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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - AVX512 Regression: KNL backend does not use KNL processor features, SKX works"
href="https://llvm.org/bugs/show_bug.cgi?id=31817">31817</a>
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<th>Summary</th>
<td>AVX512 Regression: KNL backend does not use KNL processor features, SKX works
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>wenzel.jakob@epfl.ch
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>I've noticed a serious performance and code generation regression on Clang's
KNL (= Knight's Landing) target.
With the latest trunk version of Clang & LLVM, the compiler no longer makes use
of any of the new AVX512 architectural features when working with non-512bit
registers (XMM & ZMM). This includes making full use of all 32 registers
(instead of 16) and using extensions such as builtin broadcasting of constants.
Losing half of the registers in particular is a serious performance regression
-- this wasn't the case with older versions of Clang.
Interestingly, the SKX == skylake-avx512 target remains unaffected.
Here is an example for the broadcasting of constants. Consider the following
snippet:
#include <immintrin.h>
__m256 mul_constant(__m256 x) {
return _mm256_mul_ps(x, _mm256_set1_ps(1234.f));
}
Compiling this for the "SKX" target produces the following code:
$ clang-5.0 -march=knl -O3 -S -fomit-frame-pointer test.c -o test.o
Assembly (cleaned up):
_mul_constant:
vmulps LCPI0_0(%rip){1to8}, %ymm0, %ymm0 <----- good!
retq
On the other hand, compiling for "KNL" yields
$ clang-5.0 -march=knl -O3 -S -fomit-frame-pointer test.c -o test.o
Assembly (cleaned up):
_mul_constant:
vbroadcastss LCPI0_0(%rip), %ymm1 <----- ?!?
vmulps %ymm1, %ymm0, %ymm0 <----- code should be
identical
retq</pre>
</div>
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