[SCCP] add a code comment about sitofp -> uitofp; NFC

D134975 would have added this fold, but we decided it's
not worth doing without some evidence of benefit.

diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
index 422a41e..f6868cd 100644 (file)
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -173,6 +173,8 @@ static bool replaceSignedInst(SCCPSolver &Solver,
 
   Instruction *NewInst = nullptr;
   switch (Inst.getOpcode()) {
+  // Note: We do not fold sitofp -> uitofp here because that could be more
+  // expensive in codegen and may not be reversible in the backend.
   case Instruction::SExt: {
     // If the source value is not negative, this is a zext.
     Value *Op0 = Inst.getOperand(0);