[all-commits] [llvm/llvm-project] ad5067: [InstCombine] Only fold bitcast(fptrunc) if destin...

[Victor Mustya via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ad50676055cf03fd4fecdda5736c7ac1a940231f
      https://github.com/llvm/llvm-project/commit/ad50676055cf03fd4fecdda5736c7ac1a940231f
  Author: Victor Mustya <victor.mustya at intel.com>
  Date:   2024-01-16 (Tue, 16 Jan 2024)

  Changed paths:
    M llvm/lib/IR/Instructions.cpp
    M llvm/test/Transforms/InstCombine/fptrunc.ll

  Log Message:
  -----------
  [InstCombine] Only fold bitcast(fptrunc) if destination type matches fptrunc result type. (#77046)

It's not enough to just make sure destination type is floating point,
because the following chain may be incorrectly optimized:
```LLVM
  %trunc = fptrunc float %src to bfloat
  %cast = bitcast bfloat %trunc to half
```
Before the fix, the instruction sequence mentioned above used to be
translated into single fptrunc instruction as follows:
```LLVM
  %trunc = fptrunc float %src to half
```

Such transformation was semantically incorrect.