[llvm] [TTI][AArch64] Detect OperandInfo from scalable splats. (PR #122469)

[Sander de Smalen via llvm-commits]

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@@ -893,7 +893,8 @@ TargetTransformInfo::getOperandInfo(const Value *V) {
 
   // Check for a splat of a constant or for a non uniform vector of constants
   // and check if the constant(s) are all powers of two.
-  if (isa<ConstantVector>(V) || isa<ConstantDataVector>(V)) {
+  if (isa<ConstantVector>(V) || isa<ConstantDataVector>(V) ||
+      isa<ConstantExpr>(V)) {
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sdesmalen-arm wrote:

Rather than adding another case, which I think makes this sensitive to the plethora of constant subclasses, I think the logic here would be easier to follow if it would be structured as:

```
// Handle splats first since those are all uniform
if (const Value *Splat = getSplatValue(V)) {
  if (auto *CI = dyn_cast<ConstantInt>(Splat)) {
    OpInfo = OK_UniformConstantValue;
    ...
  } else if (isa<Constant>(Splat))
    OpInfo = OK_UniformConstantValue;
  else if (isa<Argument>(Splat) || isa<GlobalValue>(Splat))
    OpInfo = OK_UniformValue;
} else if (isa<Constant>(V)) {
   OpInfo = OK_NonUniformConstantValue;
   if (const auto *CDS = dyn_cast<ConstantDataSequential>(V)) {
     ...
   }
}
```

This also allows folding in the argument/global value splats case.

https://github.com/llvm/llvm-project/pull/122469