aboutsummaryrefslogtreecommitdiff
path: root/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
diff options
context:
space:
mode:
Diffstat (limited to 'llvm/lib/Transforms/Utils/SimplifyCFG.cpp')
-rw-r--r--llvm/lib/Transforms/Utils/SimplifyCFG.cpp173
1 files changed, 135 insertions, 38 deletions
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index cbc604e..37c048f 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -778,8 +778,10 @@ private:
return false;
// Add all values from the range to the set
- for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
+ APInt Tmp = Span.getLower();
+ do
Vals.push_back(ConstantInt::get(I->getContext(), Tmp));
+ while (++Tmp != Span.getUpper());
UsedICmps++;
return true;
@@ -5212,8 +5214,7 @@ bool SimplifyCFGOpt::simplifyBranchOnICmpChain(BranchInst *BI,
// We don't have any info about this condition.
auto *Br = TrueWhenEqual ? Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB)
: Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB);
- setExplicitlyUnknownBranchWeightsIfProfiled(*Br, *NewBB->getParent(),
- DEBUG_TYPE);
+ setExplicitlyUnknownBranchWeightsIfProfiled(*Br, DEBUG_TYPE);
OldTI->eraseFromParent();
@@ -6020,6 +6021,8 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
const DataLayout &DL) {
Value *Cond = SI->getCondition();
KnownBits Known = computeKnownBits(Cond, DL, AC, SI);
+ SmallPtrSet<const Constant *, 4> KnownValues;
+ bool IsKnownValuesValid = collectPossibleValues(Cond, KnownValues, 4);
// We can also eliminate cases by determining that their values are outside of
// the limited range of the condition based on how many significant (non-sign)
@@ -6039,15 +6042,18 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
UniqueSuccessors.push_back(Successor);
++It->second;
}
- const APInt &CaseVal = Case.getCaseValue()->getValue();
+ ConstantInt *CaseC = Case.getCaseValue();
+ const APInt &CaseVal = CaseC->getValue();
if (Known.Zero.intersects(CaseVal) || !Known.One.isSubsetOf(CaseVal) ||
- (CaseVal.getSignificantBits() > MaxSignificantBitsInCond)) {
- DeadCases.push_back(Case.getCaseValue());
+ (CaseVal.getSignificantBits() > MaxSignificantBitsInCond) ||
+ (IsKnownValuesValid && !KnownValues.contains(CaseC))) {
+ DeadCases.push_back(CaseC);
if (DTU)
--NumPerSuccessorCases[Successor];
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal
<< " is dead.\n");
- }
+ } else if (IsKnownValuesValid)
+ KnownValues.erase(CaseC);
}
// If we can prove that the cases must cover all possible values, the
@@ -6058,33 +6064,41 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
const unsigned NumUnknownBits =
Known.getBitWidth() - (Known.Zero | Known.One).popcount();
assert(NumUnknownBits <= Known.getBitWidth());
- if (HasDefault && DeadCases.empty() &&
- NumUnknownBits < 64 /* avoid overflow */) {
- uint64_t AllNumCases = 1ULL << NumUnknownBits;
- if (SI->getNumCases() == AllNumCases) {
+ if (HasDefault && DeadCases.empty()) {
+ if (IsKnownValuesValid && all_of(KnownValues, IsaPred<UndefValue>)) {
createUnreachableSwitchDefault(SI, DTU);
return true;
}
- // When only one case value is missing, replace default with that case.
- // Eliminating the default branch will provide more opportunities for
- // optimization, such as lookup tables.
- if (SI->getNumCases() == AllNumCases - 1) {
- assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
- IntegerType *CondTy = cast<IntegerType>(Cond->getType());
- if (CondTy->getIntegerBitWidth() > 64 ||
- !DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
- return false;
- uint64_t MissingCaseVal = 0;
- for (const auto &Case : SI->cases())
- MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
- auto *MissingCase =
- cast<ConstantInt>(ConstantInt::get(Cond->getType(), MissingCaseVal));
- SwitchInstProfUpdateWrapper SIW(*SI);
- SIW.addCase(MissingCase, SI->getDefaultDest(), SIW.getSuccessorWeight(0));
- createUnreachableSwitchDefault(SI, DTU, /*RemoveOrigDefaultBlock*/ false);
- SIW.setSuccessorWeight(0, 0);
- return true;
+ if (NumUnknownBits < 64 /* avoid overflow */) {
+ uint64_t AllNumCases = 1ULL << NumUnknownBits;
+ if (SI->getNumCases() == AllNumCases) {
+ createUnreachableSwitchDefault(SI, DTU);
+ return true;
+ }
+ // When only one case value is missing, replace default with that case.
+ // Eliminating the default branch will provide more opportunities for
+ // optimization, such as lookup tables.
