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[Reland][SCEV] teach isImpliedViaOperations about samesign #133711

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merged 3 commits into from
Apr 2, 2025
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[Reland][SCEV] teach isImpliedViaOperations about samesign #133711

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39148ec
SCEV: teach isImpliedViaOperations about samesign (#124270)
artagnon Feb 6, 2025
e3b5501
[SCEV] Add pre-commit tests. NFC.
dtcxzyw Mar 31, 2025
1387a68
[SCEV] Do not preserve samesign after truncation
dtcxzyw Mar 31, 2025
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39 changes: 21 additions & 18 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11804,8 +11804,10 @@ bool ScalarEvolution::isImpliedCond(CmpPredicate Pred, const SCEV *LHS,
MaxValue)) {
const SCEV *TruncFoundLHS = getTruncateExpr(FoundLHS, NarrowType);
const SCEV *TruncFoundRHS = getTruncateExpr(FoundRHS, NarrowType);
if (isImpliedCondBalancedTypes(Pred, LHS, RHS, FoundPred, TruncFoundLHS,
TruncFoundRHS, CtxI))
// We cannot preserve samesign after truncation.
if (isImpliedCondBalancedTypes(
Pred, LHS, RHS, static_cast<ICmpInst::Predicate>(FoundPred),
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I kind of wonder whether we should have some more explicit API for this like FoundPred.withoutSameSign(). Doing this by going through a static_cast is a bit subtle...

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I will fix it in a follow up patch.

TruncFoundLHS, TruncFoundRHS, CtxI))
return true;
}
}
Expand Down Expand Up @@ -11862,15 +11864,13 @@ bool ScalarEvolution::isImpliedCondBalancedTypes(
}

// Check whether the found predicate is the same as the desired predicate.
// FIXME: use CmpPredicate::getMatching here.
if (FoundPred == static_cast<CmpInst::Predicate>(Pred))
return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS, CtxI);
if (auto P = CmpPredicate::getMatching(FoundPred, Pred))
return isImpliedCondOperands(*P, LHS, RHS, FoundLHS, FoundRHS, CtxI);

// Check whether swapping the found predicate makes it the same as the
// desired predicate.
// FIXME: use CmpPredicate::getMatching here.
if (ICmpInst::getSwappedCmpPredicate(FoundPred) ==
static_cast<CmpInst::Predicate>(Pred)) {
if (auto P = CmpPredicate::getMatching(
ICmpInst::getSwappedCmpPredicate(FoundPred), Pred)) {
// We can write the implication
// 0. LHS Pred RHS <- FoundLHS SwapPred FoundRHS
// using one of the following ways:
Expand All @@ -11881,22 +11881,23 @@ bool ScalarEvolution::isImpliedCondBalancedTypes(
// Forms 1. and 2. require swapping the operands of one condition. Don't
// do this if it would break canonical constant/addrec ordering.
if (!isa<SCEVConstant>(RHS) && !isa<SCEVAddRecExpr>(LHS))
return isImpliedCondOperands(FoundPred, RHS, LHS, FoundLHS, FoundRHS,
CtxI);
return isImpliedCondOperands(ICmpInst::getSwappedCmpPredicate(*P), RHS,
LHS, FoundLHS, FoundRHS, CtxI);
if (!isa<SCEVConstant>(FoundRHS) && !isa<SCEVAddRecExpr>(FoundLHS))
return isImpliedCondOperands(Pred, LHS, RHS, FoundRHS, FoundLHS, CtxI);
return isImpliedCondOperands(*P, LHS, RHS, FoundRHS, FoundLHS, CtxI);

// There's no clear preference between forms 3. and 4., try both. Avoid
// forming getNotSCEV of pointer values as the resulting subtract is
// not legal.
if (!LHS->getType()->isPointerTy() && !RHS->getType()->isPointerTy() &&
isImpliedCondOperands(FoundPred, getNotSCEV(LHS), getNotSCEV(RHS),
FoundLHS, FoundRHS, CtxI))
isImpliedCondOperands(ICmpInst::getSwappedCmpPredicate(*P),
getNotSCEV(LHS), getNotSCEV(RHS), FoundLHS,
FoundRHS, CtxI))
return true;

if (!FoundLHS->getType()->isPointerTy() &&
!FoundRHS->getType()->isPointerTy() &&
isImpliedCondOperands(Pred, LHS, RHS, getNotSCEV(FoundLHS),
isImpliedCondOperands(*P, LHS, RHS, getNotSCEV(FoundLHS),
getNotSCEV(FoundRHS), CtxI))
return true;

