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[SCEV] Rewrite more SCEVAddExpr when applying guards. #159942

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[SCEV] Rewrite more SCEVAddExpr when applying guards. #159942

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c424404
[SCEV] Rewrite more SCEVAddExpr when applying guards.
fhahn Sep 19, 2025
8dad351
!fixup getMinsSCEV -> getAddExpr + getMinusOne.
fhahn Oct 9, 2025
277d277
!fixup undo test changes, remove stray period.
fhahn Oct 11, 2025
3a98db6
!fixup update after rebase
fhahn Nov 2, 2025
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37 changes: 27 additions & 10 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Expand Up @@ -16006,6 +16006,9 @@ const SCEV *ScalarEvolution::LoopGuards::rewrite(const SCEV *Expr) const {
}

const SCEV *visitAddExpr(const SCEVAddExpr *Expr) {
if (const SCEV *S = Map.lookup(Expr))
return S;

// Helper to check if S is a subtraction (A - B) where A != B, and if so,
// return UMax(S, 1).
auto RewriteSubtraction = [&](const SCEV *S) -> const SCEV * {
Expand All @@ -16030,17 +16033,31 @@ const SCEV *ScalarEvolution::LoopGuards::rewrite(const SCEV *Expr) const {
// (Const + A + B). There may be guard info for A + B, and if so, apply
// it.
// TODO: Could more generally apply guards to Add sub-expressions.
if (isa<SCEVConstant>(Expr->getOperand(0)) &&
Expr->getNumOperands() == 3) {
const SCEV *Add =
SE.getAddExpr(Expr->getOperand(1), Expr->getOperand(2));
if (const SCEV *Rewritten = RewriteSubtraction(Add))
return SE.getAddExpr(
Expr->getOperand(0), Rewritten,
ScalarEvolution::maskFlags(Expr->getNoWrapFlags(), FlagMask));
if (const SCEV *S = Map.lookup(Add))
return SE.getAddExpr(Expr->getOperand(0), S);
if (isa<SCEVConstant>(Expr->getOperand(0))) {
if (Expr->getNumOperands() == 3) {
const SCEV *Add =
SE.getAddExpr(Expr->getOperand(1), Expr->getOperand(2));
if (const SCEV *Rewritten = RewriteSubtraction(Add))
return SE.getAddExpr(
Expr->getOperand(0), Rewritten,
ScalarEvolution::maskFlags(Expr->getNoWrapFlags(), FlagMask));
if (const SCEV *S = Map.lookup(Add))
return SE.getAddExpr(Expr->getOperand(0), S);
}

// For expressions of the form (Const + A), check if we have guard info
// for (Const + 1 + A), and rewrite to ((Const + 1 + A) - 1). This makes
// sure we don't lose information when rewriting expressions based on
// back-edge taken counts in some cases.
if (Expr->getNumOperands() == 2) {
auto *NewC =
SE.getAddExpr(Expr->getOperand(0), SE.getOne(Expr->getType()));
if (const SCEV *S =
Map.lookup(SE.getAddExpr(NewC, Expr->getOperand(1))))
return SE.getAddExpr(S, SE.getMinusOne(Expr->getType()));
}
}

