PHI elimination should not break back edge. It can cause some significant code placement issues. rdar://8263994
good:
LBB0_2:
mov r2, r0
. . .
mov r1, r2
bne LBB0_2
bad:
LBB0_2:
mov r2, r0
. . .
@ BB#3:
mov r1, r2
b LBB0_2
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111221 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/MachineBasicBlock.cpp b/lib/CodeGen/MachineBasicBlock.cpp
index a27ee47..895653d 100644
--- a/lib/CodeGen/MachineBasicBlock.cpp
+++ b/lib/CodeGen/MachineBasicBlock.cpp
@@ -439,6 +439,14 @@
if (TII->AnalyzeBranch(*this, TBB, FBB, Cond))
return NULL;
+ // Avoid splitting backedges of loops. It would introduce small out-of-line
+ // blocks into the loop which is very bad for code placement.
+ if (this == Succ)
+ return NULL;
+ MachineLoopInfo *MLI = P->getAnalysisIfAvailable<MachineLoopInfo>();
+ if (MLI->isLoopHeader(Succ))
+ return NULL;
+
MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
MF->insert(llvm::next(MachineFunction::iterator(this)), NMBB);
DEBUG(dbgs() << "PHIElimination splitting critical edge:"
@@ -471,8 +479,7 @@
P->getAnalysisIfAvailable<MachineDominatorTree>())
MDT->addNewBlock(NMBB, this);
- if (MachineLoopInfo *MLI =
- P->getAnalysisIfAvailable<MachineLoopInfo>())
+ if (MLI)
if (MachineLoop *TIL = MLI->getLoopFor(this)) {
// If one or the other blocks were not in a loop, the new block is not
// either, and thus LI doesn't need to be updated.