+ if (SI->getNumCases() == AllNumCases - 1) {
+ assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
+ IntegerType *CondTy = cast<IntegerType>(Cond->getType());
+ if (CondTy->getIntegerBitWidth() > 64 ||
+ !DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
+ return false;
+
+ uint64_t MissingCaseVal = 0;
+ for (const auto &Case : SI->cases())
+ MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
+ auto *MissingCase = cast<ConstantInt>(
+ ConstantInt::get(Cond->getType(), MissingCaseVal));
+ SwitchInstProfUpdateWrapper SIW(*SI);
+ SIW.addCase(MissingCase, SI->getDefaultDest(),
+ SIW.getSuccessorWeight(0));
+ createUnreachableSwitchDefault(SI, DTU,
+ /*RemoveOrigDefaultBlock*/ false);
+ SIW.setSuccessorWeight(0, 0);
+ return true;
+ }
}
}
@@ -7570,6 +7584,81 @@ static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
return true;
}
+/// Tries to transform the switch when the condition is umin with a constant.
+/// In that case, the default branch can be replaced by the constant's branch.
+/// This method also removes dead cases when the simplification cannot replace
+/// the default branch.
+///
+/// For example:
+/// switch(umin(a, 3)) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// case 3:
+/// case 4:
+/// // ...
+/// default:
+/// unreachable
+/// }
+///
+/// Transforms into:
+///
+/// switch(a) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// default:
+/// // This is case 3
+/// }
+static bool simplifySwitchWhenUMin(SwitchInst *SI, DomTreeUpdater *DTU) {
+ Value *A;
+ ConstantInt *Constant;
+
+ if (!match(SI->getCondition(), m_UMin(m_Value(A), m_ConstantInt(Constant))))
+ return false;
+
+ SmallVector<DominatorTree::UpdateType> Updates;
+ SwitchInstProfUpdateWrapper SIW(*SI);
+ BasicBlock *BB = SIW->getParent();
+
+ // Dead cases are removed even when the simplification fails.
+ // A case is dead when its value is higher than the Constant.
+ for (auto I = SI->case_begin(), E = SI->case_end(); I != E;) {
+ if (!I->getCaseValue()->getValue().ugt(Constant->getValue())) {
+ ++I;
+ continue;
+ }
+ BasicBlock *DeadCaseBB = I->getCaseSuccessor();
+ DeadCaseBB->removePredecessor(BB);
+ Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
+ I = SIW->removeCase(I);
+ E = SIW->case_end();
+ }
+
+ auto Case = SI->findCaseValue(Constant);
+ // If the case value is not found, `findCaseValue` returns the default case.
+ // In this scenario, since there is no explicit `case 3:`, the simplification
+ // fails. The simplification also fails when the switch’s default destination
+ // is reachable.
+ if (!SI->defaultDestUnreachable() || Case == SI->case_default()) {
+ if (DTU)
+ DTU->applyUpdates(Updates);
+ return !Updates.empty();
+ }
+
+ BasicBlock *Unreachable = SI->getDefaultDest();
+ SIW.replaceDefaultDest(Case);
+ SIW.removeCase(Case);
+ SIW->setCondition(A);
+
+ Updates.push_back({DominatorTree::Delete, BB, Unreachable});
+
+ if (DTU)
+ DTU->applyUpdates(Updates);
+
+ return true;
+}
+
/// Tries to transform switch of powers of two to reduce switch range.
/// For example, switch like:
/// switch (C) { case 1: case 2: case 64: case 128: }
@@ -7642,19 +7731,24 @@ static bool simplifySwitchOfPowersOfTwo(SwitchInst *SI, IRBuilder<> &Builder,
// label. The other is those powers of 2 that don't appear in the case
// statement. We don't know the distribution of the values coming in, so
// the safest is to split 50-50 the original probability to `default`.
- uint64_t OrigDenominator = sum_of(map_range(
- Weights, [](const auto &V) { return static_cast<uint64_t>(V); }));
+ uint64_t OrigDenominator =
+ sum_of(map_range(Weights, StaticCastTo<uint64_t>));
SmallVector<uint64_t> NewWeights(2);
NewWeights[1] = Weights[0] / 2;
NewWeights[0] = OrigDenominator - NewWeights[1];
setFittedBranchWeights(*BI, NewWeights, /*IsExpected=*/false);
-
- // For the original switch, we reduce the weight of the default by the
- // amount by which the previous branch contributes to getting to default,
- // and then make sure the remaining weights have the same relative ratio
- // wrt eachother.
+ // The probability of executing the default block stays constant. It was
+ // p_d = Weights[0] / OrigDenominator
+ // we rewrite as W/D
+ // We want to find the probability of the default branch of the switch
+ // statement. Let's call it X. We have W/D = W/2D + X * (1-W/2D)
+ // i.e. the original probability is the probability we go to the default
+ // branch from the BI branch, or we take the default branch on the SI.
+ // Meaning X = W / (2D - W), or (W/2) / (D - W/2)
+ // This matches using W/2 for the default branch probability numerator and
+ // D-W/2 as the denominator.
+ Weights[0] = NewWeights[1];
uint64_t CasesDenominator = OrigDenominator - Weights[0];
- Weights[0] /= 2;
for (auto &W : drop_begin(Weights))
W = NewWeights[0] * static_cast<double>(W) / CasesDenominator;
@@ -8037,6 +8131,9 @@ bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
if (simplifyDuplicateSwitchArms(SI, DTU))
return requestResimplify();
+ if (simplifySwitchWhenUMin(SI, DTU))
+ return requestResimplify();
+
return false;
}
generated by cgit v1.1 at 2025-11-06 21:14:42 +0000