[PEI] Remove support for register scavenging during forwards frame index elimination

[foad]

Summary

All targets that require register scavenging now use backwards frame
index elimination.

[foad]

Release notes.

llvm/docs/ReleaseNotes.rst

 
 * Added ``LLVMGetTailCallKind`` and ``LLVMSetTailCallKind`` to
   allow getting and setting ``tail``, ``musttail``, and ``notail``
   attributes on call instructions.
 
 Changes to the CodeGen infrastructure
 -------------------------------------
 
+* ``PrologEpilogInserter`` no longer supports register scavenging
+  during forwards frame index elimination. Targets should use
+  backwards frame index elimination instead.
+
 Changes to the Metadata Info
 ---------------------------------
 
 Changes to the Debug Info
 ---------------------------------
 
 Changes to the LLVM tools
 ---------------------------------

llvm/lib/CodeGen/PrologEpilogInserter.cpp

 void PEI::replaceFrameIndices(MachineBasicBlock *BB, MachineFunction &MF,
                               int &SPAdj) {
   assert(MF.getSubtarget().getRegisterInfo() &&
          "getRegisterInfo() must be implemented!");
   const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
   const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
 
-  if (RS && FrameIndexEliminationScavenging)
-    RS->enterBasicBlock(*BB);
-
   bool InsideCallSequence = false;
 
   for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ) {
     if (TII.isFrameInstr(*I)) {
       InsideCallSequence = TII.isFrameSetup(*I);
       SPAdj += TII.getSPAdjust(*I);
       I = TFI->eliminateCallFramePseudoInstr(MF, *BB, I);
       continue;
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
       // iterator at the point before insertion so that we can
       // revisit them in full.
       bool AtBeginning = (I == BB->begin());
       if (!AtBeginning) --I;
 
       // If this instruction has a FrameIndex operand, we need to
       // use that target machine register info object to eliminate
       // it.
-      TRI.eliminateFrameIndex(MI, SPAdj, i,
-                              FrameIndexEliminationScavenging ?  RS : nullptr);
+      TRI.eliminateFrameIndex(MI, SPAdj, i);
 
       // Reset the iterator if we were at the beginning of the BB.
       if (AtBeginning) {
         I = BB->begin();
         DoIncr = false;
       }
 
       DidFinishLoop = false;
       break;
     }
 
     // If we are looking at a call sequence, we need to keep track of
     // the SP adjustment made by each instruction in the sequence.
     // This includes both the frame setup/destroy pseudos (handled above),
     // as well as other instructions that have side effects w.r.t the SP.
     // Note that this must come after eliminateFrameIndex, because
     // if I itself referred to a frame index, we shouldn't count its own
     // adjustment.
     if (DidFinishLoop && InsideCallSequence)
       SPAdj += TII.getSPAdjust(MI);
 
-    if (DoIncr && I != BB->end()) ++I;
-
-    // Update register states.
-    if (RS && FrameIndexEliminationScavenging && DidFinishLoop)
-      RS->forward(MI);
+    if (DoIncr && I != BB->end())
+      ++I;
   }
 }