[PATCH] D127854: [InstCombine] Use known bits to determine exact int->fp cast

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:1945
     auto *FPCast = cast<CastInst>(Src);
-    if (isKnownExactCastIntToFP(*FPCast))
+    if (isKnownExactCastIntToFP(*FPCast, *this))
       return CastInst::Create(FPCast->getOpcode(), FPCast->getOperand(0), Ty);
----------------
We should have at least one test providing coverage for this path (and fpext too?). 

IIUC, this patch will improve a case like this:

```
define half @masked_int_to_fp_trunc(i32 %A) {
  %m = and i32 %A, 16777215
  %B = sitofp i32 %m to float
  %C = fptrunc float %B to half
  ret half %C
}

```
But we don't actually need a mask:
https://alive2.llvm.org/ce/z/iaEX2i
...so the fold conditions still aren't quite right.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D127854/new/

https://reviews.llvm.org/D127854