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[ConstraintElimination] Use SCEV ranges information for Loop counter #91457

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c41b5b5
[ConstraintSystem] FM Elimination loop code simplification and comments
May 7, 2024
9027268
[ConstraintSystem] Relax system if elimination not possible
May 7, 2024
7062594
[ConstraintElim] Use SCEV inferred ranges of loop counter
May 3, 2024
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11 changes: 10 additions & 1 deletion llvm/include/llvm/Analysis/ConstraintSystem.h
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Expand Up @@ -43,6 +43,9 @@ class ConstraintSystem {
return 0;
}

// WARNING: it represents rather the maximum number of coefficients in the
// constraints which is actually the number of variables PLUS one (for the
// constant part).
size_t NumVariables = 0;

/// Current linear constraints in the system.
Expand All @@ -54,7 +57,13 @@ class ConstraintSystem {
/// constraint system.
DenseMap<Value *, unsigned> Value2Index;

// Eliminate constraints from the system using Fourier–Motzkin elimination.
// Eliminate the last variable from the system using Fourier–Motzkin
// elimination, while possibly relaxing it if it is beyond
// acceptable means (too many created constraints, overflow when
// computing the coefficients)
//
// return true if the updated system is equivalent, otherwise return
// false if it is relaxed.
bool eliminateUsingFM();

/// Returns true if there may be a solution for the constraints in the system.
Expand Down
62 changes: 33 additions & 29 deletions llvm/lib/Analysis/ConstraintSystem.cpp
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Expand Up @@ -47,67 +47,67 @@ bool ConstraintSystem::eliminateUsingFM() {
}
}

// Track if an overflow occurred when computing coefficents
bool Overflow = false;

// Process rows where the variable is != 0.
unsigned NumRemainingConstraints = RemainingRows.size();
for (unsigned R1 = 0; R1 < NumRemainingConstraints; R1++) {
// FIXME do not use copy
for (unsigned R2 = R1 + 1; R2 < NumRemainingConstraints; R2++) {
if (R1 == R2)
continue;

int64_t UpperLast = getLastCoefficient(RemainingRows[R2], LastIdx);
int64_t LowerLast = getLastCoefficient(RemainingRows[R1], LastIdx);
assert(
UpperLast != 0 && LowerLast != 0 &&
"RemainingRows should only contain rows where the variable is != 0");

// ensure signs are different then swap if necessary to only
// deal with UpperLast > 0
if ((LowerLast < 0 && UpperLast < 0) || (LowerLast > 0 && UpperLast > 0))
continue;

unsigned LowerR = R1;
unsigned UpperR = R2;
if (UpperLast < 0) {
std::swap(LowerR, UpperR);
std::swap(LowerLast, UpperLast);
}

// The following loop does the element-wise operation on sparse
// vectors:
//
// NR = - UpperRow * LowerLast + LowerRow * UpperLast
SmallVector<Entry, 8> NR;
unsigned IdxUpper = 0;
unsigned IdxLower = 0;
auto &LowerRow = RemainingRows[LowerR];
auto &UpperRow = RemainingRows[UpperR];
while (true) {
if (IdxUpper >= UpperRow.size() || IdxLower >= LowerRow.size())
break;

while (IdxUpper < UpperRow.size() && IdxLower < LowerRow.size()) {
int64_t M1, M2, N;
int64_t UpperV = 0;
int64_t LowerV = 0;
uint16_t CurrentId = std::numeric_limits<uint16_t>::max();
if (IdxUpper < UpperRow.size()) {
CurrentId = std::min(UpperRow[IdxUpper].Id, CurrentId);
}
if (IdxLower < LowerRow.size()) {
CurrentId = std::min(LowerRow[IdxLower].Id, CurrentId);
}
uint16_t CurrentId =
std::min(UpperRow[IdxUpper].Id, LowerRow[IdxLower].Id);

if (IdxUpper < UpperRow.size() && UpperRow[IdxUpper].Id == CurrentId) {
if (UpperRow[IdxUpper].Id == CurrentId) {
UpperV = UpperRow[IdxUpper].Coefficient;
IdxUpper++;
IdxUpper += 1;
}

