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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Nested loop unroll bug on skylake avx512"
href="https://bugs.llvm.org/show_bug.cgi?id=44544">44544</a>
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<th>Summary</th>
<td>Nested loop unroll bug on skylake avx512
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</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>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>jakobschwarz@yahoo.com
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>I think, I found a bug in clang, tested on local machines and on godbolt with
clang 7, 8 and 9. It only occurs with -O3 optimization and
-march=skylake-avx512. With GCC and Intel the code produces correct results.
Disabling loop nesting in the example is also fine with Clang. The code should
return just zeros in the cout print.
#include <iostream>
int main(int argc, char *argv[])
{
static constexpr uint32_t mult = 4u;
static constexpr uint64_t MASK_H = 0x000000000000FFFFull;
uint64_t arr2[16][4];
for(auto i=0; i<16; i++) for(auto j=0; j<4; j++) arr2[i][j] = ~uint64_t(0);
uint64_t* mm =&arr2[0][0];
for(uint32_t zz=0; zz<16; zz++){
// #pragma clang loop unroll(disable)
for(uint32_t yy=0; yy<16; yy++){
const uint32_t ID = yy+zz*16;
const uint64_t mask = ~(MASK_H<<(ID%mult*16));
mm[ID/mult] &= mask;
}
}
for(auto i=0; i<16; i++) {
for(auto j=0; j<4; j++) std::cout << arr2[i][j] << " ";
std::cout << std::endl;
}
return 0;
}</pre>
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