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<== Date ==> <== Thread ==>

Subject: Re: C strict aliasing rules
From: Eric Norum <norume@aps.anl.gov>
To: Benjamin Franksen <benjamin.franksen@bessy.de>
Cc: core-talk@aps.anl.gov
Date: Mon, 27 Nov 2006 20:02:38 -0600
On Nov 27, 2006, at 7:20 PM, Benjamin Franksen wrote:
Cheers
Ben
PS: whatever happened to the notorious compiler warning 'xyz disables
strict aliasing rules'? Seems to me as if compilers once were able to
detect cases where the assumptions broke down. Hmmm.

The explicit cast is what's suppressing any warning. Unfortunately I expect that we have lots of casts in EPICS which might cause similar (and silent) problems. FWIW, here's a version of the code that actually does produce the warning to which Benjamin is referring:
norume@gnarly 704> cat a.c
void
writethree(short *p)
{
    short *palias = p;
   *palias = 3;
}

int
testalias()
{
    int value = 12345678;
    writethree((short *)&value);
    return value;
}
norume@gnarly 705> m68k-rtems4.7-gcc -O4 -Wall -S a.c
a.c: In function 'testalias':
a.c:12: warning: dereferencing type-punned pointer will break strict- aliasing rules


So, do we:
1) Ensure that EPICS is built with an optimization level that doesn't use the strict aliasing rules?
or
2) Explictly prevent the compiler from using the strict aliasing rules (-fnostrict-aliasing for gcc, ??? for other compilers)?
or
3) Bury our heads in the sand and hope that the problem never arises in our code?
--
Eric Norum <norume@aps.anl.gov>
Advanced Photon Source
Argonne National Laboratory
(630) 252-4793



Replies:
Re: C strict aliasing rules Benjamin Franksen
References:
C strict aliasing rules Eric Norum
Re: C strict aliasing rules Andrew Johnson
Re: C strict aliasing rules Eric Norum
Re: C strict aliasing rules Benjamin Franksen

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