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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization for multiplication on 1.5 and 1.25"
href="https://bugs.llvm.org/show_bug.cgi?id=43257">43257</a>
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<th>Summary</th>
<td>Missed optimization for multiplication on 1.5 and 1.25
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>zamazan4ik@tut.by
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>On x86_32 for any number X of type (unsigned, unsigned short, unsigned char)
multiplication by 1.5 with a conversion back to unsigned with any rounding mode
produces the exactly same result as if X + (X >> 1).
Same holds for 1.25:
unsigned(X * 1.25) == unsigned(X + (X >> 2))
The above transformation allows to emit a short code without floating point
computations:
test2(unsigned int): # @test2(unsigned int)
mov eax, edi
shr eax
lea eax, [rax + rdi]
ret
Instead of:
test(unsigned int): # @test(unsigned int)
mov eax, edi
cvtsi2sd xmm0, rax
mulsd xmm0, qword ptr [rip + .LCPI0_0]
cvttsd2si rax, xmm0
ret
Same issue in GCC: <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91709">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91709</a>
Godbolt playground: <a href="https://godbolt.org/z/zVQuwP">https://godbolt.org/z/zVQuwP</a></pre>
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