From: Daniel Colascione <dancol@dancol.org>
To: Eli Zaretskii <eliz@gnu.org>
Cc: eggert@cs.ucla.edu, emacs-devel@gnu.org
Subject: Re: [Emacs-diffs] trunk r114593: * lisp.h (eassert): Don't use 'assume'.
Date: Fri, 11 Oct 2013 01:19:21 -0700 [thread overview]
Message-ID: <5257B489.2050609@dancol.org> (raw)
In-Reply-To: <83eh7sp6v0.fsf@gnu.org>
On 10/11/13 1:08 AM, Eli Zaretskii wrote:
> Let me rephrase: assertions are used in unoptimized code, and compile
> to nothing in optimized code. 'assume' is not needed in unoptimized
> code, since that code is grossly inefficient anyway. Thus, it sounds
> like the two are almost perfectly orthogonal
Say we have this function:
void foo (int *a, int *b) { *a = *a / *b; }
Suppose it's part of foo's contract that it never be called with *b ==
0. In checked builds, we add an assertion to catch callers that violate
the contract:
void foo (int *a, int *b) { eassert (*b != 0); *a = *a / *b; }
Now suppose we also want the optimizer to take advantage of the fact
that *b != 0. Because we have the assertion, we're confident that
callers never actually pass a *b equal to 0.
void foo (int *a, int *b)
{ eassert (*b != 0); assume (*b != 0); *a = *a / *b; }
Now we've said *b != 0 twice. That's silly. We should just combine the
eassert and assume into a single construct:
void foo (int *a, int *b) { eassume (*b != 0); *a = *a / *b; }
The connection between assume and assert is that both of them accept an
expression that we expect to always evaluate to true. It's only what
the program does with that information that differs: in the assert case,
we check that the constraint is met. In the assume case, we optimize
assuming the constraint has been met. The condition itself is almost
always the same.
> , and lumping them
> together into a single construct is likely to continue bumping upon
> problems that stem from basic incompatibility between the use cases,
> which target two different non-overlapping build types.
Only when we have side effects. Looking through the code just now, I
only saw a few assertions that weren't obviously free of side effects.
> IOW, you should almost _never_ need to use 'assume' where you use
> 'eassert', and vice versa. So why do we need a single macro that does
> both?
I'd actually argue that you should almost always combine assert and
assume. You've gone through the trouble of spelling out constraints on
program execution: why not let the optimizer take advantage of that
information?
next prev parent reply other threads:[~2013-10-11 8:19 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <E1VTxwB-0001h8-7E@vcs.savannah.gnu.org>
2013-10-11 2:31 ` [Emacs-diffs] trunk r114593: * lisp.h (eassert): Don't use 'assume' Daniel Colascione
2013-10-11 6:36 ` Paul Eggert
2013-10-11 7:00 ` Eli Zaretskii
2013-10-11 7:41 ` Daniel Colascione
2013-10-11 8:08 ` Eli Zaretskii
2013-10-11 8:19 ` Daniel Colascione [this message]
2013-10-11 8:59 ` Stephen J. Turnbull
2013-10-11 9:10 ` Daniel Colascione
2013-10-11 10:27 ` Stephen J. Turnbull
2013-10-11 12:42 ` Stefan Monnier
2013-10-11 15:24 ` Stephen J. Turnbull
2013-10-11 9:06 ` Eli Zaretskii
2013-10-11 9:18 ` Daniel Colascione
2013-10-11 9:36 ` Eli Zaretskii
2013-10-11 9:55 ` Daniel Colascione
2013-10-11 10:31 ` Dmitry Antipov
2013-10-11 15:22 ` Paul Eggert
2013-10-11 15:41 ` Daniel Colascione
2013-10-12 7:37 ` Paul Eggert
2013-10-11 11:19 ` Eli Zaretskii
2013-10-11 15:57 ` Daniel Colascione
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