[llvm] [AArch64] Cache repeated subtarget queries (NFC) (PR #222637)

[Cullen Rhodes via llvm-commits]

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@@ -436,6 +439,12 @@ AArch64TargetMachine::~AArch64TargetMachine() = default;
 
 const AArch64Subtarget *
 AArch64TargetMachine::getSubtargetImpl(const Function &F) const {
+  // Constructing the subtarget key is not cheap, avoid rebuilding it for
+  // repeated queries with the same function attributes.
+  AttributeSet FnAttrs = F.getAttributes().getFnAttrs();
+  if (LastSubtarget && LastSubtargetAttrs == FnAttrs)
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c-rhodes wrote:

I think those are the global options Ben was referring to. They are fixed per compiler invocation, so cannot change between subtarget queries.

https://github.com/llvm/llvm-project/pull/222637