Re: GCC bug when -ffast-math is set on armel
On Thu, Mar 19, 2009 at 09:40:47AM +0000, Martin Guy wrote:
> I thought I should mention this here too in case anyone else gets
> bitten by this armel-specific bug.
> A GCC bug has just turned up that affects the arm-*-gnueabi
> architecture in gcc-4.[123]. While debugging libvorbisenc (which
> produces silent output files on armel), it turns out that when -O
> -ffast-math are set, GCC can produce incorrect code for the max(x,y)
> macro applied to floating point values.
Thanks for finding this out. Has a bug been filed to the GCC bugtracker?
> Thanks to Erik de Castro Lopo for some incisive debugging and for
> producing a minimal example (should print "0 0" but doesn't)
>
> /*
> ** This file is in the Public Domain.
> **
> ** This program demonstrates a bug in the -ffast-math option of the gcc
> ** armel compiler : gcc version 4.3.2 (Debian 4.3.2-1.1)
> **
> ** This works as expected:
> **
> ** > gcc -Wall -O3 gcc-test.c -o gcc-test && ./gcc-test
> ** min : 0.0000 max : 0.0000
> **
> ** Compile with -ffast-math and things goes screwy.
> **
> ** > gcc -Wall -O3 -ffast-math gcc-test.c -o gcc-test && ./gcc-test
> ** min : 99999.0000 max : 0.0000
> */
>
> #include <stdio.h>
>
> #define COUNT 10
>
> #define test_max(x,y) ((x) < (y) ? (y) : (x))
> #define test_min(x,y) ((x) > (y) ? (y) : (x))
>
> int
> main (void)
> { /* C Standard says static data gets initialized to zero. */
> static float data [COUNT] ;
> float max = -99999.0, min = 99999.0 ;
> int k ;
>
> for (k = 0 ; k < COUNT ; k++)
> { max = test_max (max, data [k]) ;
> min = test_min (min, data [k]) ;
> } ;
>
> printf ("min : %12.4f max : %12.4f\n", min, max) ;
>
> return 0 ;
> }
>
> Full details at http://bugs.debian.org/515949
>
> M
>
>
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