Date: Wed, 14 Jun 2000 02:40:05 -0700 (PDT) From: Bruce Evans <bde@zeta.org.au> To: freebsd-bugs@FreeBSD.org Subject: Re: i386/19245: -fexpensive-optimizations buggy (even with -O) Message-ID: <200006140940.CAA76259@freefall.freebsd.org>
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The following reply was made to PR i386/19245; it has been noted by GNATS.
From: Bruce Evans <bde@zeta.org.au>
To: mi@aldan.algebra.com
Cc: FreeBSD-gnats-submit@FreeBSD.ORG
Subject: Re: i386/19245: -fexpensive-optimizations buggy (even with -O)
Date: Wed, 14 Jun 2000 19:37:28 +1000 (EST)
On Tue, 13 Jun 2000, Mikhail Teterin wrote:
> >Description:
>
> The attached piece of code, when compiled with
> ``-O -fexpensive-optimizations'', produces incorrect
> binary on FreeBSD-4.0 .
>
> I tested the same compiler line on Mandrake Linux (an
> identical machine hardware-wise) and it compiles correctly.
This is hard to explain, since the bug shown by your example is in gcc
(2.95.2), not in the assembler or linker.
> static off_t
> rfc1035QuestionPack(char *buf,
> size_t sz,
> const char *hostname,
> unsigned short class
> )
> {
> off_t off = 0;
> unsigned short s;
> printf("In rfc1035QuestionPack: hostname is %p (%s)\n",
> hostname, hostname);
> s = htons(class);
> memcpy(buf + off, &s, sizeof(s));
> off += sizeof(s);
> assert(off <= sz);
> return off;
> }
gcc -O -fexpensive-optimizations reuses the stack space for `hostname' and
`class', and zeros this space to initialize `off' before loading `hostname'
or `class'.
> Yet, with the -fexpensive-optimizations flag, the hostname
> argument is passed in the register, which, apparently, is
> sometimes not loaded with the value and remains zero, resulting
> in:
No, -fexpensive-optimizations doesn't affect the function call protocol.
Args are still passed on the stack.
> >Fix:
> Get the new assembler/binutils and add -fno-expensive-optimizations
> to all CFLAGS in the meantime. Anything else?
Don't use -O2 (which enables -fexpensive-optimizations) unless you want to
find bugs like this :-).
Bruce
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