Re: [flac-dev] Two questions about RG in flac
Ozkan Sezer wrote: With gcc-3,3,6, 3,4,6, 4.3.0 and gcc-4.9.1 (svn r210839) the output is normal: Sum = 64.00 (should be equal to 64) With gcc-4.8.3 (release version) it's broken: Sum = 206158430272.00 (should be equal to 64) With clang-3.4.1 (compiled with gcc-4.8.3) the output is normal again. This is on i686-linux (fedora9, glibc-2.8, kernel-2.6.27.35) Thank you for testing. However, I compiled my test with gcc 4.9.1 20140604 from dongsheng-daily ( http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/dongsheng-daily/4.9/ , file gcc-4.9-win32_4.9.1-20140604.7z) and it still fails... ___ flac-dev mailing list flac-dev@xiph.org http://lists.xiph.org/mailman/listinfo/flac-dev
Re: [flac-dev] Two questions about RG in flac
Robert Kausch wrote: 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. Thank you for testing. You should file a bug report with the GCC team. Done: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61423 ___ flac-dev mailing list flac-dev@xiph.org http://lists.xiph.org/mailman/listinfo/flac-dev
Re: [flac-dev] Two questions about RG in flac
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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Re: [flac-dev] Two questions about RG in flac
On 6/3/14, Robert Kausch robert.kau...@freac.org wrote: 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. With gcc-3,3,6, 3,4,6, 4.3.0 and gcc-4.9.1 (svn r210839) the output is normal: Sum = 64.00 (should be equal to 64) With gcc-4.8.3 (release version) it's broken: Sum = 206158430272.00 (should be equal to 64) With clang-3.4.1 (compiled with gcc-4.8.3) the output is normal again. This is on i686-linux (fedora9, glibc-2.8, kernel-2.6.27.35) ___ flac-dev mailing list flac-dev@xiph.org http://lists.xiph.org/mailman/listinfo/flac-dev