if (MulOverflow(UpperV, -1 * LowerLast, M1))
return false;
if (IdxLower < LowerRow.size() && LowerRow[IdxLower].Id == CurrentId) {
if (LowerRow[IdxLower].Id == CurrentId) {
LowerV = LowerRow[IdxLower].Coefficient;
IdxLower++;
IdxLower += 1;
}

if (MulOverflow(UpperV, -1 * LowerLast, M1) ||
MulOverflow(LowerV, UpperLast, M2) || AddOverflow(M1, M2, N)) {
Overflow = true;
continue;
}

if (MulOverflow(LowerV, UpperLast, M2))
return false;
if (AddOverflow(M1, M2, N))
return false;
// useless to add a 0 to a sparse vector
if (N == 0)
continue;

NR.emplace_back(N, CurrentId);
}
if (NR.empty())
Expand All @@ -120,13 +120,17 @@ bool ConstraintSystem::eliminateUsingFM() {
}
NumVariables -= 1;

return true;
return !Overflow;
}

bool ConstraintSystem::mayHaveSolutionImpl() {
while (!Constraints.empty() && NumVariables > 1) {
if (!eliminateUsingFM())
return true;
while (!Constraints.empty() && Constraints.size() <= 500 &&
NumVariables > 1) {
if (!eliminateUsingFM()) {
LLVM_DEBUG(
dbgs()
<< "Some new constraints has been ignored during elimination.\n");
};
}

if (Constraints.empty() || NumVariables > 1)
Expand Down
44 changes: 39 additions & 5 deletions llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
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Expand Up @@ -916,6 +916,45 @@ void State::addInfoForInductions(BasicBlock &BB) {
!SE.isSCEVable(PN->getType()))
return;

auto *AR = dyn_cast_or_null<SCEVAddRecExpr>(SE.getSCEV(PN));
BasicBlock *LoopPred = L->getLoopPredecessor();
if (!AR || AR->getLoop() != L || !LoopPred)
return;

// If SCEV provides relevant range information, we push those facts
// to the worklist relatively to the header node itself (and not its
// successor).
auto UnsignedRange = SE.getUnsignedRange(AR);
auto SignedRange = SE.getSignedRange(AR);
DomTreeNode *DTNHeader = DT.getNode(&BB);

// The range can wrap thus we take Min/Max instead of Lower/Upper
auto UnsignedMin = UnsignedRange.getUnsignedMin();
auto UnsignedMax = UnsignedRange.getUnsignedMax();
if (!UnsignedMax.isMaxValue()) {
WorkList.push_back(FactOrCheck::getConditionFact(
DTNHeader, CmpInst::ICMP_ULE, PN,
Constant::getIntegerValue(PN->getType(), UnsignedMax)));
}
if (!UnsignedMin.isMinValue()) {
WorkList.push_back(FactOrCheck::getConditionFact(
DTNHeader, CmpInst::ICMP_UGE, PN,
Constant::getIntegerValue(PN->getType(), UnsignedMin)));
}

auto SignedMin = SignedRange.getSignedMin();
auto SignedMax = SignedRange.getSignedMax();
if (!SignedMax.isMaxSignedValue()) {
WorkList.push_back(FactOrCheck::getConditionFact(
DTNHeader, CmpInst::ICMP_SLE, PN,
Constant::getIntegerValue(PN->getType(), SignedMax)));
}
if (!SignedMin.isMinSignedValue()) {
WorkList.push_back(FactOrCheck::getConditionFact(
DTNHeader, CmpInst::ICMP_SGE, PN,
Constant::getIntegerValue(PN->getType(), SignedMin)));
}