Expand Down Expand Up @@ -12572,14 +12573,16 @@ bool ScalarEvolution::isImpliedViaOperations(CmpPredicate Pred, const SCEV *LHS,
return false;

// We only want to work with GT comparison so far.
if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_SLT) {
if (ICmpInst::isLT(Pred)) {
Pred = ICmpInst::getSwappedCmpPredicate(Pred);
std::swap(LHS, RHS);
std::swap(FoundLHS, FoundRHS);
}

CmpInst::Predicate P = Pred.getPreferredSignedPredicate();

// For unsigned, try to reduce it to corresponding signed comparison.
if (Pred == ICmpInst::ICMP_UGT)
if (P == ICmpInst::ICMP_UGT)
// We can replace unsigned predicate with its signed counterpart if all
// involved values are non-negative.
// TODO: We could have better support for unsigned.
Expand All @@ -12592,10 +12595,10 @@ bool ScalarEvolution::isImpliedViaOperations(CmpPredicate Pred, const SCEV *LHS,
FoundRHS) &&
isImpliedCondOperands(ICmpInst::ICMP_SGT, RHS, MinusOne, FoundLHS,
FoundRHS))
Pred = ICmpInst::ICMP_SGT;
P = ICmpInst::ICMP_SGT;
}

if (Pred != ICmpInst::ICMP_SGT)
if (P != ICmpInst::ICMP_SGT)
return false;

auto GetOpFromSExt = [&](const SCEV *S) {
Expand Down
4 changes: 2 additions & 2 deletions llvm/test/Analysis/ScalarEvolution/exit-count-samesign.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -5,9 +5,9 @@
define i32 @exit_count_samesign(i32 %iter.count, ptr %ptr) {
; CHECK-LABEL: 'exit_count_samesign'
; CHECK-NEXT: Determining loop execution counts for: @exit_count_samesign
; CHECK-NEXT: Loop %inner.loop: backedge-taken count is (-1 + (1 smax {(-1 + %iter.count)<nsw>,+,-1}<nsw><%outer.loop>))<nsw>
; CHECK-NEXT: Loop %inner.loop: backedge-taken count is {(-2 + %iter.count),+,-1}<nw><%outer.loop>
; CHECK-NEXT: Loop %inner.loop: constant max backedge-taken count is i32 2147483646
; CHECK-NEXT: Loop %inner.loop: symbolic max backedge-taken count is (-1 + (1 smax {(-1 + %iter.count)<nsw>,+,-1}<nsw><%outer.loop>))<nsw>
; CHECK-NEXT: Loop %inner.loop: symbolic max backedge-taken count is {(-2 + %iter.count),+,-1}<nw><%outer.loop>
; CHECK-NEXT: Loop %inner.loop: Trip multiple is 1
; CHECK-NEXT: Loop %outer.loop: <multiple exits> Unpredictable backedge-taken count.
; CHECK-NEXT: Loop %outer.loop: Unpredictable constant max backedge-taken count.
Expand Down
164 changes: 131 additions & 33 deletions llvm/test/Analysis/ScalarEvolution/implied-via-division.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -2,12 +2,10 @@
; RUN: opt < %s -disable-output -passes="print<scalar-evolution>" \
; RUN: -scalar-evolution-classify-expressions=0 2>&1 | FileCheck %s

declare void @llvm.experimental.guard(i1, ...)

