Re-apply 101075 and fix it properly. Just reuse the debug info of the branch instruction being optimized. There is no need to --I which can deref off start of the BB.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101162 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index e67bf3d..66b6562 100644
--- a/lib/Target/X86/X86InstrInfo.cpp
+++ b/lib/Target/X86/X86InstrInfo.cpp
@@ -1684,6 +1684,7 @@
// Start from the bottom of the block and work up, examining the
// terminator instructions.
MachineBasicBlock::iterator I = MBB.end();
+ MachineBasicBlock::iterator UnCondBrIter = MBB.end();
while (I != MBB.begin()) {
--I;
if (I->isDebugValue())
@@ -1701,6 +1702,8 @@
// Handle unconditional branches.
if (I->getOpcode() == X86::JMP_4) {
+ UnCondBrIter = I;
+
if (!AllowModify) {
TBB = I->getOperand(0).getMBB();
continue;
@@ -1718,10 +1721,11 @@
TBB = 0;
I->eraseFromParent();
I = MBB.end();
+ UnCondBrIter = MBB.end();
continue;
}
- // TBB is used to indicate the unconditinal destination.
+ // TBB is used to indicate the unconditional destination.
TBB = I->getOperand(0).getMBB();
continue;
}
@@ -1733,6 +1737,45 @@
// Working from the bottom, handle the first conditional branch.
if (Cond.empty()) {
+ MachineBasicBlock *TargetBB = I->getOperand(0).getMBB();
+ if (AllowModify && UnCondBrIter != MBB.end() &&
+ MBB.isLayoutSuccessor(TargetBB)) {
+ // If we can modify the code and it ends in something like:
+ //
+ // jCC L1
+ // jmp L2
+ // L1:
+ // ...
+ // L2:
+ //
+ // Then we can change this to:
+ //
+ // jnCC L2
+ // L1:
+ // ...
+ // L2:
+ //
+ // Which is a bit more efficient.
+ // We conditionally jump to the fall-through block.
+ BranchCode = GetOppositeBranchCondition(BranchCode);
+ unsigned JNCC = GetCondBranchFromCond(BranchCode);
+ MachineBasicBlock::iterator OldInst = I;
+
+ BuildMI(MBB, UnCondBrIter, MBB.findDebugLoc(I), get(JNCC))
+ .addMBB(UnCondBrIter->getOperand(0).getMBB());
+ BuildMI(MBB, UnCondBrIter, MBB.findDebugLoc(I), get(X86::JMP_4))
+ .addMBB(TargetBB);
+ MBB.addSuccessor(TargetBB);
+
+ OldInst->eraseFromParent();
+ UnCondBrIter->eraseFromParent();
+
+ // Restart the analysis.
+ UnCondBrIter = MBB.end();
+ I = MBB.end();
+ continue;
+ }
+
FBB = TBB;
TBB = I->getOperand(0).getMBB();
Cond.push_back(MachineOperand::CreateImm(BranchCode));