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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect swap of fptrunc with fast-math instructions"
href="https://bugs.llvm.org/show_bug.cgi?id=49080">49080</a>
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<th>Summary</th>
<td>Incorrect swap of fptrunc with fast-math instructions
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Keywords</th>
<td>miscompilation
</td>
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<th>Severity</th>
<td>normal
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>nunoplopes@sapo.pt
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<th>CC</th>
<td>lebedev.ri@gmail.com, llvm-bugs@lists.llvm.org, Matthew.Arsenault@amd.com, regehr@cs.utah.edu, spatel+llvm@rotateright.com
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<pre>See the example below where fptrunc & fabs are swapped. The issue is that fabs
has the nninf tag, which becomes poison because its input is inf.
Two solutions come to mind: drop nninf from fabs in the solution, or add
fast-math flags to fptrunc.
define half @test_shrink_intrin_fabs_fast_double_src(float %D) {
%E = fabs fast float %D
%F = fptrunc float %E to half
ret half %F
}
=>
define half @test_shrink_intrin_fabs_fast_double_src(float %D) {
%1 = fptrunc float %D to half
%F = fabs fast half %1
ret half %F
}
Transformation doesn't verify!
ERROR: Target is more poisonous than source
Example:
float %D = #x477ff080 (65520.5)
Source:
float %E = #x477ff080 (65520.5)
half %F = #x7c00 (+oo)
Target:
half %1 = #x7c00 (+oo)
half %F = poison
Source value: #x7c00 (+oo)
Target value: poison
File: llvm/test/Transforms/InstCombine/double-float-shrink-2.ll
A similar test also fails in Transforms/InstCombine/fpcast.ll (test4-fast)</pre>
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