c/6893: gcc generates invalid code for x86 subarches.

[warp at debian dot org]

>Number:         6893
>Category:       c
>Synopsis:       gcc generates invalid code for x86 subarches.
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Sat Jun 01 06:16:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     warp@debian.org
>Release:        3.0 and 3.1
>Organization:
>Environment:
Debian SID, 1GHz Athlon Thunderbird
>Description:
gcc generates code which does not seem to behave correctly for the source below when using -march=pentium, pentium-mmx, pentium2, athlon, and athlon-tbird.

It may also generate bad code for other flags but I have not been able to test such yet.
>How-To-Repeat:
Compile with 'gcc-3.1 -o bad_gcc bad_gcc.c -Wall -O2 -march=athlon' or the like, use with './bad_gcc 0.5 0.2 0.3 0.5'.
The program should covert four floats given on the command line, which should be in the 0-1 range, to bytes in the 0-255 range.

Source starts here:

#include <stdio.h>
#include <stdlib.h>

#define min(a,b) ((a) < (b) ? (a) : (b))

float source[4];
float tmp[4];
unsigned char out[4];

extern void inline FtoUB (float *in, char *out, int num)
{
    int         i, *icolor;
    float       *fcolor_to;

    fcolor_to = tmp;

    // shift float to have 8bit fraction at base of number
    for (i = 0; i < num; i += 4) {
        fcolor_to[i    ] = in[i    ] + 32768.0f;
        fcolor_to[i + 1] = in[i + 1] + 32768.0f;
        fcolor_to[i + 2] = in[i + 2] + 32768.0f;
        fcolor_to[i + 3] = in[i + 3] + 32768.0f;
    }

    icolor = (int *) tmp;

    // then read as integer and kill float bits...
    for (i = 0; i < num; i += 4) {
        out[i    ] = (char) min(icolor[i    ] & 0x7FFFFF, 255);
        out[i + 1] = (char) min(icolor[i + 1] & 0x7FFFFF, 255);
        out[i + 2] = (char) min(icolor[i + 2] & 0x7FFFFF, 255);
        out[i + 3] = (char) min(icolor[i + 3] & 0x7FFFFF, 255);
    }
}

int
main (int argc, char *argv[])
{
    source[0] = atof(argv[1]);
    source[1] = atof(argv[2]);
    source[2] = atof(argv[3]);
    source[3] = atof(argv[4]);

    FtoUB(source, out, 4);

    printf("in: %f %f %f %f\n", source[0], source[1], source[2], source[3]);
    printf("out: %d %d %d %d\n", out[0], out[1], out[2], out[3]);
    exit (0);
}
>Fix:
Work around is to either use gcc 2.95 or to not use -march.
>Release-Note:
>Audit-Trail:
>Unformatted: