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DeadBranchElim: Improve algorithm to only remove blocks with no predecessors

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Must be careful not to remove blocks pointed at by unreachable blocks
This commit is contained in:
GregF 2017-07-10 17:20:35 -06:00 committed by David Neto
parent 06d4fd52c2
commit e2544ddc90
1 changed files with 11 additions and 22 deletions
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33
source/opt/dead_branch_elim_pass.cpp
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@ -218,27 +218,11 @@ bool DeadBranchElimPass::EliminateDeadBranches(ir::Function* func) {
def_use_mgr_->KillInst(mergeInst);
// Iterate to merge block adding dead blocks to elimination set
std::unordered_set<uint32_t> deadSuccIds;
deadSuccIds.insert(deadLabId);
auto dbi = bi;
++dbi;
uint32_t dLabId = (*dbi)->id();
while (dLabId != mergeLabId) {
if (deadSuccIds.find(dLabId) != deadSuccIds.end()) {
// Add successor blocks to dead successor set
(*dbi)->ForEachSuccessorLabel([&deadSuccIds](uint32_t succ) {
deadSuccIds.insert(succ);
});
// If dead block is header block, add its merge block to dead
// successor set in case it has no predecessors. Add continue
// target if loop header.
uint32_t cbid;
const uint32_t dMergeLabId = MergeBlockIdIfAny(**dbi, &cbid);
if (dMergeLabId != 0) {
deadSuccIds.insert(dMergeLabId);
if (cbid != 0)
deadSuccIds.insert(cbid);
}
if (!HasNonPhiRef(dLabId)) {
// Kill use/def for all instructions and mark block for elimination
KillAllInsts(*dbi);
elimBlocks.insert(*dbi);
@ -248,14 +232,19 @@ bool DeadBranchElimPass::EliminateDeadBranches(ir::Function* func) {
}
// Process phi instructions in merge block.
// deadLabIds are now blocks which cannot precede merge block.
// If eliminated branch is to merge label, add current block to dead blocks.
// elimBlocks are now blocks which cannot precede merge block. Also,
// if eliminated branch is to merge label, remember the conditional block
// also cannot precede merge block.
uint32_t deadCondLabId = 0;
if (deadLabId == mergeLabId)
deadSuccIds.insert((*bi)->id());
(*dbi)->ForEachPhiInst([&deadSuccIds, this](ir::Instruction* phiInst) {
deadCondLabId = (*bi)->id();
(*dbi)->ForEachPhiInst([&elimBlocks, &deadCondLabId, this](
ir::Instruction* phiInst) {
const uint32_t phiLabId0 =
phiInst->GetSingleWordInOperand(kPhiLab0IdInIdx);
const bool useFirst = deadSuccIds.find(phiLabId0) == deadSuccIds.end();
const bool useFirst =
elimBlocks.find(id2block_[phiLabId0]) == elimBlocks.end() &&
phiLabId0 != deadCondLabId;
const uint32_t phiValIdx =
useFirst ? kPhiVal0IdInIdx : kPhiVal1IdInIdx;
const uint32_t replId = phiInst->GetSingleWordInOperand(phiValIdx);