define void @test_1(i32 %n) nounwind {
; Prove that (n > 1) ===> (n / 2 > 0).
; CHECK-LABEL: 'test_1'
; CHECK-NEXT: Determining loop execution counts for: @test_1
define void @implied1(i32 %n) {
; Prove that (n s> 1) ===> (n / 2 s> 0).
; CHECK-LABEL: 'implied1'
; CHECK-NEXT: Determining loop execution counts for: @implied1
; CHECK-NEXT: Loop %header: backedge-taken count is (-1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741822
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (-1 + %n.div.2)<nsw>
Expand All @@ -29,10 +27,35 @@ exit:
ret void
}

define void @test_1neg(i32 %n) nounwind {
; Prove that (n > 0) =\=> (n / 2 > 0).
; CHECK-LABEL: 'test_1neg'
; CHECK-NEXT: Determining loop execution counts for: @test_1neg
define void @implied1_samesign(i32 %n) {
; Prove that (n > 1) ===> (n / 2 s> 0).
; CHECK-LABEL: 'implied1_samesign'
; CHECK-NEXT: Determining loop execution counts for: @implied1_samesign
; CHECK-NEXT: Loop %header: backedge-taken count is (-1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741822
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (-1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: Trip multiple is 1
;
entry:
%cmp1 = icmp samesign ugt i32 %n, 1
%n.div.2 = sdiv i32 %n, 2
call void @llvm.assume(i1 %cmp1)
br label %header

header:
%indvar = phi i32 [ %indvar.next, %header ], [ 0, %entry ]
%indvar.next = add i32 %indvar, 1
%exitcond = icmp sgt i32 %n.div.2, %indvar.next
br i1 %exitcond, label %header, label %exit

exit:
ret void
}

define void @implied1_neg(i32 %n) {
; Prove that (n s> 0) =\=> (n / 2 s> 0).
; CHECK-LABEL: 'implied1_neg'
; CHECK-NEXT: Determining loop execution counts for: @implied1_neg
; CHECK-NEXT: Loop %header: backedge-taken count is (-1 + (1 smax %n.div.2))<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741822
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (-1 + (1 smax %n.div.2))<nsw>
Expand All @@ -54,10 +77,10 @@ exit:
ret void
}

define void @test_2(i32 %n) nounwind {
; Prove that (n >= 2) ===> (n / 2 > 0).
; CHECK-LABEL: 'test_2'
; CHECK-NEXT: Determining loop execution counts for: @test_2
define void @implied2(i32 %n) {
; Prove that (n s>= 2) ===> (n / 2 s> 0).
; CHECK-LABEL: 'implied2'
; CHECK-NEXT: Determining loop execution counts for: @implied2
; CHECK-NEXT: Loop %header: backedge-taken count is (-1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741822
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (-1 + %n.div.2)<nsw>
Expand All @@ -79,10 +102,35 @@ exit:
ret void
}

define void @test_2neg(i32 %n) nounwind {
; Prove that (n >= 1) =\=> (n / 2 > 0).
; CHECK-LABEL: 'test_2neg'
; CHECK-NEXT: Determining loop execution counts for: @test_2neg
define void @implied2_samesign(i32 %n) {
; Prove that (n >= 2) ===> (n / 2 s> 0).
; CHECK-LABEL: 'implied2_samesign'
; CHECK-NEXT: Determining loop execution counts for: @implied2_samesign
; CHECK-NEXT: Loop %header: backedge-taken count is (-1 + (1 smax %n.div.2))<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741822
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (-1 + (1 smax %n.div.2))<nsw>
; CHECK-NEXT: Loop %header: Trip multiple is 1
;
entry:
%cmp1 = icmp samesign uge i32 %n, 2
%n.div.2 = sdiv i32 %n, 2
call void @llvm.assume(i1 %cmp1)
br label %header

header:
%indvar = phi i32 [ %indvar.next, %header ], [ 0, %entry ]
%indvar.next = add i32 %indvar, 1
%exitcond = icmp sgt i32 %n.div.2, %indvar.next
br i1 %exitcond, label %header, label %exit

exit:
ret void
}

define void @implied2_neg(i32 %n) {
; Prove that (n s>= 1) =\=> (n / 2 s> 0).
; CHECK-LABEL: 'implied2_neg'
; CHECK-NEXT: Determining loop execution counts for: @implied2_neg
; CHECK-NEXT: Loop %header: backedge-taken count is (-1 + (1 smax %n.div.2))<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741822
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (-1 + (1 smax %n.div.2))<nsw>
Expand All @@ -104,10 +152,10 @@ exit:
ret void
}

