[PowerPC] Avoid RegScavenger::forward in PPCFrameLowering

[foad]

Summary

RegScavenger::backward is preferred because it does not rely on accurate
kill flags.

[foad]

llvm/lib/Target/PowerPC/PPCFrameLowering.cpp

469–473

This code seems confused to me. I tried to follow (and document) the intent of it, rather than the exact logic here.

llvm/lib/Target/PowerPC/PPCFrameLowering.cpp

if (SR2) {
     *SR2 = R12;
   }
 
   // If MBB is an entry or exit block, use R0 and R12 as the scratch registers.
   if ((UseAtEnd && MBB->isReturnBlock()) ||
       (!UseAtEnd && (&MBB->getParent()->front() == MBB)))
     return true;
 
-  RS.enterBasicBlock(*MBB);
-
-  if (UseAtEnd && !MBB->empty()) {
-    // The scratch register will be used at the end of the block, so must
-    // consider all registers used within the block
-
+  if (UseAtEnd) {
+    // The scratch register will be used before the first terminator (or at the
+    // end of the block if there are no terminators).
     MachineBasicBlock::iterator MBBI = MBB->getFirstTerminator();
-    // If no terminator, back iterator up to previous instruction.
-    if (MBBI == MBB->end())
-      MBBI = std::prev(MBBI);
-
-    if (MBBI != MBB->begin())
-      RS.forward(MBBI);
+    if (MBBI == MBB->begin()) {
+      RS.enterBasicBlock(*MBB);
+    } else {
+      RS.enterBasicBlockEnd(*MBB);
+      RS.backward(std::prev(MBBI));
+    }
+  } else {
+    // The scratch register will be used at the start of the block.
+    RS.enterBasicBlock(*MBB);
   }
 
   // If the two registers are available, we're all good.
   // Note that we only return here if both R0 and R12 are available because
   // although the function may not require two unique registers, it may benefit
   // from having two so we should try to provide them.
   if (!RS.isRegUsed(R0) && !RS.isRegUsed(R12))
     return true;