From: Pip Cet <pipcet@gmail.com>
To: Bruno Haible <bruno@clisp.org>
Cc: 36370@debbugs.gnu.org, Paul Eggert <eggert@cs.ucla.edu>,
bug-gnulib@gnu.org
Subject: bug#36370: 27.0.50; XFIXNAT called on negative numbers
Date: Fri, 28 Jun 2019 13:51:24 +0000 [thread overview]
Message-ID: <CAOqdjBfS99UpLZ-qLe4=FMXMsr+T3LUvJEsf_gfmF6wwLbqgOw__12120.8733807848$1561734682$gmane$org@mail.gmail.com> (raw)
In-Reply-To: <8979488.cRkkfcT1mV@omega>
On Fri, Jun 28, 2019 at 12:14 PM Bruno Haible <bruno@clisp.org> wrote:
> Pip Cet wrote:
> > This makes it safe to use function expressions in eassume, whether the
> > function is inlined or not.
>
> By "safe" you mean that you want the function call to not be evaluated.
Sorry, sloppy wording there. You're right.
> You are mentioning a limitation:
>
> > eassume(i >= 0 && i < complicated_function ());
> >
> > will not "split" the && expression, so it'll behave differently from
> >
> > eassume(i >= 0);
> > eassume(i < complicated_function ());
>
> And I would mention a regression: When -flto is in use and the expression
> invokes an external potentially-inlined function, the old 'assume' would
> work fine, i.e. do optimizations across compilation-unit boundaries.
Sorry, can't reproduce that here. I'm sure the changes I need to make
are obvious once I've found them, but can you let me know your gcc
version?
> > But even in those cases, this approach is better than the old approach
> > of actually evaluating complicated_function.
>
> I disagree that it is better:
Sorry to be pedantic, but do you disagree that it is better in these
cases, or in general? The latter is a question that I'm trying to find
the answer to, but in these specific cases, it clearly is better.
(Just in the interest of full disclosure, I described the idea in a
different context; I think it's a neat hack, and I'm trying to figure
out whether it has practical applications, but if it doesn't then I
won't feel there's continuing disagreement).
> 1. The new 'assume' is worse when -flto is in use.
Maybe. Even if it is, though, that's a GCC limitation which I consider
likely to be fixable; your estimation of that may vary, of course.
> 2. You recommend to users to split assume(A && B) into assume(A); assume(B);
> which is unnatural.
I make that recommendation independently of which assume is in use.
In practice, combining a complicated expression with a simple one in
an eassume is almost always not what you want to do. It's way too easy
to do something like
eassume(ptr->field >= 0 && f(ptr));
when what you mean is
eassume(ptr->field >= 0);
eassume(f(ptr));
(As an unusual special case, consider:
{
printf("%d\n", i & 0x80000000);
assume(i >= 0 && complicated_function());
}
which would generate different code from
{
printf("%d\n", i & 0x80000000);
assume(i >= 0);
assume(complicated_function());
})
Combining two simple expressions and not getting the right result
appears, at this point, to run into a GCC limitation, but I'm not sure
where.
> > At first, I thought it would be better to have a __builtin_assume
> > expression at the GCC level, but even that would have to have "either
> > evaluate the entire condition expression, or evaluate none of it"
> > semantics.
>
> No. At GCC level, it could have a "make the maximum of inferences - across
> all optimization phases -, but evaluate none of it" semantics.
There's no contradiction there: I'm saying that the programmer is
allowed to assume that the expression passed to assume either has been
evaluated, or hasn't been, with no in-between interpretations allowed
to the compiler. That means assume (A && B) isn't equivalent, in
general, to assume (A); assume (B); My suspicion is that the latter is
almost always what is intended.
next prev parent reply other threads:[~2019-06-28 13:51 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-25 5:36 bug#36370: 27.0.50; XFIXNAT called on negative numbers Pip Cet
2019-06-27 1:10 ` Paul Eggert
2019-06-27 6:16 ` Pip Cet
2019-06-27 8:28 ` Paul Eggert
2019-06-27 13:17 ` Pip Cet
2019-06-27 13:37 ` Eli Zaretskii
2019-06-27 19:38 ` Paul Eggert
2019-06-27 19:56 ` Pip Cet
2019-06-27 21:13 ` Paul Eggert
[not found] ` <5284eb58-3560-da42-d1d1-3bdb930eae49@cs.ucla.edu>
2019-06-27 21:37 ` Pip Cet
2019-06-27 23:45 ` Bruno Haible
[not found] ` <2715311.ceefYqj39C@omega>
2019-06-28 0:04 ` Paul Eggert
2019-06-28 11:06 ` Pip Cet
2019-06-28 12:14 ` Bruno Haible
[not found] ` <8979488.cRkkfcT1mV@omega>
2019-06-28 12:29 ` Bruno Haible
2019-06-28 13:51 ` Pip Cet [this message]
[not found] ` <CAOqdjBfS99UpLZ-qLe4=FMXMsr+T3LUvJEsf_gfmF6wwLbqgOw@mail.gmail.com>
2019-06-28 17:46 ` Paul Eggert
2019-06-28 19:11 ` Bruno Haible
[not found] ` <a293f2fe-99b3-3776-f27b-35e3a93d1d34@cs.ucla.edu>
2019-06-28 19:15 ` Pip Cet
2019-06-28 19:56 ` Bruno Haible
2019-06-28 21:08 ` Pip Cet
2019-06-29 5:41 ` Paul Eggert
[not found] ` <87168b28-192b-6666-e9b6-9cdc2ed3917a@cs.ucla.edu>
2019-06-29 6:48 ` Pip Cet
[not found] ` <CAOqdjBfcNbXFw3Fb0wgRR10PNbkJQ+88ObE9KEghLSb-ptdrbA@mail.gmail.com>
2019-06-29 17:31 ` Paul Eggert
[not found] ` <791ae316-3a6f-605a-0da5-874fe3d224c5@cs.ucla.edu>
2019-06-30 9:21 ` Pip Cet
[not found] ` <11002295.LrvMqknVDZ@omega>
2019-06-28 21:07 ` Pip Cet
2019-06-28 23:30 ` Bruno Haible
[not found] ` <2067160.1HRgjLhtDS@omega>
2019-06-29 5:40 ` Paul Eggert
2019-06-29 5:44 ` Pip Cet
[not found] ` <CAOqdjBcNA4mDiwsd_jbeePGMdUwPvkFCNdgtZvmiQnYmJNR3pA@mail.gmail.com>
2019-06-29 10:31 ` Bruno Haible
[not found] ` <2515002.Q0mBYvUW8C@omega>
2019-06-29 17:11 ` Paul Eggert
[not found] ` <99bacb9f-1192-1315-85d7-5ab4924dfef8@cs.ucla.edu>
2019-06-29 17:48 ` Bruno Haible
2019-06-30 15:30 ` Pip Cet
[not found] ` <CAOqdjBeiMno7nGKwk7SSZQob+CTyG39KRTM9EEebq7NQavLR-Q@mail.gmail.com>
2019-06-30 15:45 ` Bruno Haible
2019-07-02 23:39 ` Paul Eggert
2019-07-01 1:46 ` Richard Stallman
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