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From: "joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/102930] equal values appear to be different due to missing correct rounding in libc
Date: Tue, 26 Oct 2021 00:59:19 +0000 [thread overview]
Message-ID: <bug-102930-4-7rTS5bJWZM@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-102930-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102930
--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Mon, 25 Oct 2021, vincent-gcc at vinc17 dot net via Gcc-bugs wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102930
>
> --- Comment #3 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
> (In reply to Jakub Jelinek from comment #2)
> > I think there is nothing that can be done about this, and something like
> > this has been there since forever. While perhaps some double routines in
> > glibc used to be at some point correctly roundded, many others (e.g. float
> > or long double) never were, there is always a chance you get different
> > results from compile time folding vs. runtime evaluation.
>
> In this case, for its optimizations, GCC shouldn't assume that these results
> are equal.
Indeed. It's fine that different executions of a call to such a function
*in the abstract machine* return different values. But each execution of
such a call in the abstract machine must return some particular value
(possibly different for different executions); it's not valid to optimize
the code in a way that's inconsistent with a single return value for each
call to the function in the abstract machine. (Likewise for other cases
discussed in other bugs such as out-of-range floating-point-to-integer
conversions.)
next prev parent reply other threads:[~2021-10-26 0:59 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-25 13:43 [Bug middle-end/102930] New: " vincent-gcc at vinc17 dot net
2021-10-25 13:51 ` [Bug middle-end/102930] " vincent-gcc at vinc17 dot net
2021-10-25 13:57 ` jakub at gcc dot gnu.org
2021-10-25 14:01 ` vincent-gcc at vinc17 dot net
2021-10-26 0:59 ` joseph at codesourcery dot com [this message]
2021-10-26 7:06 ` rguenth at gcc dot gnu.org
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