[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(
+          MF.getFunction().getCallingConv(), MF.getFunction().isVarArg()))
+    return false;
+
+  // The layout of the va_list struct is specified in the AArch64 Procedure Call
+  // Standard, section 10.1.5.
+
+  const AArch64FunctionInfo *FuncInfo = MF.getInfo<AArch64FunctionInfo>();
+  const unsigned PtrSize = STI.isTargetILP32() ? 4 : 8;
+  const auto *PtrRegClass =
+      STI.isTargetILP32() ? &AArch64::GPR32RegClass : &AArch64::GPR64RegClass;
+
+  const MCInstrDesc &MCIDAddAddr =
+      TII.get(STI.isTargetILP32() ? AArch64::ADDWri : AArch64::ADDXri);
+  const MCInstrDesc &MCIDStoreAddr =
+      TII.get(STI.isTargetILP32() ? AArch64::STRWui : AArch64::STRXui);
+
+  /*
+   * typedef struct va_list {
+   *  void * stack; // next stack param
+   *  void * gr_top; // end of GP arg reg save area
+   *  void * vr_top; // end of FP/SIMD arg reg save area
+   *  int gr_offs; // offset from gr_top to next GP register arg
+   *  int vr_offs; // offset from vr_top to next FP/SIMD register arg
+   * } va_list;
+   */
+  const auto VAList = I.getOperand(0).getReg();
+
+  // Our current offset in bytes from the va_list struct (VAList).
+  unsigned OffsetBytes = 0;
+
+  // Helper function to store (FrameIndex + Imm) to VAList at offset OffsetBytes
+  // and increment OffsetBytes by PtrSize.
+  const auto PushAddress = [&](const int FrameIndex, const int64_t Imm) {
+    const Register Top = MRI.createVirtualRegister(PtrRegClass);
+    auto MIB = BuildMI(*I.getParent(), I, I.getDebugLoc(), MCIDAddAddr)
+                   .addDef(Top)
+                   .addFrameIndex(FrameIndex)
+                   .addImm(Imm)
+                   .addImm(0);
+    constrainSelectedInstRegOperands(*MIB, TII, TRI, RBI);
+
+    MIB = BuildMI(*I.getParent(), I, I.getDebugLoc(), MCIDStoreAddr)
+              .addUse(Top)
+              .addUse(VAList)
+              .addImm(OffsetBytes / PtrSize)
+              .addMemOperand(MF.getMachineMemOperand(
+                  (*I.memoperands_begin())
+                      ->getPointerInfo()
+                      .getWithOffset(OffsetBytes),
+                  MachineMemOperand::MOStore, PtrSize, Align(PtrSize)));
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arsenm wrote:

Should do commonAlignment of the original alignments and the offset 

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