sec%s)%s")))
               (message format-string
                        (ert-string-for-test-result result
                                                    (ert-test-result-expected-p
@@ -1501,13 +1547,19 @@ ert-run-tests-batch
                        (1+ (ert--stats-test-pos stats test))
                        (ert-test-name test)
                        (ert-test-result-duration result)
+                       (if (ert-test-result-benchmark result)
+                           (format ", %f sec in GC, %d GCs"
+                                   (caddr (ert-test-result-benchmark result))
+                                   (cadr (ert-test-result-benchmark result)))
+                         "")
                        (if (ert-test-result-expected-p test result)
                            ""
                          (concat " " (ert-test-location test))))))))))
-   nil))
+   nil
+   benchmark))
 
 ;;;###autoload
-(defun ert-run-tests-batch-and-exit (&optional selector)
+(defun ert-run-tests-batch-and-exit (&optional selector benchmark)
   "Like `ert-run-tests-batch', but exits Emacs when done.
 
 The exit status will be 0 if all test results were as expected, 1
@@ -1525,7 +1577,7 @@ ert-run-tests-batch-and-exit
     (setq attempt-stack-overflow-recovery nil
           attempt-orderly-shutdown-on-fatal-signal nil)
     (unwind-protect
-        (let ((stats (ert-run-tests-batch selector)))
+        (let ((stats (ert-run-tests-batch selector benchmark)))
           (when eln-dir
             (ignore-errors
               (delete-directory eln-dir t)))
@@ -1726,7 +1778,10 @@ ert-summarize-tests-batch-and-exit
 If HIGH is a natural number, the HIGH long lasting tests are summarized."
   (or noninteractive
       (user-error "This function is only for use in batch mode"))
-  (or (natnump high) (setq high 0))
+  (cond
+   ;; FIXME: ntake doesn't allow an infinity argument
+   ((eql high 1.0e+INF) (setq high most-positive-fixnum))
+   ((not (natnump high)) (setq high 0)))
   ;; Better crash loudly than attempting to recover from undefined
   ;; behavior.
   (setq attempt-stack-overflow-recovery nil
-- 
2.47.0

From 8cd4053967a0aa6521039ba887c911daa13b0cf0 Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@protonmail.com>
Date: Sat, 14 Dec 2024 10:59:38 +0000
Subject: [PATCH 3/3] Add "make benchmark" rule

* Makefile.in (benchmark): New recipe.
* test/Makefile.in (SELECTOR_BENCHMARK): New selector.
(SELECTOR_ALL, SELECTOR_EXPENSIVE, SELECTOR_DEFAULT): Modify selectors
not to include benchmark tests.
(check-benchmark): New recipe.
---
 Makefile.in      |  7 +++++++
 test/Makefile.in | 25 +++++++++++++++++--------
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/Makefile.in b/Makefile.in
index 30a762ed03b..13a55452da2 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -69,6 +69,9 @@
 #      check-expensive includes additional tests that can be slow.
 #      check-all runs all tests, including ones that can be slow, or
 #        fail unpredictably
+#
+# make benchmark
+#      Run the Emacs benchmark suite.
 
 SHELL = @SHELL@
 
@@ -1138,6 +1141,10 @@ $(CHECK_TARGETS):
 test/%:
 	$(MAKE) -C test $*
 
+BENCHMARK_TARGETS = benchmark
+.PHONY: $(BENCHMARK_TARGETS)
+$(BENCHMARK_TARGETS): all
+	$(MAKE) SUMMARIZE_TESTS="1.0e+INF" BENCHMARKP="t" -C test check-benchmark
 
 dist:
 	cd ${srcdir}; ./make-dist
diff --git a/test/Makefile.in b/test/Makefile.in
index 7a3178546a1..18a478b3e6c 100644
--- a/test/Makefile.in
+++ b/test/Makefile.in
@@ -78,9 +78,9 @@ TEST_TIMEOUT =
 TEST_INTERACTIVE ?= no
 
 ifeq ($(TEST_INTERACTIVE),yes)
-TEST_RUN_ERT = --eval '(ert (quote ${SELECTOR_ACTUAL}))'
+TEST_RUN_ERT = --eval '(ert (quote ${SELECTOR_ACTUAL}) ${BENCHMARKP})'
 else
-TEST_RUN_ERT = --batch --eval '(ert-run-tests-batch-and-exit (quote ${SELECTOR_ACTUAL}))' ${WRITE_LOG}
+TEST_RUN_ERT = --batch --eval '(ert-run-tests-batch-and-exit (quote ${SELECTOR_ACTUAL}) ${BENCHMARKP})' ${WRITE_LOG}
 endif
 
