[llvm] [X86] Support EGPR (R16-R31) for APX (PR #67702)

[Phoebe Wang via llvm-commits]

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@@ -92,6 +92,37 @@ X86InstrInfo::X86InstrInfo(X86Subtarget &STI)
       Subtarget(STI), RI(STI.getTargetTriple()) {
 }
 
+const TargetRegisterClass *
+X86InstrInfo::getRegClass(const MCInstrDesc &MCID, unsigned OpNum,
+                          const TargetRegisterInfo *TRI,
+                          const MachineFunction &MF) const {
+  auto *RC = TargetInstrInfo::getRegClass(MCID, OpNum, TRI, MF);
+  // If the target does not have egpr, then r16-r31 will be resereved for all
+  // instructions.
+  if (!RC || !Subtarget.hasEGPR())
+    return RC;
+
+  if (X86II::canUseApxExtendedReg(MCID))
+    return RC;
+
+  switch (RC->getID()) {
+  default:
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phoebewang wrote:

Is this optimism for other cases? I think we have other classes like `GR8_ABCD_LRegClassID`, `GR16_ABCDRegClassID` etc.

https://github.com/llvm/llvm-project/pull/67702