BasicBlock *InLoopSucc = nullptr;
if (Pred == CmpInst::ICMP_NE)
InLoopSucc = cast<BranchInst>(BB.getTerminator())->getSuccessor(0);
Expand All @@ -927,11 +966,6 @@ void State::addInfoForInductions(BasicBlock &BB) {
if (!L->contains(InLoopSucc) || !L->isLoopExiting(&BB) || InLoopSucc == &BB)
return;

auto *AR = dyn_cast_or_null<SCEVAddRecExpr>(SE.getSCEV(PN));
BasicBlock *LoopPred = L->getLoopPredecessor();
if (!AR || AR->getLoop() != L || !LoopPred)
return;

const SCEV *StartSCEV = AR->getStart();
Value *StartValue = nullptr;
if (auto *C = dyn_cast<SCEVConstant>(StartSCEV)) {
Expand Down
3 changes: 1 addition & 2 deletions llvm/test/Transforms/ConstraintElimination/constraint-overflow.ll
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Expand Up @@ -13,8 +13,7 @@ define i32 @f(i64 %a3, i64 %numElements) {
; CHECK-NEXT: [[CMP:%.*]] = icmp ugt i64 [[A1]], [[A3]]
; CHECK-NEXT: br i1 [[CMP]], label [[IF_END_I:%.*]], label [[ABORT:%.*]]
; CHECK: if.end.i:
; CHECK-NEXT: [[CMP2_NOT_I:%.*]] = icmp ult i64 [[A1]], [[A3]]
; CHECK-NEXT: br i1 [[CMP2_NOT_I]], label [[ABORT]], label [[EXIT:%.*]]
; CHECK-NEXT: br i1 false, label [[ABORT]], label [[EXIT:%.*]]
; CHECK: abort:
; CHECK-NEXT: ret i32 -1
; CHECK: exit:
Expand Down
15 changes: 5 additions & 10 deletions llvm/test/Transforms/ConstraintElimination/loops-bottom-tested-base.ll
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Expand Up @@ -11,10 +11,8 @@ define void @loop_iv_cond_variable_bound(i32 %n) {
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[T_1:%.*]] = icmp ule i32 [[IV]], [[N:%.*]]
; CHECK-NEXT: call void @use(i1 [[T_1]])
; CHECK-NEXT: [[T_2:%.*]] = icmp sge i32 [[IV]], 0
; CHECK-NEXT: call void @use(i1 [[T_2]])
; CHECK-NEXT: [[T_3:%.*]] = icmp sge i32 [[IV]], -1
; CHECK-NEXT: call void @use(i1 [[T_3]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[C_1:%.*]] = icmp ult i32 [[IV]], [[N]]
; CHECK-NEXT: call void @use(i1 [[C_1]])
; CHECK-NEXT: [[C_2:%.*]] = icmp ugt i32 [[IV]], 1
Expand Down Expand Up @@ -56,12 +54,9 @@ define void @loop_iv_cond_constant_bound() {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[T_1:%.*]] = icmp ule i32 [[IV]], 2
; CHECK-NEXT: call void @use(i1 [[T_1]])
; CHECK-NEXT: [[T_2:%.*]] = icmp sge i32 [[IV]], 0
; CHECK-NEXT: call void @use(i1 [[T_2]])
; CHECK-NEXT: [[T_3:%.*]] = icmp sge i32 [[IV]], -1
; CHECK-NEXT: call void @use(i1 [[T_3]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[C_1:%.*]] = icmp ult i32 [[IV]], 2
; CHECK-NEXT: call void @use(i1 [[C_1]])
; CHECK-NEXT: [[C_2:%.*]] = icmp ugt i32 [[IV]], 1
Expand Down
37 changes: 13 additions & 24 deletions llvm/test/Transforms/ConstraintElimination/loops-header-tested-base.ll
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Expand Up @@ -14,15 +14,11 @@ define void @loop_phi_pos_start_value(i32 %y, i1 %c, i32 %n) {
; CHECK: loop.latch:
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 false)
; CHECK-NEXT: [[T_2:%.*]] = icmp sge i32 [[X]], 10
; CHECK-NEXT: call void @use(i1 [[T_2]])
; CHECK-NEXT: [[C_2:%.*]] = icmp sle i32 [[X]], 9
; CHECK-NEXT: call void @use(i1 [[C_2]])
; CHECK-NEXT: [[C_3:%.*]] = icmp sgt i32 [[X]], 9
