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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

llvm-svn: 31006
This commit is contained in:
Chris Lattner 2006-10-17 18:16:40 +00:00
parent b7f67a23e0
commit bca3e297fe
1 changed files with 3 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
llvm/lib/CodeGen/BranchFolding.cpp
View File

@ -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