[Bug target/43703] New: Unexpected floating point precision loss due to ARM NEON autovectorization

[siarhei dot siamashka at gmail dot com]

Using gcc-4.5.0-RC-20100406.tar.bz2

/************************************************************/
#include <stdio.h>

void __attribute__((noinline)) f(float * __restrict c,
                                 float * __restrict a,
                                 float * __restrict b)
{
    int i;
    for (i = 0; i < 4; i++) {
        c[i] = a[i] * b[i];
    }
}

int main()
{
    float a[4], b[4], c[4];

    a[0] = 1e-40;
    b[0] = 1e+38;

    f(c, a, b);

    printf("c[0]=%f\n", (double)c[0]);
    if (c[0] < 0.001)
        printf("precision problem: c[0] was flushed to zero\n");

    return 0;
}
/************************************************************/

# gcc -mcpu=cortex-a8 -mfloat-abi=softfp -mfpu=neon -O2 -fno-fast-math test.c
# ./a.out
c[0]=0.010000

# gcc -mcpu=cortex-a8 -mfloat-abi=softfp -mfpu=neon -O3 -fno-fast-math test.c
# ./a.out
c[0]=0.000000
precision problem: c[0] was flushed to zero


Using -O3 option turns on autovectorization, and the results of operations
involving denormals get flushed to zero. This happens even if no "-ffast-math"
or any other precision sacrificing options are enabled.


-- 
           Summary: Unexpected floating point precision loss due to ARM NEON
                    autovectorization
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: siarhei dot siamashka at gmail dot com
 GCC build triplet: armv7l-unknown-linux-gnueabi
  GCC host triplet: armv7l-unknown-linux-gnueabi
GCC target triplet: armv7l-unknown-linux-gnueabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43703