; CHECK-NEXT: call void @use(i1 [[C_3]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[C_5:%.*]] = icmp sge i32 [[X]], 9
; CHECK-NEXT: call void @use(i1 [[C_5]])
; CHECK-NEXT: call void @use(i1 false)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[X_NEXT]] = add nsw i32 [[X]], 1
; CHECK-NEXT: br label [[LOOP_HEADER]]
; CHECK: exit:
Expand Down Expand Up @@ -75,8 +71,7 @@ define void @loop_phi_neg_start_value(i32 %y, i1 %c, i32 %n) {
; CHECK: loop.latch:
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 false)
; CHECK-NEXT: [[T_2:%.*]] = icmp sge i32 [[X]], -10
; CHECK-NEXT: call void @use(i1 [[T_2]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[C_2:%.*]] = icmp sle i32 [[X]], 9
; CHECK-NEXT: call void @use(i1 [[C_2]])
; CHECK-NEXT: [[C_3:%.*]] = icmp sgt i32 [[X]], 9
Expand Down Expand Up @@ -248,12 +243,11 @@ define void @loop_latch_not_executed_constant_bound(i32 %y, i1 %c) {
; CHECK-NEXT: br i1 [[C:%.*]], label [[LOOP_HEADER:%.*]], label [[EXIT:%.*]]
; CHECK: loop.header:
; CHECK-NEXT: [[X:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[X_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[C_1:%.*]] = icmp ugt i32 [[X]], 10
; CHECK-NEXT: br i1 [[C_1]], label [[LOOP_LATCH]], label [[EXIT]]
; CHECK-NEXT: br i1 false, label [[LOOP_LATCH]], label [[EXIT]]
; CHECK: loop.latch:
; CHECK-NEXT: call void @use(i1 false)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 false)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[X_NEXT]] = add i32 [[X]], 1
; CHECK-NEXT: br label [[LOOP_HEADER]]
Expand Down Expand Up @@ -299,10 +293,8 @@ define void @loop_iv_cond_variable_bound(i32 %n) {
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[T_1:%.*]] = icmp ule i32 [[IV]], [[N:%.*]]
; CHECK-NEXT: call void @use(i1 [[T_1]])
; CHECK-NEXT: [[T_2:%.*]] = icmp sge i32 [[IV]], 0
; CHECK-NEXT: call void @use(i1 [[T_2]])
; CHECK-NEXT: [[T_3:%.*]] = icmp sge i32 [[IV]], -1
; CHECK-NEXT: call void @use(i1 [[T_3]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[C_1:%.*]] = icmp ult i32 [[IV]], [[N]]
; CHECK-NEXT: call void @use(i1 [[C_1]])
; CHECK-NEXT: [[C_2:%.*]] = icmp ugt i32 [[IV]], 1
Expand Down Expand Up @@ -362,12 +354,9 @@ define void @loop_iv_cond_constant_bound() {
; CHECK-NEXT: br label [[LOOP_HEADER:%.*]]
; CHECK: loop.header:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[T_1:%.*]] = icmp ule i32 [[IV]], 2
; CHECK-NEXT: call void @use(i1 [[T_1]])
; CHECK-NEXT: [[T_2:%.*]] = icmp sge i32 [[IV]], 0
; CHECK-NEXT: call void @use(i1 [[T_2]])
; CHECK-NEXT: [[T_3:%.*]] = icmp sge i32 [[IV]], -1
; CHECK-NEXT: call void @use(i1 [[T_3]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[C_1:%.*]] = icmp ult i32 [[IV]], 2
; CHECK-NEXT: call void @use(i1 [[C_1]])
; CHECK-NEXT: [[C_2:%.*]] = icmp ugt i32 [[IV]], 1
Expand Down
11 changes: 4 additions & 7 deletions llvm/test/Transforms/ConstraintElimination/loops.ll
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Expand Up @@ -107,15 +107,12 @@ define i32 @loop_header_dom_successors_flipped(i32 %y, i1 %c) {
; CHECK-NEXT: br i1 [[C:%.*]], label [[LOOP_HEADER:%.*]], label [[EXIT:%.*]]
; CHECK: loop.header:
; CHECK-NEXT: [[X:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[X_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[C_1:%.*]] = icmp ule i32 [[X]], 10
