[MBP] Only run the BranchFolder after RA

[joey]

Summary

The Branch Folder can only run after RA. According to the comment at the top of the file:

// Note that this pass must be run after register allocation, it cannot handle
// SSA form.

[joey]

Ping!

[arsenm]

This is actually a lie I found. I was experimenting enabling this before RA and was able to get it to work

[arsenm]

I think the problem is by referring it to as post-RA. I interpret that is meaning physical registers, not post-SSA

[joey]

After talking to Matthias and Matt, this seems like the wrong solution.

CodeGen/MachineBlockPlacement.cpp

 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachinePostDominators.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/TailDuplicator.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetSubtargetInfo.h"

 
   // Changing the layout can create new tail merging opportunities.
   TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
   // TailMerge can create jump into if branches that make CFG irreducible for
   // HW that requires structured CFG.
   bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
                          PassConfig->getEnableTailMerge() &&
                          BranchFoldPlacement;
+  bool isPreRegAlloc = MF.getRegInfo().isSSA();
   // No tail merging opportunities if the block number is less than four.
-  if (MF.size() > 3 && EnableTailMerge) {
+  if (MF.size() > 3 && EnableTailMerge && !isPreRegAlloc) {
     unsigned TailMergeSize = TailDupPlacementThreshold + 1;
     BranchFolder BF(/*EnableTailMerge=*/true, /*CommonHoist=*/false, *MBFI,
                     *MBPI, TailMergeSize);
 
     if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
                             getAnalysisIfAvailable<MachineModuleInfo>(), MLI,
                             /*AfterBlockPlacement=*/true)) {
       // Redo the layout if tail merging creates/removes/moves blocks.