[llvm] [AArch64][GlobalISel] Implement selectVaStartAAPCS (PR #106979)

[Matt Arsenault via llvm-commits]

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@@ -1994,7 +1994,108 @@ bool AArch64InstructionSelector::selectVectorAshrLshr(
 
 bool AArch64InstructionSelector::selectVaStartAAPCS(
     MachineInstr &I, MachineFunction &MF, MachineRegisterInfo &MRI) const {
-  return false;
+
+  if (MF.getSubtarget<AArch64Subtarget>().isCallingConvWin64(
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arsenm wrote:

I assume the subtarget is already available as a member in AArch64InstructionSelector 

https://github.com/llvm/llvm-project/pull/106979