; CHECK-NEXT: br i1 [[C_1]], label [[EXIT]], label [[LOOP_LATCH]]
; CHECK-NEXT: br i1 true, label [[EXIT]], label [[LOOP_LATCH]]
; CHECK: loop.latch:
; CHECK-NEXT: call void @use(i1 false)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[C_2:%.*]] = icmp ugt i32 [[X]], 11
; CHECK-NEXT: call void @use(i1 [[C_2]])
; CHECK-NEXT: [[C_3:%.*]] = icmp ule i32 [[X]], 11
; CHECK-NEXT: call void @use(i1 [[C_3]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[X_NEXT]] = add i32 [[X]], 1
; CHECK-NEXT: br label [[LOOP_HEADER]]
; CHECK: exit:
Expand Down
3 changes: 1 addition & 2 deletions llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-decrement.ll
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Expand Up @@ -197,8 +197,7 @@ define void @add_rec_decreasing_2_cond_true_constant(i8 noundef %len) {
; CHECK-NEXT: [[CMP2_NOT:%.*]] = icmp eq i8 [[K_0]], 0
; CHECK-NEXT: br i1 [[CMP2_NOT]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
; CHECK: loop.latch:
; CHECK-NEXT: [[CMP_NOT_I:%.*]] = icmp ult i8 [[K_0]], 5
; CHECK-NEXT: call void @use(i1 [[CMP_NOT_I]])
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: [[K_DEC]] = add i8 [[K_0]], -2
; CHECK-NEXT: br label [[LOOP_HEADER]]
; CHECK: exit:
Expand Down
9 changes: 3 additions & 6 deletions llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-wrapping.ll
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Expand Up @@ -49,8 +49,7 @@ define void @test_iv_nuw_nsw_1_uge_start(i8 %len.n, i8 %a) {
; CHECK-NEXT: br label [[LOOP_HEADER:%.*]]
; CHECK: loop.header:
; CHECK-NEXT: [[IV:%.*]] = phi i8 [ -1, [[LOOP_PH:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[C:%.*]] = icmp eq i8 [[IV]], 1
; CHECK-NEXT: br i1 [[C]], label [[EXIT:%.*]], label [[FOR_BODY:%.*]]
; CHECK-NEXT: br i1 false, label [[EXIT:%.*]], label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[C_2:%.*]] = call i1 @cond()
; CHECK-NEXT: br i1 [[C_2]], label [[LOOP_LATCH]], label [[EXIT]]
Expand Down Expand Up @@ -90,8 +89,7 @@ define void @test_iv_nuw_nsw_2_uge_start(i8 %len.n, i8 %a) {
; CHECK-NEXT: br label [[LOOP_HEADER:%.*]]
; CHECK: loop.header:
; CHECK-NEXT: [[IV:%.*]] = phi i8 [ [[START]], [[LOOP_PH:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[C:%.*]] = icmp eq i8 [[IV]], 1
; CHECK-NEXT: br i1 [[C]], label [[EXIT:%.*]], label [[FOR_BODY:%.*]]
; CHECK-NEXT: br i1 false, label [[EXIT:%.*]], label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[C_2:%.*]] = call i1 @cond()
; CHECK-NEXT: br i1 [[C_2]], label [[LOOP_LATCH]], label [[EXIT]]
Expand Down Expand Up @@ -131,8 +129,7 @@ define void @test_iv_nsw_nuw_1_ult_end(i8 %len.n, i8 %a) {
; CHECK-NEXT: br label [[LOOP_HEADER:%.*]]
; CHECK: loop.header:
; CHECK-NEXT: [[IV:%.*]] = phi i8 [ -2, [[LOOP_PH:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT: [[C:%.*]] = icmp eq i8 [[IV]], 1
; CHECK-NEXT: br i1 [[C]], label [[EXIT:%.*]], label [[FOR_BODY:%.*]]
; CHECK-NEXT: br i1 false, label [[EXIT:%.*]], label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[C_2:%.*]] = call i1 @cond()
; CHECK-NEXT: br i1 [[C_2]], label [[LOOP_LATCH]], label [[EXIT]]
Expand Down
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