From ffddf6ba1c58b42dcfd071972754649c3ca5dff7 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Tue, 17 Oct 2006 18:16:40 +0000
Subject: [PATCH] Enable deleting branches to successor blocks.  With the
 previous patches, branch folding can now compile stuff like this:

void foo(int W, int X, int Y, int Z) {
  if (W & 1) {
    for (; X;--X) bar();
  } else if (W & 2) {
    for (; Y;--Y) bar();
  } else if (W & 4) {
    for (; Z;--Z) bar();
  } else if (W & 8) {
    for (; W;--W) bar();
  }

  if (W) {
    bar();
  }
}

contrived testcase where loops exits all end up merging together.  To have
the loop merges be:

...
        cmplw cr0, r30, r27
        bne cr0, LBB1_14        ;bb38
LBB1_16:        ;cond_next48.loopexit
        mr r27, r29
LBB1_20:        ;cond_next48
        cmplwi cr0, r27, 0
        beq cr0, LBB1_22        ;UnifiedReturnBlock
...

instead of:


...
        cmplw cr0, r30, r27
        bne cr0, LBB1_14        ;bb38
LBB1_16:        ;cond_next48.loopexit
        mr r27, r29
        b LBB1_20       ;cond_next48
LBB1_17:        ;cond_next48.loopexit1
        b LBB1_20       ;cond_next48
LBB1_18:        ;cond_next48.loopexit2
        b LBB1_20       ;cond_next48
LBB1_19:        ;cond_next48.loopexit3
LBB1_20:        ;cond_next48
        cmplwi cr0, r27, 0
        beq cr0, LBB1_22        ;UnifiedReturnBlock
...


This is CodeGen/PowerPC/branch-opt.ll


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31006 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/BranchFolding.cpp | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index b589c03f56d..725317378c4 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -137,10 +137,11 @@ void BranchFolder::OptimizeBlock(MachineFunction::iterator MBB) {
 
   // Check to see if we can simplify the terminator of the block before this
   // one.
-#if 0
+  MachineBasicBlock &PrevBB = *prior(MBB);
+
   MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
   std::vector<MachineOperand> PriorCond;
-  if (!TII->AnalyzeBranch(*prior(MBB), PriorTBB, PriorFBB, PriorCond)) {
+  if (!TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond)) {
     // If the previous branch is conditional and both conditions go to the same
     // destination, remove the branch, replacing it with an unconditional one.
     if (PriorTBB && PriorTBB == PriorFBB) {
@@ -160,8 +161,6 @@ void BranchFolder::OptimizeBlock(MachineFunction::iterator MBB) {
       return OptimizeBlock(MBB);
     }
   }
-#endif
-  
   
 #if 0
 
-- 
2.11.0