Am 03.06.2014 16:45, schrieb lvqcl:
2) to ALL:
I attached a small program. Compile and run it.
* Does it work correctly when compiled with -O3 -msse2 options?
* If yes, does it work correctly when compiled with -O3 -funroll-loops
-msse2 options?
( and what is the version of your GCC? )
I further reduced the testcase (attached).
The bug only occurs if N >= 64; presumably the second loop is only SSE2
optimized if that's the case.
The problem seems to be that sum is interpreted as a 64 bit value if
SSE2 was used in the loop (the lower 32 bits of the result give the
expected value). If sum is evaluated another time before or after (!)
the printf, the problem goes away. For example, changing the last line
to "return sum + 1;" lets the problem disappear.
I confirmed the bug with GCC 4.6.3 on Ubuntu. As on Windows, only 32 bit
code generation is affected.
You should file a bug report with the GCC team.
#include <stdio.h>
#define N 64 /* problem is triggered only if N >= 64 */
unsigned A[N];
int main()
{
unsigned i, sum = 0; /* both sum and A[] need to be unsigned for the
bug to happen */
for (i = 0; i < N; i++) A[i] = 1;
for (i = 0; i < N; i++) sum += A[i];
printf("Sum = %f (should be equal to %i)\n", (float) sum, N);
return 0;
}
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