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Oct 6, 2025
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[SimplifyCFG] Fold the contiguous wrapping cases into ICmp. #161000

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dc8ea1e
Pre-commit test cases
dianqk Sep 27, 2025
4010221
[SimplifyCFG] Fold the contiguous wrapping cases into ICmp.
dianqk Sep 26, 2025
c56155d
Fix the count of pruning incoming values
dianqk Sep 27, 2025
41c4da0
[SimplifyCFG] Cases to the default destination cannot be contiguous c…
dianqk Oct 2, 2025
0ce3e6e
Add more tests
dianqk Oct 3, 2025
dd4fdf6
[SimplifyCFG] Improve one case
dianqk Oct 3, 2025
bfc9e92
Add more tests
dianqk Oct 3, 2025
9864e17
[SimplifyCFG] Use icmp eq for one case
dianqk Oct 3, 2025
2288724
Address feedback
dianqk Oct 4, 2025
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dianqk Oct 6, 2025
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149 changes: 112 additions & 37 deletions llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5718,15 +5718,66 @@ bool SimplifyCFGOpt::simplifyUnreachable(UnreachableInst *UI) {
return Changed;
}

static bool casesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
struct ContiguousCasesResult {
ConstantInt *Min;
ConstantInt *Max;
BasicBlock *Dest;
BasicBlock *OtherDest;
SmallVectorImpl<ConstantInt *> *Cases;
SmallVectorImpl<ConstantInt *> *OtherCases;
};

static std::optional<ContiguousCasesResult>
findContiguousCases(Value *Condition, SmallVectorImpl<ConstantInt *> &Cases,
SmallVectorImpl<ConstantInt *> &OtherCases,
BasicBlock *Dest, BasicBlock *OtherDest) {
assert(Cases.size() >= 1);

array_pod_sort(Cases.begin(), Cases.end(), constantIntSortPredicate);
for (size_t I = 1, E = Cases.size(); I != E; ++I) {
if (Cases[I - 1]->getValue() != Cases[I]->getValue() + 1)
return false;
APInt Min = Cases.back()->getValue();
APInt Max = Cases.front()->getValue();
APInt Offset = Max - Min;
size_t ContiguousOffset = Cases.size() - 1;
if (Offset == ContiguousOffset) {
return ContiguousCasesResult{
/*Min=*/Cases.back(),
/*Max=*/Cases.front(),
/*Dest=*/Dest,
/*OtherDest=*/OtherDest,
/*Cases=*/&Cases,
/*OtherCases=*/&OtherCases,
};
}
return true;
ConstantRange CR = computeConstantRange(Condition, /*ForSigned=*/false);
// If this is a wrapping contiguous range, that is, [Min, OtherMin] +
// [OtherMax, Max] (also [OtherMax, OtherMin]), [OtherMin+1, OtherMax-1] is a
// contiguous range for the other destination. N.B. If CR is not a full range,
// Max+1 is not equal to Min. It's not continuous in arithmetic.
if (Max == CR.getUnsignedMax() && Min == CR.getUnsignedMin()) {
assert(Cases.size() >= 2);
auto *It =
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How about something as follows before the logic here?

// Find the first non-consecutive pair, and ensure this pair
// happens to be unique.

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(I meant this before the std::adjacent_find, which is not appearing)

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I think this is what you want. std::adjacent_find finds one non-consecutive pair and checks their distance to check if this pair is unique.

std::adjacent_find(Cases.begin(), Cases.end(), [](auto L, auto R) {
return L->getValue() != R->getValue() + 1;
});
if (It == Cases.end())
return std::nullopt;
auto [OtherMax, OtherMin] = std::make_pair(*It, *std::next(It));
if ((Max - OtherMax->getValue()) + (OtherMin->getValue() - Min) ==
Cases.size() - 2) {
return ContiguousCasesResult{
/*Min=*/cast<ConstantInt>(
ConstantInt::get(OtherMin->getType(), OtherMin->getValue() + 1)),
/*Max=*/
cast<ConstantInt>(
ConstantInt::get(OtherMax->getType(), OtherMax->getValue() - 1)),
/*Dest=*/OtherDest,
/*OtherDest=*/Dest,
/*Cases=*/&OtherCases,
/*OtherCases=*/&Cases,
};
}
}
return std::nullopt;
}

static void createUnreachableSwitchDefault(SwitchInst *Switch,
Expand Down Expand Up @@ -5763,7 +5814,6 @@ bool SimplifyCFGOpt::turnSwitchRangeIntoICmp(SwitchInst *SI,
bool HasDefault = !SI->defaultDestUnreachable();

auto *BB = SI->getParent();

