================
@@ -1363,6 +1363,14 @@ bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB)
{
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough);
}
+ // Add rest successors of MBB to successors of FallThrough. Those
+ // successors are not directly reachable via MBB, so it should be
+ // landing-pad.
+ for (auto SI = MBB->succ_begin(), SE = MBB->succ_end(); SI != SE; ++SI)
+ if (*SI != &*FallThrough && !FallThrough->isSuccessor(*SI)) {
+ assert((*SI)->isEHPad() && "Bad CFG");
+ FallThrough->copySuccessor(MBB, SI);
+ }
----------------
HaohaiWen wrote:
Let's see what happened before/after this code:
Before:
```
bb.0:
successors: %bb.3(0x7ffff800), %bb.5(0x00000800); %bb.3(100.00%), %bb.5(0.00%)
JMP_1 %bb.3
bb.2:
successors: %bb.3(0x7ffff800), %bb.4(0x00000800); %bb.3(100.00%), %bb.4(0.00%)
bb.3:
; predecessors: %bb.2, %bb.0
successors: %bb.6(0x80000000); %bb.6(100.00%)
JMP_1 %bb.6
bb.4 (machine-block-address-taken, landing-pad, ehfunclet-entry):
; predecessors: %bb.2
successors: %bb.5(0x80000000); %bb.5(100.00%)
CLEANUPRET
bb.5 (landing-pad, ehfunclet-entry):
; predecessors: %bb.0, %bb.4
CLEANUPRET
bb.6:
; predecessors: %bb.3
RET 0
# End machine code for function main.
```
After
```
bb.0:
successors: %bb.3(0x7ffff800), %bb.5(0x00000800); %bb.3(100.00%), %bb.5(0.00%)
JMP_1 %bb.3
bb.2:
successors: %bb.3(0x7ffff800), %bb.4(0x00000800); %bb.3(100.00%), %bb.4(0.00%)
bb.3:
; predecessors: %bb.2, %bb.0
successors: %bb.6(0x80000000), %bb.4(0x00000800); %bb.6(100.00%), %bb.4(0.00%)
JMP_1 %bb.6
bb.4 (machine-block-address-taken, landing-pad, ehfunclet-entry):
; predecessors: %bb.2, %bb.3
successors: %bb.5(0x80000000); %bb.5(100.00%)
CLEANUPRET
bb.5 (landing-pad, ehfunclet-entry):
; predecessors: %bb.0, %bb.4
CLEANUPRET
bb.6:
; predecessors: %bb.3
RET 0
# End machine code for function main.
```
As we can see, No BB is removed at this time. We're adding edge from one live
BB to another live BB.
In fact, dead BB was removed by BranchFolder::RemoveDeadBlock later. The CHECK
is the final result after many optimization.
https://github.com/llvm/llvm-project/pull/77608
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