[llvm] [AArch64][GlobalISel] Make G_DUP immediate 32-bits or larger (PR #96780)

[Amara Emerson via llvm-commits]

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@@ -774,8 +793,12 @@ AArch64RegisterBankInfo::getInstrMapping(const MachineInstr &MI) const {
              (getRegBank(ScalarReg, MRI, TRI) == &AArch64::FPRRegBank ||
               onlyDefinesFP(*ScalarDef, MRI, TRI)))
       OpRegBankIdx = {PMI_FirstFPR, PMI_FirstFPR};
-    else
+    else {
+      if (ScalarTy.getSizeInBits() < 32 &&
+          getRegBank(ScalarReg, MRI, TRI) == &AArch64::GPRRegBank)
+        MappingID = 1;
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aemerson wrote:

Can you make `1` here a `const unsigned` with a name like `CustomID` so it's clearer? Also add a comment here so the reader knows it causes `applyMappingImpl` to be called.

https://github.com/llvm/llvm-project/pull/96780