 # Whether to run tests from .el files in preference to .elc, we do
@@ -136,13 +136,15 @@ TEST_NATIVE_COMP =
 TEST_NATIVE_COMP = no
 endif
 ifeq ($(TEST_NATIVE_COMP),yes)
-SELECTOR_DEFAULT = (not (or (tag :expensive-test) (tag :unstable)))
-SELECTOR_EXPENSIVE = (not (tag :unstable))
-SELECTOR_ALL = t
+SELECTOR_DEFAULT = (not (or (tag :expensive-test) (tag :unstable) (tag :benchmark)))
+SELECTOR_EXPENSIVE = (not (or (tag :unstable) (tag :benchmark)))
+SELECTOR_ALL = (not (tag :benchmark))
+SELECTOR_BENCHMARK = (tag :benchmark)
 else
-SELECTOR_DEFAULT = (not (or (tag :expensive-test) (tag :unstable) (tag :nativecomp)))
-SELECTOR_EXPENSIVE = (not (or (tag :unstable) (tag :nativecomp)))
-SELECTOR_ALL = (not (tag :nativecomp))
+SELECTOR_DEFAULT = (not (or (tag :expensive-test) (tag :unstable) (tag :nativecomp) (tag :benchmark)))
+SELECTOR_EXPENSIVE = (not (or (tag :unstable) (tag :nativecomp) (tag :benchmark)))
+SELECTOR_ALL = (not (or (tag :nativecomp) (tag :benchmark)))
+SELECTOR_BENCHMARK = (and (tag :benchmark) (not (tag :nativecomp)))
 endif
 ifdef SELECTOR
 SELECTOR_ACTUAL=$(SELECTOR)
@@ -154,6 +156,8 @@ SELECTOR_ACTUAL=
 SELECTOR_ACTUAL=$(SELECTOR_DEFAULT)
 else ifeq ($(MAKECMDGOALS),check-maybe)
 SELECTOR_ACTUAL=$(SELECTOR_DEFAULT)
+else ifeq ($(MAKECMDGOALS),check-benchmark)
+SELECTOR_ACTUAL=$(SELECTOR_BENCHMARK)
 else
 SELECTOR_ACTUAL=$(SELECTOR_EXPENSIVE)
 endif
@@ -323,6 +327,11 @@ .PHONY:
 check-all: mostlyclean check-no-automated-subdir
 	@${MAKE} check-doit SELECTOR="${SELECTOR_ALL}"
 
+## Run all benchmark tests, regardless of tag.
+.PHONY: check-benchmark
+check-benchmark: mostlyclean check-no-automated-subdir
+	@${MAKE} check-doit SELECTOR="${SELECTOR_BENCHMARK}"
+
 ## Re-run all tests which are outdated. A test is outdated if its
 ## logfile is out-of-date with either the test file, or the source
 ## files that the tests depend on.  See test_template.
-- 
2.47.0

Re: New "make benchmark" target

[Stefan Kangas]

Pip Cet <pipcet@protonmail.com> writes:

> FIXME BEFORE MERGING: Should we add a link to
> https://git.savannah.gnu.org/gitweb/?p=emacs/elpa.git;a=history;\
> f=benchmarks/bubble.el;h=d7101b1b99b60a3bd6945909d1f0125215f4ce1c;\
> hb=refs/heads/externals/elisp-benchmarks
> here?  Losing git history because we copy a file from elpa to emacs
> seems suboptimal...

Instead of copying the file, it might be preferable to import the entire
git history into emacs.git, like we did for use-package and eglot.  Then
the old branch on GNU ELPA can be dropped, as we won't lose any history.

João has some scripts that he used for eglot, and I adapted them for
use-package.  Note that he also had some copyright assignment issues to
take care of, so it could probably be simplified.

Please take a look here:
https://gist.github.com/joaotavora/2ed97f2ec85958986983d5cb78202770

Re: New "make benchmark" target

[João Távora]

On Sat, Dec 14, 2024 at 8:07 PM Pip Cet <pipcet@protonmail.com> wrote:

> Joao, I think it would be a good idea to keep the modified script in
> admin/ or somewhere for future reference, but I don't know whether you
> consider it an Emacs contribution (and, thus, covered by your copyright
> assignment).

Sure, use it.  But there's a catch that I'm fairly sure I didn't write part
of that code.  Someone on the emacs-devel list helped me and wrote
the first versions of it (following my requirements), and I'm very sorry
but I can't remember who it was. Github handle is 'bhankas.  Good luck!