define void @test_3(i32 %n) nounwind {
; Prove that (n > -2) ===> (n / 2 >= 0).
; CHECK-LABEL: 'test_3'
; CHECK-NEXT: Determining loop execution counts for: @test_3
define void @implied3(i32 %n) {
; Prove that (n s> -2) ===> (n / 2 s>= 0).
; CHECK-LABEL: 'implied3'
; CHECK-NEXT: Determining loop execution counts for: @implied3
; CHECK-NEXT: Loop %header: backedge-taken count is (1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741824
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (1 + %n.div.2)<nsw>
Expand All @@ -129,10 +177,35 @@ exit:
ret void
}

define void @test_3neg(i32 %n) nounwind {
define void @implied3_samesign(i32 %n) {
; Prove that (n > -2) ===> (n / 2 s>= 0).
; CHECK-LABEL: 'implied3_samesign'
; CHECK-NEXT: Determining loop execution counts for: @implied3_samesign
; CHECK-NEXT: Loop %header: backedge-taken count is (1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: Trip multiple is 1
;
entry:
%cmp1 = icmp samesign ugt i32 %n, -2
%n.div.2 = sdiv i32 %n, 2
call void @llvm.assume(i1 %cmp1)
br label %header

header:
%indvar = phi i32 [ %indvar.next, %header ], [ 0, %entry ]
%indvar.next = add i32 %indvar, 1
%exitcond = icmp sge i32 %n.div.2, %indvar
br i1 %exitcond, label %header, label %exit

exit:
ret void
}

define void @implied3_neg(i32 %n) {
; Prove that (n > -3) =\=> (n / 2 >= 0).
; CHECK-LABEL: 'test_3neg'
; CHECK-NEXT: Determining loop execution counts for: @test_3neg
; CHECK-LABEL: 'implied3_neg'
; CHECK-NEXT: Determining loop execution counts for: @implied3_neg
; CHECK-NEXT: Loop %header: backedge-taken count is (0 smax (1 + %n.div.2)<nsw>)
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741824
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (0 smax (1 + %n.div.2)<nsw>)
Expand All @@ -154,10 +227,10 @@ exit:
ret void
}

define void @test_4(i32 %n) nounwind {
; Prove that (n >= -1) ===> (n / 2 >= 0).
; CHECK-LABEL: 'test_4'
; CHECK-NEXT: Determining loop execution counts for: @test_4
define void @implied4(i32 %n) {
; Prove that (n s>= -1) ===> (n / 2 s>= 0).
; CHECK-LABEL: 'implied4'
; CHECK-NEXT: Determining loop execution counts for: @implied4
; CHECK-NEXT: Loop %header: backedge-taken count is (1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741824
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (1 + %n.div.2)<nsw>
Expand All @@ -179,10 +252,35 @@ exit:
ret void
}

define void @test_4neg(i32 %n) nounwind {
; Prove that (n >= -2) =\=> (n / 2 >= 0).
; CHECK-LABEL: 'test_4neg'
; CHECK-NEXT: Determining loop execution counts for: @test_4neg
define void @implied4_samesign(i32 %n) {
; Prove that (n >= -1) ===> (n / 2 s>= 0).
; CHECK-LABEL: 'implied4_samesign'
; CHECK-NEXT: Determining loop execution counts for: @implied4_samesign
; CHECK-NEXT: Loop %header: backedge-taken count is (1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (1 + %n.div.2)<nsw>
; CHECK-NEXT: Loop %header: Trip multiple is 1
;
entry:
%cmp1 = icmp samesign uge i32 %n, -1
%n.div.2 = sdiv i32 %n, 2
call void @llvm.assume(i1 %cmp1)
br label %header

header:
%indvar = phi i32 [ %indvar.next, %header ], [ 0, %entry ]
%indvar.next = add i32 %indvar, 1
%exitcond = icmp sge i32 %n.div.2, %indvar
br i1 %exitcond, label %header, label %exit

exit:
ret void
}

define void @implied4_neg(i32 %n) {
; Prove that (n s>= -2) =\=> (n / 2 s>= 0).
; CHECK-LABEL: 'implied4_neg'
; CHECK-NEXT: Determining loop execution counts for: @implied4_neg
; CHECK-NEXT: Loop %header: backedge-taken count is (0 smax (1 + %n.div.2)<nsw>)
; CHECK-NEXT: Loop %header: constant max backedge-taken count is i32 1073741824
; CHECK-NEXT: Loop %header: symbolic max backedge-taken count is (0 smax (1 + %n.div.2)<nsw>)
Expand Down
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