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swizzles_avx fails with GCC 5.1 #9

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mattkretz opened this issue Jul 14, 2015 · 5 comments
Closed

swizzles_avx fails with GCC 5.1 #9

mattkretz opened this issue Jul 14, 2015 · 5 comments
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type: bug type: miscompilation
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Vc 0.7.5

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@mattkretz
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https://cdash.gsi.de/testDetails.php?test=272280&build=51127

 FAIL: ┍ at /home/mkretz/.Vc-Test/Vc-0.7/tests/swizzles.cpp:115 (0x40da5b):
 FAIL: │ test.badc() ([23591, -7970, 18436, -13100, 29690, 24409, 12135, 29166]) == scalarSwizzle(test, BADC) ([23591, -7970, 18436, -13100, 29690, -7970, 12135, 29166]) -> m[1111 1011] 
 FAIL: ┕ testSwizzle<short_v>
 FAIL: ┍ at /home/mkretz/.Vc-Test/Vc-0.7/tests/swizzles.cpp:115 (0x40f367):
 FAIL: │ test.badc() ([57770, 9820, 42369, 36174, 26448, 46473, 35535, 47990]) == scalarSwizzle(test, BADC) ([57770, 9820, 42369, 36174, 26448, 9820, 35535, 47990]) -> m[1111 1011] 
 FAIL: ┕ testSwizzle<ushort_v>
@mattkretz mattkretz self-assigned this Jul 14, 2015
@mattkretz mattkretz added this to the Vc 0.7.5 milestone Jul 14, 2015
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The following indicates that the Vc function works correctly, but the reference value is wrong.

Vc:  [23591, -7970, 18436, -13100, 29690, 24409, 12135, 29166]
Ref: [23591, -7970, 18436, -13100, 29690, -7970, 12135, 29166]
Orig:[-7970, 23591, -13100, 18436, 24409, 29690, 29166, 12135]

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GCC 5.1.0 miscompiles the reference function, which seems to indicate that AVX::Vector<short>::operator[] is broken.

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This is the relevant bug in GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66866

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A possible workaround for Vc is a store to (u)short[8] and subsequent array dereference in Vector<T>::operator[](size_t) const. I'll wait for the feedback on the GCC issue first, though.

mattkretz added a commit that referenced this issue Jul 17, 2015
@mattkretz
AVX: subscript operator workaround for GCC 5.1
db35c7e 
This works around pr66866 in GCC, where GCC messes up the member
extraction.

Fixes gh-9.

Signed-off-by: Matthias Kretz <kretz@kde.org>
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This bug is fixed in GCC on all branches starting from 4.9. I implemented a workaround for the compilers that fail.

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