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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [x86-64] Suboptimal codegen for (!x) as opposed to (0 == x)"
href="https://bugs.llvm.org/show_bug.cgi?id=39968">39968</a>
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<th>Summary</th>
<td>[x86-64] Suboptimal codegen for (!x) as opposed to (0 == x)
</td>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>arthur.j.odwyer@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<div>
<pre><a href="https://godbolt.org/z/Z2afe8">https://godbolt.org/z/Z2afe8</a>
#include <stdint.h>
struct u128 { uint64_t x, y; };
u128 shl2(uint64_t rdi, uint64_t rdx, int n)
{
if (n & 64) rdx = rdi;
#if WORSE
if (!(n & 64)) rdi = 0;
#else
if (0 == (n & 64)) rdi = 0;
#endif
return {rdi,rdx};
}
Without -DWORSE, Clang generates perfect codegen.
andl $64, %edx
xorl %eax, %eax
cmovneq %rdi, %rsi
cmovneq %rdi, %rax
movq %rsi, %rdx
retq
But with -DWORSE, Clang generates a dead-store "shr" instruction:
andl $64, %edx
xorl %eax, %eax
shrl $6, %edx // USELESS INSTRUCTION!
cmovneq %rdi, %rsi
cmovneq %rdi, %rax
movq %rsi, %rdx
retq
This is surprising, given that the only difference is using (0 == x) versus
(!x). I would expect these to have equivalent codegen.</pre>
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