Re: Improving EQ

[Óscar Fuentes]

Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
writes:

> I looked at the "new" code generated for our EQ macro, and decided that
> a fix was in order.  I'm therefore sending a first proposal to explain
> what I think should be done, and some numbers.
>
> This patch:
> * moves the "slow path" of EQ into a NO_INLINE function
> * exits early if the arguments to EQ are actually BASE_EQ
> * returns quickly (after a single memory access which cannot be avoided
> until we fix our tagging scheme to distinguish exotic objects from
> ordinary ones) when symbols_with_pos_enabled isn't true.
>
> The effect on the code size of the stripped emacs binary is small, but
> significant: 8906336 bytes instead of 8955488 bytes on this machine.
> (The effect on the code size of the emacs binary with debugging
> information is much larger, reducing it from 32182000 bytes to 31125832
> bytes on this system.)  There is no effect on the size of the .pdmp
> file, which is expected.
>
> What's missing here is a benchmark, but unless there's a really nasty
> surprise when that happens, I'm quite confident that we can improve the
> code here.

I've seen too many cases where *removing* instructions (mind you,
literally removing, not changing!) made the code significantly slower.

Modern CPUs are insanely complex and combined with compilers make
intuition-based predictions even more futile.

But reading your message makes me wonder if EQ and some other "simple"
fundamental functions are not lowered by nativecomp? If not, maybe
that's a significant opportunity for improvement.

As for your patch, one thing that would be easy to do and might save
quite a lot of head scratching is to count the fraction of the calls to
EQ that benefit from the fast path on a "representative" Emacs run. Then
you would have hard data to decide if fighting the compiler/CPU on that
case is a worthy cause.

Re: Improving EQ

[Andrea Corallo]

Óscar Fuentes <ofv@wanadoo.es> writes:

> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> writes:
>
>> I looked at the "new" code generated for our EQ macro, and decided that
>> a fix was in order.  I'm therefore sending a first proposal to explain
>> what I think should be done, and some numbers.
>>
>> This patch:
>> * moves the "slow path" of EQ into a NO_INLINE function
>> * exits early if the arguments to EQ are actually BASE_EQ
>> * returns quickly (after a single memory access which cannot be avoided
>> until we fix our tagging scheme to distinguish exotic objects from
>> ordinary ones) when symbols_with_pos_enabled isn't true.
>>
>> The effect on the code size of the stripped emacs binary is small, but
>> significant: 8906336 bytes instead of 8955488 bytes on this machine.
>> (The effect on the code size of the emacs binary with debugging
>> information is much larger, reducing it from 32182000 bytes to 31125832
>> bytes on this system.)  There is no effect on the size of the .pdmp
>> file, which is expected.
>>
>> What's missing here is a benchmark, but unless there's a really nasty
>> surprise when that happens, I'm quite confident that we can improve the
>> code here.
>
> I've seen too many cases where *removing* instructions (mind you,
> literally removing, not changing!) made the code significantly slower.
>
> Modern CPUs are insanely complex and combined with compilers make
> intuition-based predictions even more futile.

That's why the patch needs to be benchmarked anyway.

> But reading your message makes me wonder if EQ and some other "simple"
> fundamental functions are not lowered by nativecomp? If not, maybe
> that's a significant opportunity for improvement.

Nativecomp only compiles eq for Lisp code, the one discussed here is the
eq used in C (and bytecode).

BTW ATM nativecomp generates code with the same layout of the eq we had
in C till my last change of few weeks ago.  When eq will be stable in C
I guess I'll replicate the layout for generated code for Lisp as well.

  Andrea

Re: Improving EQ

[Óscar Fuentes]

Andrea Corallo <acorallo@gnu.org> writes:

>> But reading your message makes me wonder if EQ and some other "simple"
>> fundamental functions are not lowered by nativecomp? If not, maybe
>> that's a significant opportunity for improvement.
>
> Nativecomp only compiles eq for Lisp code, the one discussed here is the
> eq used in C (and bytecode).

Ok, thanks.

Of course this change also affects Emacs running with nativecomp, as
many calls to EQ are made by C functions not lowered by nativecomp.

My guess is that nativecomp's performance would benefit quite a bit from
the general approach of this patch, as every point where nativecomp
calls C is a pessimization spot, but that's another topic.

Re: Improving EQ

[Pip Cet via Emacs development discussions.]

Óscar Fuentes <ofv@wanadoo.es> writes:

> Andrea Corallo <acorallo@gnu.org> writes:
>
>>> But reading your message makes me wonder if EQ and some other "simple"
>>> fundamental functions are not lowered by nativecomp? If not, maybe
>>> that's a significant opportunity for improvement.
>>
>> Nativecomp only compiles eq for Lisp code, the one discussed here is the
>> eq used in C (and bytecode).

Does nativecomp actually call emit_EQ for anything but lowering ELC jump
tables into a sequence of conditional branches?  I don't see any code to
do so, and Emacs builds fine without emit_EQ if
byte-compile-cond-use-jump-table is disabled.

