Date: Wed, 14 May 2003 02:42:25 -0700 From: David Schultz <das@FreeBSD.ORG> To: Mikhail Teterin <mi+mx@aldan.algebra.com> Cc: freebsd-bugs@FreeBSD.ORG Subject: Re: bin/43299: march=pentium4 miscompiles msun/src/e_pow.c Message-ID: <20030514094225.GA7355@HAL9000.homeunix.com> In-Reply-To: <200305131630.12451.mi%2Bmx@aldan.algebra.com> References: <200305131610.h4DGA8vx007871@freefall.freebsd.org> <20030513180417.GA4917@HAL9000.homeunix.com> <200305131630.12451.mi%2Bmx@aldan.algebra.com>
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On Tue, May 13, 2003, Mikhail Teterin wrote:
> It seems like the problem is inside the SET_HIGH_WORD(ax, ix) macro
> on line e_pow.c:214, which expands into
> into
>
> do {
> ieee_double_shape_type sh_u;
>
> sh_u.value = (ax);
> sh_u.parts.msw = (ix);
> (ax) = sh_u.value;
> } while (0);
>
> for whatever reason, if compiled for pentium4 with -O1, the line
>
> sh_u.value = (ax)
>
> is SKIPPED -- according to the debugger. ax ends up filled with junk,
> which screws everything up afterwards... I'm attaching the simple test
> file (the skipped line is 34 in it). If compiled for p3 or for p4 with
> -O0, the result is 0.5. When compiled for p4 with -O1, the result
> is junk. The junk is persistent -- I'm getting ``5.29463e-315'' on
> both FreeBSD (gcc version 3.2.2 20030205) and Linux (gcc version 3.2
> 20020903).
The C standard does not make any provisions for type punning,
except for chars. However, SET_HIGH_WORD is doing it in the
generally accepted way, which gcc claims to support. This looks
like a bug in gcc, and you seem to have determined that it is not
FreeBSD specific. It should probably be reported to the gcc folks
if it is still a problem in a recent gcc 3.3 snapshot. (It's
great that you found a short test case; they appreciate that.)
BTW, thanks for taking the time to look into this! I don't have a
P4 to test on or enough free cycles at the moment.
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