Is LiveVariables updated correctly when TII->RemoveBranch and TII->InsertBranch are called in the following piece of code?
- MachineBasicBlock::updateTerminator() line 307 of MachineBasicBlock.cpp:
if (FBB) {
// The block has a non-fallthrough conditional branch. If one of its
// successors is its layout successor, rewrite it to a fallthrough
// conditional branch.
if (isLayoutSuccessor(TBB)) {
if (TII->ReverseBranchCondition(Cond))
return;
TII->RemoveBranch(*this);
TII->InsertBranch(*this, FBB, 0, Cond, dl);
} else if (isLayoutSuccessor(FBB)) {
I am investigating an assertion (shown below) that is fired when I run llc with -march=mipsel -mcpu=4ke. It seems to me that the root cause of this assertion is the piece of code shown above that is called during PHI nodes elimination (llc exits normally if I add -disable-phi-elim-edge-splitting).
llc: llvm/lib/CodeGen/SplitKit.cpp:170: bool llvm::SplitAnalysis::calcLiveBlockInfo(): Assertion `BI.FirstUse >= Start’ failed.
The following is the Machine IR and LiveVariables::ValInfo dump before and after PHI nodes elimination.
- Before PHI nodes elimination.
-Machine IR:
BB#14: derived from LLVM BB %for.cond151.preheader
Predecessors according to CFG: BB#12 BB#13
%vreg29 = PHI %vreg25, <BB#12>, %vreg28, <BB#13>; CPURegs:%vreg29,%vreg25,%vreg28
%vreg30 = PHI %vreg26, <BB#12>, %vreg27, <BB#13>; CPURegs:%vreg30,%vreg26,%vreg27
BNE %vreg81, %ZERO, <BB#17>; CPURegs:%vreg81
J <BB#15>
Successors according to CFG: BB#15 BB#17
BB#15: derived from LLVM BB %for.body156.preheader
Predecessors according to CFG: BB#14
BB#17: derived from LLVM BB %for.end241
Predecessors according to CFG: BB#14 BB#16
-vreg81’s VarInfo:
Alive in blocks: 5, 6, 7, 8, 10, 12, 13, 19,
Killed by:
#0: BNE %vreg81, %ZERO, <BB#17>; CPURegs:%vreg81
-
During PHI nodes elimination, critical edge BB#14-#17 is split and BB#20 is inserted. The two terminators of BB#14 (BNE and J ) are replaced with a conditional branch (BEQ) when MachineBasicBlock::updateTerminator() is called.
-
After PHI nodes elimination.
- Machine IR:
BB#14: derived from LLVM BB %for.cond151.preheader
Predecessors according to CFG: BB#13 BB#19
%vreg29 = COPY %vreg180; CPURegs:%vreg29,%vreg180
%vreg30 = COPY %vreg181; CPURegs:%vreg30,%vreg181
BEQ %vreg81, %ZERO, <BB#15>; CPURegs:%vreg81
Successors according to CFG: BB#15 BB#20
BB#20:
Predecessors according to CFG: BB#14
%vreg187 = COPY %vreg14; CPURegs:%vreg187,%vreg14
%vreg188 = COPY %vreg29; CPURegs:%vreg188,%vreg29
J <BB#17>
Successors according to CFG: BB#17
BB#15: derived from LLVM BB %for.body156.preheader
Predecessors according to CFG: BB#14
BB#17: derived from LLVM BB %for.end241
Predecessors according to CFG: BB#16 BB#20
- vreg81’s VarInfo:
Alive in blocks: 5, 6, 7, 8, 10, 12, 13, 19,
Killed by:
#0: J <BB#17>
As you can see, VarInfo vreg81 is killed by the unconditional jump instruction of BB#20 when it should be killed by the newly created conditional branch in BB#14 (BEQ). Is this a bug in updateTerminator() or is the back-end responsible for updating LiveVariables?