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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Division by a constant could be optimized for known variables value range"
href="https://bugs.llvm.org/show_bug.cgi?id=43432">43432</a>
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<th>Summary</th>
<td>Division by a constant could be optimized for known variables value range
</td>
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<th>Product</th>
<td>clang
</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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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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>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>zamazan4ik@tut.by
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Consider the example:
unsigned long long kBorder = (1ull<<62);
unsigned long long sample(unsigned long long m) {
if (m >= kBorder) __builtin_unreachable();
return m / 10;
}
It produces the following assembly:
sample(unsigned long long):
movabs rdx, -3689348814741910323
mov rax, rdi
mul rdx
mov rax, rdx
shr rax, 3
ret
However, knowing that the higher bits are always 0, the constant could be
adjusted to avoid the `shr rax, 3`:
sample(unsigned long long):
movabs rax, 1844674407370955162
mul rdi
mov rax, rdx
ret
Godbolt playground: <a href="https://godbolt.org/z/YU2yAC">https://godbolt.org/z/YU2yAC</a>
This issue is probably related to <a class="bz_bug_link
bz_status_NEW "
title="NEW - Value range knowledge of higher bits not used in optimizations"
href="show_bug.cgi?id=43431">https://bugs.llvm.org/show_bug.cgi?id=43431</a>
Same issue for GCC: <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91883">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91883</a>
P.S.: that optimization is important for std::to_chars(..., double) like
functions, where a significant of a double is extracted into an unsigned long
long variable, so its upper bits are always zero.</pre>
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