[PATCH] D104612: [CGP][RISCV] Teach CodeGenPrepare::optimizeSwitchInst to honor isSExtCheaperThanZExt.

[Jessica Clarke via Phabricator via llvm-commits]

jrtc27 added inline comments.


================
Comment at: llvm/lib/CodeGen/CodeGenPrepare.cpp:7014-7016
+  // Some targets prefer SExt over ZExt, honor that.
+  if (TLI->isSExtCheaperThanZExt(OldVT, RegType))
+    ExtType = Instruction::SExt;
----------------
jrtc27 wrote:
> Do we not want to avoid this in the case this is a function argument that is already being zero-extended? i.e. something like:
> 
> ```
>   Instruction::CastOps ExtType = Instruction::CastOpsEnd;
>   if (auto *Arg = dyn_cast<Argument>(Cond)) {
>     if (Arg->hasSExtAttr())
>       ExtType = Instruction::SExt;
>     else if (Arg->hasZExtAttr())
>       ExtType = Instruction::ZExt;
>   }
>   if (ExtType == Instruction::CastOpsEnd) {
>     if (TLI->isSExtCheaperThanZExt(OldVT, RegType))
>       ExtType = Instruction::SExt;
>     else
>       ExtType = Instruction::ZExt;
>   }
> ```
> (slightly abusing CastOpsEnd as being one past the last valid op, and thus a sentinel, though you could write it in other ways too if you wanted)
> 
Or maybe the fact that sext is cheaper means the saving for each case's extending to match outweighs the cost of having to also extend the argument?


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  https://reviews.llvm.org/D104612/new/

https://reviews.llvm.org/D104612