> Of course this change also affects Emacs running with nativecomp, as
> many calls to EQ are made by C functions not lowered by nativecomp.

My impression is that nativecomp usually ends up calling Feq for
eq-based conditions.

My point is that emit_EQ is used very rarely, when emitting a switch
statement (and switch statements should usually use
maybe_remove_pos_from_symbol + BASE_EQ rather than EQ).

So I won't bother doing anything with emit_EQ.

Pip

Re: Improving EQ

[Pip Cet via Emacs development discussions.]

"Andrea Corallo" <acorallo@gnu.org> writes:
> Óscar Fuentes <ofv@wanadoo.es> writes:
>> I've seen too many cases where *removing* instructions (mind you,
>> literally removing, not changing!) made the code significantly slower.

Yes, there are many ways in which that can happen. Removing prefetches
or branch hints is the most obvious example, but I don't claim to know
all the ways, and ultimately if an expected performance improvement does
not materialize we might have to decide this one on code size reasons
alone (of course, if performance is drastically worse, we shouldn't
apply the patch).

>> Modern CPUs are insanely complex and combined with compilers make
>> intuition-based predictions even more futile.

The compiler isn't the issue here, since I checked the assembly code
that was generated. Totally agree about CPUs. For example, moving code
out of line will change many conditional branch locations to a single
one (the one in the out-of-line function), which may help or hurt branch
prediction, and that's just one of many ways in which inline functions
often lose.  So we should also benchmark whether this might be one of
those cases, in which case we'd want to move all of EQ to a non-inlined
function...

> That's why the patch needs to be benchmarked anyway.

Absolute agreement there. I tried some initial benchmarks and it's lost
in the noise, but that was while running on battery on a laptop, and I
need to test on a machine with a proper fixed clock rate.

>> But reading your message makes me wonder if EQ and some other "simple"
>> fundamental functions are not lowered by nativecomp? If not, maybe
>> that's a significant opportunity for improvement.
>
> Nativecomp only compiles eq for Lisp code, the one discussed here is the
> eq used in C (and bytecode).
>
> BTW ATM nativecomp generates code with the same layout of the eq we had
> in C till my last change of few weeks ago.  When eq will be stable in C
> I guess I'll replicate the layout for generated code for Lisp as well.

Thanks, that would be great. Yes, it makes most sense to test with and
without nativecomp, expecting improvement to be more significant in the
latter case (but as EQ is used by C code used by native-compiled Lisp, I
expect a small improvement there, too).

Pip

Re: Improving EQ

[John ff]

[-- Attachment #1: Type: text/plain, Size: 2352 bytes --]




-------- Original Message --------
From: Andrea Corallo <acorallo@gnu.org>
Sent: Thu Dec 12 10:50:04 GMT 2024
To: "Óscar Fuentes" <ofv@wanadoo.es>
Cc: emacs-devel@gnu.org, Pip Cet <pipcet@protonmail.com>
Subject: Re: Improving EQ

Óscar Fuentes <ofv@wanadoo.es> writes:

> Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
> writes:
>
>> I looked at the "new" code generated for our EQ macro, and decided that
>> a fix was in order.  I'm therefore sending a first proposal to explain
>> what I think should be done, and some numbers.
>>
>> This patch:
>> * moves the "slow path" of EQ into a NO_INLINE function
>> * exits early if the arguments to EQ are actually BASE_EQ
>> * returns quickly (after a single memory access which cannot be avoided
>> until we fix our tagging scheme to distinguish exotic objects from
>> ordinary ones) when symbols_with_pos_enabled isn't true.
>>
>> The effect on the code size of the stripped emacs binary is small, but
>> significant: 8906336 bytes instead of 8955488 bytes on this machine.
>> (The effect on the code size of the emacs binary with debugging
>> information is much larger, reducing it from 32182000 bytes to 31125832
>> bytes on this system.)  There is no effect on the size of the .pdmp
>> file, which is expected.
>>
>> What's missing here is a benchmark, but unless there's a really nasty
>> surprise when that happens, I'm quite confident that we can improve the
>> code here.
>
> I've seen too many cases where *removing* instructions (mind you,
> literally removing, not changing!) made the code significantly slower.
>
> Modern CPUs are insanely complex and combined with compilers make
> intuition-based predictions even more futile.

That's why the patch needs to be benchmarked anyway.

> But reading your message makes me wonder if EQ and some other "simple"
> fundamental functions are not lowered by nativecomp? If not, maybe
> that's a significant opportunity for improvement.

Nativecomp only compiles eq for Lisp code, the one discussed here is the
eq used in C (and bytecode).

BTW ATM nativecomp generates code with the same layout of the eq we had
in C till my last change of few weeks ago.  When eq will be stable in C
I guess I'll replicate the layout for generated code for Lisp as well.

  Andrea


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