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

[Allen zhong via Phabricator via llvm-commits]

Allen 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);
----------------
spatel wrote:
> 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.
hi @spatel, I think the case in your link https://alive2.llvm.org/ce/z/iaEX2i is related to another top, maybe make use of the fptrunc?

* In this patch, we hope fold the case
```
  %m = and i25 %A, smallMaskValue
  %B = uitofp i25 %m to float
  %C = fptoui float %B to i25
```

* base on the bt we can see, it has both FPToUI and FPtoI.
```
#0  isKnownExactCastIntToFP (I=..., IC=...)
    at /home/zhongyunde/llvm-project-init-dev_12x/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:1762
#1  0x0000aaaaae7ee18c in llvm::InstCombinerImpl::foldItoFPtoI (this=0xffffffffbce0, FI=...)
    at /home/zhongyunde/llvm-project-init-dev_12x/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:1986
#2  0x0000aaaaae7ee398 in llvm::InstCombinerImpl::visitFPToUI (this=0xffffffffbce0, FI=...)
    at /home/zhongyunde/llvm-project-init-dev_12x/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:2011
```


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

https://reviews.llvm.org/D127854