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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstCombine: frem X, (C ? 0 : Y) -> frem X, Y changes result"
href="https://llvm.org/bugs/show_bug.cgi?id=26943">26943</a>
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<th>Summary</th>
<td>InstCombine: frem X, (C ? 0 : Y) -> frem X, Y changes result
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>MacOS X
</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>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>andres.noetzli@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The following program:
$ cat ../example.ll
define i1 @main() {
%w = call float @foo(i1 1, float 17.0)
%w2 = frem float 17.0, 42.0
%rr = fcmp oeq float %w, %w2
ret i1 %rr
}
define float @foo(i1 %c, float %x) {
%r = select i1 %c, float 0.0, float 42.0
%rr = frem float %x, %r
ret float %rr
}
gets optimized to:
$ bin/opt < ../example.ll -instcombine -S > ../example2.ll && cat
../example2.ll
; ModuleID = '<stdin>'
define i1 @main() {
%w = call float @foo(i1 true, float 1.700000e+01)
%rr = fcmp oeq float %w, 1.700000e+01
ret i1 %rr
}
define float @foo(i1 %c, float %x) {
%rr = frem float %x, 4.200000e+01
ret float %rr
}
The output of the original program and the optimized program is not the same:
$ bin/lli ../example.ll; echo $?
0
$ bin/lli ../example2.ll; echo $?
1
It seems like the optimization assumes that a frem %x, 0.0 results in undefined
behavior but according to [0], the result should just be NaN and an error flag
should be set. My understanding is that frem corresponds to C's fmod (see e.g.
[1]), so it seems like the optimization is invalid.
[0] <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fmod.html">http://pubs.opengroup.org/onlinepubs/9699919799/functions/fmod.html</a>
[1]
<a href="http://llvm.org/docs/doxygen/html/classllvm_1_1APFloat.html#ab715d4ba706dd8dd1c7d51e43d067d4c">http://llvm.org/docs/doxygen/html/classllvm_1_1APFloat.html#ab715d4ba706dd8dd1c7d51e43d067d4c</a></pre>
</div>
</p>
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