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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang/LLVM optimizes division and modulo worse than MSVC"
href="https://bugs.llvm.org/show_bug.cgi?id=37983">37983</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Clang/LLVM optimizes division and modulo worse than MSVC
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sfinae@hotmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=20488" name="attach_20488" title="Test case">attachment 20488</a> <a href="attachment.cgi?id=20488&action=edit" title="Test case">[details]</a></span>
Test case
This affects the Ryu algorithm for printing floating-point numbers
(<a href="https://github.com/ulfjack/ryu">https://github.com/ulfjack/ryu</a> ) and therefore affects C++17 floating-point
std::to_chars(). This is possibly the same bug as
<a class="bz_bug_link
bz_status_NEW "
title="NEW - Division followed by modulo generates longer machine code than vice versa"
href="show_bug.cgi?id=23106">https://bugs.llvm.org/show_bug.cgi?id=23106</a> "Division followed by modulo
generates longer machine code than vice versa".
I observe that MSVC's codegen is unaffected by USE_MODULO, while Clang/LLVM
generates more assembly code (which is slower when profiled in the real
algorithm) for USE_MODULO.
C:\Temp\TESTING_X64>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.15.26504 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
usage: cl [ option... ] filename... [ /link linkoption... ]
C:\Temp\TESTING_X64>clang-cl -m64 -v
clang version 6.0.0 (tags/RELEASE_600/final)
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: S:\msvc\src\vctools\NonShip\ClangLLVM\bin
C:\Temp\TESTING_X64>type meow.cpp
unsigned long long ryu(unsigned long long vp, unsigned long long vm) {
bool vmIsTrailingZeros = true;
while (vp / 10 > vm / 10) {
#ifdef USE_MODULO
vmIsTrailingZeros &= vm % 10 == 0;
#else
// The compiler does not realize that vm % 10 can be computed from vm /
10
// as vm - (vm / 10) * 10.
vmIsTrailingZeros &= vm - (vm / 10) * 10 == 0; // vm % 10 == 0;
#endif
vp /= 10;
vm /= 10;
}
return vmIsTrailingZeros ? vp : vm;
}
C:\Temp\TESTING_X64>cl /EHsc /nologo /W4 /MT /O2 /c /FAsc
/Famsvc_workaround.cod meow.cpp
meow.cpp
C:\Temp\TESTING_X64>cl /EHsc /nologo /W4 /MT /O2 /c /FAsc /Famsvc_modulo.cod
/DUSE_MODULO meow.cpp
meow.cpp
C:\Temp\TESTING_X64>clang-cl -m64 /EHsc /nologo /W4 /MT /O2 /c /FA
/Faclang_workaround.asm meow.cpp
C:\Temp\TESTING_X64>clang-cl -m64 /EHsc /nologo /W4 /MT /O2 /c /FA
/Faclang_modulo.asm /DUSE_MODULO meow.cpp
C:\Temp\TESTING_X64></pre>
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