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)