https://bugs.llvm.org/show_bug.cgi?id=49080
Bug ID: 49080
Summary: Incorrect swap of fptrunc with fast-math instructions
Product: libraries
Version: trunk
Hardware: All
OS: All
Status: NEW
Keywords: miscompilation
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
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)
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