// Partition the cases into two sets with different destinations.
BasicBlock *DestA = HasDefault ? SI->getDefaultDest() : nullptr;
BasicBlock *DestB = nullptr;
Expand Down Expand Up @@ -5797,37 +5847,62 @@ bool SimplifyCFGOpt::turnSwitchRangeIntoICmp(SwitchInst *SI,
assert(!CasesA.empty() || HasDefault);

// Figure out if one of the sets of cases form a contiguous range.
SmallVectorImpl<ConstantInt *> *ContiguousCases = nullptr;
BasicBlock *ContiguousDest = nullptr;
BasicBlock *OtherDest = nullptr;
if (!CasesA.empty() && casesAreContiguous(CasesA)) {
ContiguousCases = &CasesA;
ContiguousDest = DestA;
OtherDest = DestB;
} else if (casesAreContiguous(CasesB)) {
ContiguousCases = &CasesB;
ContiguousDest = DestB;
OtherDest = DestA;
} else
return false;
std::optional<ContiguousCasesResult> ContiguousCases;

// Only one icmp is needed when there is only one case.
if (!HasDefault && CasesA.size() == 1)
ContiguousCases = ContiguousCasesResult{
/*Min=*/CasesA[0],
/*Max=*/CasesA[0],
/*Dest=*/DestA,
/*OtherDest=*/DestB,
/*Cases=*/&CasesA,
/*OtherCases=*/&CasesB,
};
else if (CasesB.size() == 1)
ContiguousCases = ContiguousCasesResult{
/*Min=*/CasesB[0],
/*Max=*/CasesB[0],
/*Dest=*/DestB,
/*OtherDest=*/DestA,
/*Cases=*/&CasesB,
/*OtherCases=*/&CasesA,
};
// Correctness: Cases to the default destination cannot be contiguous cases.
else if (!HasDefault)
ContiguousCases =
findContiguousCases(SI->getCondition(), CasesA, CasesB, DestA, DestB);

// Start building the compare and branch.
if (!ContiguousCases)
ContiguousCases =
findContiguousCases(SI->getCondition(), CasesB, CasesA, DestB, DestA);

Constant *Offset = ConstantExpr::getNeg(ContiguousCases->back());
Constant *NumCases =
ConstantInt::get(Offset->getType(), ContiguousCases->size());
if (!ContiguousCases)
return false;

Value *Sub = SI->getCondition();
if (!Offset->isNullValue())
Sub = Builder.CreateAdd(Sub, Offset, Sub->getName() + ".off");
auto [Min, Max, Dest, OtherDest, Cases, OtherCases] = *ContiguousCases;

Value *Cmp;
// Start building the compare and branch.

Constant *Offset = ConstantExpr::getNeg(Min);
Constant *NumCases = ConstantInt::get(Offset->getType(),
Max->getValue() - Min->getValue() + 1);
BranchInst *NewBI;
if (NumCases->isOneValue()) {
assert(Max->getValue() == Min->getValue());
Value *Cmp = Builder.CreateICmpEQ(SI->getCondition(), Min);
NewBI = Builder.CreateCondBr(Cmp, Dest, OtherDest);
}
// If NumCases overflowed, then all possible values jump to the successor.
if (NumCases->isNullValue() && !ContiguousCases->empty())
Cmp = ConstantInt::getTrue(SI->getContext());
else
Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
BranchInst *NewBI = Builder.CreateCondBr(Cmp, ContiguousDest, OtherDest);
else if (NumCases->isNullValue() && !Cases->empty()) {
NewBI = Builder.CreateBr(Dest);
} else {
Value *Sub = SI->getCondition();
if (!Offset->isNullValue())
Sub = Builder.CreateAdd(Sub, Offset, Sub->getName() + ".off");
Value *Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
NewBI = Builder.CreateCondBr(Cmp, Dest, OtherDest);
}

// Update weight for the newly-created conditional branch.
if (hasBranchWeightMD(*SI)) {
Expand All @@ -5837,7 +5912,7 @@ bool SimplifyCFGOpt::turnSwitchRangeIntoICmp(SwitchInst *SI,
uint64_t TrueWeight = 0;
uint64_t FalseWeight = 0;
for (size_t I = 0, E = Weights.size(); I != E; ++I) {
if (SI->getSuccessor(I) == ContiguousDest)
if (SI->getSuccessor(I) == Dest)
TrueWeight += Weights[I];
else
FalseWeight += Weights[I];
Expand All @@ -5852,15 +5927,15 @@ bool SimplifyCFGOpt::turnSwitchRangeIntoICmp(SwitchInst *SI,
}

// Prune obsolete incoming values off the successors' PHI nodes.
for (auto BBI = ContiguousDest->begin(); isa<PHINode>(BBI); ++BBI) {
unsigned PreviousEdges = ContiguousCases->size();
if (ContiguousDest == SI->getDefaultDest())
for (auto BBI = Dest->begin(); isa<PHINode>(BBI); ++BBI) {
unsigned PreviousEdges = Cases->size();
if (Dest == SI->getDefaultDest())
++PreviousEdges;
for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
}
for (auto BBI = OtherDest->begin(); isa<PHINode>(BBI); ++BBI) {
unsigned PreviousEdges = SI->getNumCases() - ContiguousCases->size();
unsigned PreviousEdges = OtherCases->size();
if (OtherDest == SI->getDefaultDest())
++PreviousEdges;
for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
Expand Down
4 changes: 2 additions & 2 deletions llvm/test/Transforms/Coroutines/coro-catchswitch-cleanuppad.ll
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Expand Up @@ -80,8 +80,8 @@ cleanup2:
; CHECK: cleanup2.corodispatch:
; CHECK: %1 = phi i8 [ 0, %handler2 ], [ 1, %catch.dispatch.2 ]
; CHECK: %2 = cleanuppad within %h1 []
; CHECK: %switch = icmp ult i8 %1, 1
; CHECK: br i1 %switch, label %cleanup2.from.handler2, label %cleanup2.from.catch.dispatch.2
; CHECK: %3 = icmp eq i8 %1, 0
; CHECK: br i1 %3, label %cleanup2.from.handler2, label %cleanup2.from.catch.dispatch.2

; CHECK: cleanup2.from.handler2:
; CHECK: %valueB.reload = load i32, ptr %valueB.spill.addr, align 4
Expand Down
12 changes: 4 additions & 8 deletions llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
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Expand Up @@ -7,8 +7,7 @@ declare void @foo(i32)
define void @test(i1 %a) {
; CHECK-LABEL: define void @test(
; CHECK-SAME: i1 [[A:%.*]]) {
; CHECK-NEXT: [[A_OFF:%.*]] = add i1 [[A]], true
; CHECK-NEXT: [[SWITCH:%.*]] = icmp ult i1 [[A_OFF]], true
; CHECK-NEXT: [[SWITCH:%.*]] = icmp eq i1 [[A]], true
; CHECK-NEXT: br i1 [[SWITCH]], label [[TRUE:%.*]], label [[FALSE:%.*]]
; CHECK: common.ret:
; CHECK-NEXT: ret void
Expand Down Expand Up @@ -209,8 +208,7 @@ define void @test5(i8 %a) {
; CHECK-SAME: i8 [[A:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i8 [[A]], 2
; CHECK-NEXT: call void @llvm.assume(i1 [[CMP]])
; CHECK-NEXT: [[A_OFF:%.*]] = add i8 [[A]], -1
; CHECK-NEXT: [[SWITCH:%.*]] = icmp ult i8 [[A_OFF]], 1
; CHECK-NEXT: [[SWITCH:%.*]] = icmp eq i8 [[A]], 1
; CHECK-NEXT: br i1 [[SWITCH]], label [[TRUE:%.*]], label [[FALSE:%.*]]
; CHECK: common.ret:
; CHECK-NEXT: ret void
Expand Down Expand Up @@ -243,8 +241,7 @@ define void @test6(i8 %a) {
; CHECK-NEXT: [[AND:%.*]] = and i8 [[A]], -2
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[AND]], -2
; CHECK-NEXT: call void @llvm.assume(i1 [[CMP]])
; CHECK-NEXT: [[A_OFF:%.*]] = add i8 [[A]], 1
; CHECK-NEXT: [[SWITCH:%.*]] = icmp ult i8 [[A_OFF]], 1
; CHECK-NEXT: [[SWITCH:%.*]] = icmp eq i8 [[A]], -1
; CHECK-NEXT: br i1 [[SWITCH]], label [[TRUE:%.*]], label [[FALSE:%.*]]
; CHECK: common.ret:
; CHECK-NEXT: ret void
Expand Down Expand Up @@ -279,8 +276,7 @@ define void @test7(i8 %a) {
; CHECK-NEXT: [[AND:%.*]] = and i8 [[A]], -2
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[AND]], -2
; CHECK-NEXT: call void @llvm.assume(i1 [[CMP]])
; CHECK-NEXT: [[A_OFF:%.*]] = add i8 [[A]], 1
; CHECK-NEXT: [[SWITCH:%.*]] = icmp ult i8 [[A_OFF]], 1
; CHECK-NEXT: [[SWITCH:%.*]] = icmp eq i8 [[A]], -1
; CHECK-NEXT: br i1 [[SWITCH]], label [[TRUE:%.*]], label [[FALSE:%.*]]
; CHECK: common.ret:
; CHECK-NEXT: ret void
Expand Down
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