[llvm] [PATCH] [Xtensa] Implement FrameLowering methods and stack operation lowering. (PR #92960)

[Matt Arsenault via llvm-commits]

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@@ -33,10 +34,153 @@ bool XtensaFrameLowering::hasFP(const MachineFunction &MF) const {
 }
 
 void XtensaFrameLowering::emitPrologue(MachineFunction &MF,
-                                       MachineBasicBlock &MBB) const {}
+                                       MachineBasicBlock &MBB) const {
+  assert(&MBB == &MF.front() && "Shrink-wrapping not yet implemented");
+  MachineFrameInfo &MFI = MF.getFrameInfo();
+  MachineBasicBlock::iterator MBBI = MBB.begin();
+  DebugLoc DL = MBBI != MBB.end() ? MBBI->getDebugLoc() : DebugLoc();
+  MCRegister SP = Xtensa::SP;
+  MCRegister FP = TRI->getFrameRegister(MF);
+  MachineModuleInfo &MMI = MF.getMMI();
+  const MCRegisterInfo *MRI = MMI.getContext().getRegisterInfo();
+
+  // First, compute final stack size.
+  uint64_t StackSize = MFI.getStackSize();
+  uint64_t PrevStackSize = StackSize;
+
+  // Round up StackSize to 16*N
+  StackSize += (16 - StackSize) & 0xf;
+
+  // No need to allocate space on the stack.
+  if (StackSize == 0 && !MFI.adjustsStack())
+    return;
+
+  // Adjust stack.
+  TII.adjustStackPtr(SP, -StackSize, MBB, MBBI);
+
+  // emit ".cfi_def_cfa_offset StackSize"
+  unsigned CFIIndex =
+      MF.addFrameInst(MCCFIInstruction::cfiDefCfaOffset(nullptr, StackSize));
+  BuildMI(MBB, MBBI, DL, TII.get(TargetOpcode::CFI_INSTRUCTION))
+      .addCFIIndex(CFIIndex);
+
+  const std::vector<CalleeSavedInfo> &CSI = MFI.getCalleeSavedInfo();
+
+  if (CSI.size()) {
+    // Find the instruction past the last instruction that saves a
+    // callee-saved register to the stack.
+    for (unsigned i = 0; i < CSI.size(); ++i)
+      ++MBBI;
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arsenm wrote:

This seems like a dubious way to guarantee this. Can you at least assert each instruction is a spill of the appropriate register? 

https://github.com/llvm/llvm-project/pull/92960