[llvm] eaca8c2 - [PEI][PowerPC] Switch to backwards frame index elimination

[Jay Foad via llvm-commits]

Author: Jay Foad
Date: 2023-08-01T07:56:11+01:00
New Revision: eaca8c2edfb14c4edab61c05562b472225f029e0

URL: https://github.com/llvm/llvm-project/commit/eaca8c2edfb14c4edab61c05562b472225f029e0
DIFF: https://github.com/llvm/llvm-project/commit/eaca8c2edfb14c4edab61c05562b472225f029e0.diff

LOG: [PEI][PowerPC] Switch to backwards frame index elimination

This adds support for reprocessing new instructions that were generated
by the target's eliminateFrameIndex.

Backwards frame index elimination uses backwards register scavenging,
which is preferred because it does not rely on accurate kill flags.

Differential Revision: https://reviews.llvm.org/D156690

Added: 
    

Modified: 
    llvm/lib/CodeGen/PrologEpilogInserter.cpp
    llvm/lib/Target/PowerPC/PPCRegisterInfo.h

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/PrologEpilogInserter.cpp b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
index c2b2399d56e791..d97a0e723d5cee 100644
--- a/llvm/lib/CodeGen/PrologEpilogInserter.cpp
+++ b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
@@ -1486,7 +1486,9 @@ void PEI::replaceFrameIndicesBackward(MachineBasicBlock *BB,
   if (LocalRS)
     LocalRS->enterBasicBlockEnd(*BB);
 
-  for (MachineInstr &MI : make_early_inc_range(reverse(*BB))) {
+  for (MachineBasicBlock::iterator I = BB->end(); I != BB->begin();) {
+    MachineInstr &MI = *std::prev(I);
+
     if (TII.isFrameInstr(MI)) {
       SPAdj -= TII.getSPAdjust(MI);
       TFI.eliminateCallFramePseudoInstr(MF, *BB, &MI);
@@ -1497,27 +1499,27 @@ void PEI::replaceFrameIndicesBackward(MachineBasicBlock *BB,
     if (LocalRS)
       LocalRS->backward(MI);
 
-    for (unsigned i = 0; i != MI.getNumOperands(); ++i) {
-      if (!MI.getOperand(i).isFI())
+    bool RemovedMI = false;
+    for (const auto &[Idx, Op] : enumerate(MI.operands())) {
+      if (!Op.isFI())
         continue;
 
-      if (replaceFrameIndexDebugInstr(MF, MI, i, SPAdj))
+      if (replaceFrameIndexDebugInstr(MF, MI, Idx, SPAdj))
         continue;
 
       // Eliminate this FrameIndex operand.
-      //
-      // Save and restore the scavenger's position around the call to
-      // eliminateFrameIndex in case it erases MI and invalidates the iterator.
-      MachineBasicBlock::iterator Save;
-      if (LocalRS)
-	Save = std::next(LocalRS->getCurrentPosition());
-      bool Removed = TRI.eliminateFrameIndex(MI, SPAdj, i, LocalRS);
-      if (LocalRS)
-	LocalRS->skipTo(std::prev(Save));
-
-      if (Removed)
+      RemovedMI = TRI.eliminateFrameIndex(MI, SPAdj, Idx, LocalRS);
+      if (RemovedMI)
         break;
     }
+
+    // Refresh the scavenger's internal iterator in case MI was removed or more
+    // instructions were inserted after it.
+    if (LocalRS)
+      LocalRS->skipTo(std::prev(I));
+
+    if (!RemovedMI)
+      --I;
   }
 }
 

diff  --git a/llvm/lib/Target/PowerPC/PPCRegisterInfo.h b/llvm/lib/Target/PowerPC/PPCRegisterInfo.h
index 11dbbce42f61c7..72058ea6678e3e 100644
--- a/llvm/lib/Target/PowerPC/PPCRegisterInfo.h
+++ b/llvm/lib/Target/PowerPC/PPCRegisterInfo.h
@@ -113,6 +113,8 @@ class PPCRegisterInfo : public PPCGenRegisterInfo {
 
   bool requiresFrameIndexScavenging(const MachineFunction &MF) const override;
 
+  bool supportsBackwardScavenger() const override { return true; }
+
   bool requiresVirtualBaseRegisters(const MachineFunction &MF) const override;
 
   void lowerDynamicAlloc(MachineBasicBlock::iterator II) const;