[llvm] [LLVM][InstCombine] Enable constant folding for SVE asr,lsl and lsr intrinsics. (PR #137350)

[David Sherwood via llvm-commits]

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@@ -1571,7 +1583,11 @@ simplifySVEIntrinsicBinOp(InstCombiner &IC, IntrinsicInst &II,
   else
     SimpleII = simplifyBinOp(Opc, Op1, Op2, DL);
 
-  if (!SimpleII)
+  // An SVE intrinsic's result is always defined. However, this is not the case
+  // for its equivalent IR instruction (e.g. when shifting by an amount more
+  // than the data's bitwidth). Simplifications to an undefined result must be
+  // ignored to preserve the intrinsic's expected behaviour.
+  if (!SimpleII || isa<UndefValue>(SimpleII))
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david-arm wrote:

There are multiple ways that `undef` (or `poison`?) can be returned, for example `Op1` itself could be undef. Do we have a test for this case? I assume you're taking the conservative approach here and just ignoring safe optimisations like `asr <vscale x 4 x i32> undef, <vscale x 4 x i32> splat(i32 0)` for now to reduce code complexity?

Also, is it always guaranteed to be `UndefValue` - do we ever return poison?

https://github.com/llvm/llvm-project/pull/137350