[llvm] r184680 - Fix tail merging to assign the (more) correct BasicBlock when splitting.

[Andrew Trick]

Author: atrick
Date: Sun Jun 23 20:55:01 2013
New Revision: 184680

URL: http://llvm.org/viewvc/llvm-project?rev=184680&view=rev
Log:
Fix tail merging to assign the (more) correct BasicBlock when splitting.

This makes it possible to write unit tests that are less susceptible
to minor code motion, particularly copy placement. block-placement.ll
covers this case with -pre-RA-sched=source which will soon be
default. One incorrectly named block is already fixed, but without
this fix, enabling new coalescing and scheduling would cause more
failures.

Modified:
    llvm/trunk/lib/CodeGen/BranchFolding.cpp
    llvm/trunk/lib/CodeGen/BranchFolding.h
    llvm/trunk/test/CodeGen/X86/block-placement.ll

Modified: llvm/trunk/lib/CodeGen/BranchFolding.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/BranchFolding.cpp?rev=184680&r1=184679&r2=184680&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/BranchFolding.cpp (original)
+++ llvm/trunk/lib/CodeGen/BranchFolding.cpp Sun Jun 23 20:55:01 2013
@@ -406,7 +406,8 @@ void BranchFolder::ReplaceTailWithBranch
 /// MBB so that the part before the iterator falls into the part starting at the
 /// iterator.  This returns the new MBB.
 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
-                                            MachineBasicBlock::iterator BBI1) {
+                                            MachineBasicBlock::iterator BBI1,
+                                            const BasicBlock *BB) {
   if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
     return 0;
 
@@ -414,7 +415,7 @@ MachineBasicBlock *BranchFolder::SplitMB
 
   // Create the fall-through block.
   MachineFunction::iterator MBBI = &CurMBB;
-  MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
+  MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(BB);
   CurMBB.getParent()->insert(++MBBI, NewMBB);
 
   // Move all the successors of this block to the specified block.
@@ -647,6 +648,7 @@ void BranchFolder::RemoveBlocksWithHash(
 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
 /// only of the common tail.  Create a block that does by splitting one.
 bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
+                                             MachineBasicBlock *SuccBB,
                                              unsigned maxCommonTailLength,
                                              unsigned &commonTailIndex) {
   commonTailIndex = 0;
@@ -676,7 +678,12 @@ bool BranchFolder::CreateCommonTailOnlyB
   DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
                << maxCommonTailLength);
 
-  MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
+  // If the split block unconditionally falls-thru to SuccBB, it will be
+  // merged. In control flow terms it should then take SuccBB's name. e.g. If
+  // SuccBB is an inner loop, the common tail is still part of the inner loop.
+  const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
+    SuccBB->getBasicBlock() : MBB->getBasicBlock();
+  MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
   if (!newMBB) {
     DEBUG(dbgs() << "... failed!");
     return false;
@@ -784,7 +791,7 @@ bool BranchFolder::TryTailMergeBlocks(Ma
          !SameTails[commonTailIndex].tailIsWholeBlock())) {
       // None of the blocks consist entirely of the common tail.
       // Split a block so that one does.
-      if (!CreateCommonTailOnlyBlock(PredBB,
+      if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
                                      maxCommonTailLength, commonTailIndex)) {
         RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
         continue;

Modified: llvm/trunk/lib/CodeGen/BranchFolding.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/BranchFolding.h?rev=184680&r1=184679&r2=184680&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/BranchFolding.h (original)
+++ llvm/trunk/lib/CodeGen/BranchFolding.h Sun Jun 23 20:55:01 2013
@@ -100,13 +100,15 @@ namespace llvm {
     void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
                                  MachineBasicBlock *NewDest);
     MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
-                                  MachineBasicBlock::iterator BBI1);
+                                  MachineBasicBlock::iterator BBI1,
+                                  const BasicBlock *BB);
     unsigned ComputeSameTails(unsigned CurHash, unsigned minCommonTailLength,
                               MachineBasicBlock *SuccBB,
                               MachineBasicBlock *PredBB);
     void RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock* SuccBB,
                                                 MachineBasicBlock* PredBB);
     bool CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
+                                   MachineBasicBlock *SuccBB,
                                    unsigned maxCommonTailLength,
                                    unsigned &commonTailIndex);
 

Modified: llvm/trunk/test/CodeGen/X86/block-placement.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/block-placement.ll?rev=184680&r1=184679&r2=184680&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/block-placement.ll (original)
+++ llvm/trunk/test/CodeGen/X86/block-placement.ll Sun Jun 23 20:55:01 2013
@@ -1,4 +1,4 @@
-; RUN: llc -mtriple=i686-linux < %s | FileCheck %s
+; RUN: llc -mtriple=i686-linux -pre-RA-sched=source < %s | FileCheck %s
 
 declare void @error(i32 %i, i32 %a, i32 %b)
 
@@ -997,7 +997,7 @@ define void @benchmark_heapsort(i32 %n,
 ; CHECK: %while.body
 ; CHECK: %land.lhs.true
 ; CHECK: %if.then19
-; CHECK: %if.then19
+; CHECK: %if.end20
 ; CHECK: %if.then8
 ; CHECK: ret