SmallVector<const SCEV *, 2> Operands;
bool Changed = false;
for (const auto *Op : Expr->operands()) {
Expand Down
6 changes: 3 additions & 3 deletions llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-apply-to-adds.ll
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Expand Up @@ -33,9 +33,9 @@ declare void @clobber()
define void @test_add_sub_1_guard(ptr %src, i32 %n) {
; CHECK-LABEL: 'test_add_sub_1_guard'
; CHECK-NEXT: Determining loop execution counts for: @test_add_sub_1_guard
; CHECK-NEXT: Loop %loop: backedge-taken count is (zext i32 (-1 + (%n /u 2))<nsw> to i64)
; CHECK-NEXT: Loop %loop: constant max backedge-taken count is i64 4294967295
; CHECK-NEXT: Loop %loop: symbolic max backedge-taken count is (zext i32 (-1 + (%n /u 2))<nsw> to i64)
; CHECK-NEXT: Loop %loop: backedge-taken count is i64 0
; CHECK-NEXT: Loop %loop: constant max backedge-taken count is i64 0
; CHECK-NEXT: Loop %loop: symbolic max backedge-taken count is i64 0
; CHECK-NEXT: Loop %loop: Trip multiple is 1
;
entry:
Expand Down
12 changes: 6 additions & 6 deletions llvm/test/Analysis/ScalarEvolution/trip-count-minmax.ll
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Expand Up @@ -102,12 +102,12 @@ define void @umax(i32 noundef %a, i32 noundef %b) {
; CHECK-NEXT: %cond = select i1 %cmp, i32 %mul, i32 %mul1
; CHECK-NEXT: --> ((2 * %a) umax (4 * %b)) U: [0,-1) S: [-2147483648,2147483647)
; CHECK-NEXT: %i.011 = phi i32 [ %inc, %for.body ], [ 0, %entry ]
; CHECK-NEXT: --> {0,+,1}<nuw><nsw><%for.body> U: [0,-2147483648) S: [0,-2147483648) Exits: (-1 + ((2 * %a) umax (4 * %b))) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: --> {0,+,1}<nuw><nsw><%for.body> U: [0,2147483647) S: [0,2147483647) Exits: (-1 + ((2 * %a) umax (4 * %b))) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: %inc = add nuw nsw i32 %i.011, 1
; CHECK-NEXT: --> {1,+,1}<nuw><%for.body> U: [1,-1) S: [1,-1) Exits: ((2 * %a) umax (4 * %b)) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: --> {1,+,1}<nuw><nsw><%for.body> U: [1,-2147483648) S: [1,-2147483648) Exits: ((2 * %a) umax (4 * %b)) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: Determining loop execution counts for: @umax
; CHECK-NEXT: Loop %for.body: backedge-taken count is (-1 + ((2 * %a) umax (4 * %b)))
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -3
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 2147483646
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (-1 + ((2 * %a) umax (4 * %b)))
; CHECK-NEXT: Loop %for.body: Trip multiple is 2
;
Expand Down Expand Up @@ -197,12 +197,12 @@ define void @smax(i32 noundef %a, i32 noundef %b) {
; CHECK-NEXT: %cond = select i1 %cmp, i32 %mul, i32 %mul1
; CHECK-NEXT: --> ((2 * %a)<nsw> smax (4 * %b)<nsw>) U: [0,-1) S: [-2147483648,2147483647)
; CHECK-NEXT: %i.011 = phi i32 [ %inc, %for.body ], [ 0, %entry ]
; CHECK-NEXT: --> {0,+,1}<nuw><nsw><%for.body> U: [0,-2147483648) S: [0,-2147483648) Exits: (-1 + ((2 * %a)<nsw> smax (4 * %b)<nsw>)) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: --> {0,+,1}<nuw><nsw><%for.body> U: [0,2147483647) S: [0,2147483647) Exits: (-1 + ((2 * %a)<nsw> smax (4 * %b)<nsw>)) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: %inc = add nuw nsw i32 %i.011, 1
; CHECK-NEXT: --> {1,+,1}<nuw><%for.body> U: [1,-1) S: [1,-1) Exits: ((2 * %a)<nsw> smax (4 * %b)<nsw>) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: --> {1,+,1}<nuw><nsw><%for.body> U: [1,-2147483648) S: [1,-2147483648) Exits: ((2 * %a)<nsw> smax (4 * %b)<nsw>) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: Determining loop execution counts for: @smax
; CHECK-NEXT: Loop %for.body: backedge-taken count is (-1 + ((2 * %a)<nsw> smax (4 * %b)<nsw>))
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -3
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 2147483646
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (-1 + ((2 * %a)<nsw> smax (4 * %b)<nsw>))
; CHECK-NEXT: Loop %for.body: Trip multiple is 2
;
Expand Down
6 changes: 2 additions & 4 deletions llvm/test/Transforms/IndVarSimplify/canonicalize-cmp.ll
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Expand Up @@ -343,14 +343,13 @@ define void @slt_no_smax_needed(i64 %n, ptr %dst) {
; CHECK-NEXT: [[PRE:%.*]] = icmp ult i32 [[ADD_1]], 8
; CHECK-NEXT: br i1 [[PRE]], label [[EXIT:%.*]], label [[LOOP_PREHEADER:%.*]]
; CHECK: loop.preheader:
; CHECK-NEXT: [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[SHR]], i32 1)
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i32 [[IV]]
; CHECK-NEXT: store i8 0, ptr [[GEP]], align 1
; CHECK-NEXT: [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[SMAX]]
; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[SHR]]
; CHECK-NEXT: br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
; CHECK: exit.loopexit:
; CHECK-NEXT: br label [[EXIT]]
Expand Down Expand Up @@ -385,14 +384,13 @@ define void @ult_no_umax_needed(i64 %n, ptr %dst) {
; CHECK-NEXT: [[PRE:%.*]] = icmp ult i32 [[ADD_1]], 8
; CHECK-NEXT: br i1 [[PRE]], label [[EXIT:%.*]], label [[LOOP_PREHEADER:%.*]]
; CHECK: loop.preheader:
; CHECK-NEXT: [[UMAX:%.*]] = call i32 @llvm.umax.i32(i32 [[SHR]], i32 1)
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i32 [[IV]]
; CHECK-NEXT: store i8 0, ptr [[GEP]], align 1
; CHECK-NEXT: [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[UMAX]]
; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[SHR]]
; CHECK-NEXT: br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
; CHECK: exit.loopexit:
; CHECK-NEXT: br label [[EXIT]]
Expand Down
6 changes: 3 additions & 3 deletions llvm/test/Transforms/LoopIdiom/add-nsw-zext-fold.ll
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Expand Up @@ -61,9 +61,9 @@ define void @test_memset_size_can_use_info_from_guards(i32 %x, ptr %dst) {
; CHECK: [[LOOP1_BACKEDGE]]:
; CHECK-NEXT: br label %[[LOOP1]]
; CHECK: [[LOOP2_PREHEADER]]:
; CHECK-NEXT: [[TMP0:%.*]] = zext i32 [[SUB]] to i64
; CHECK-NEXT: [[TMP1:%.*]] = lshr i64 [[TMP0]], 1
; CHECK-NEXT: [[UMAX:%.*]] = call i64 @llvm.umax.i64(i64 [[TMP1]], i64 1)
; CHECK-NEXT: [[TMP0:%.*]] = add nsw i32 [[SHR]], -1
; CHECK-NEXT: [[TMP1:%.*]] = zext i32 [[TMP0]] to i64
; CHECK-NEXT: [[UMAX:%.*]] = add nuw nsw i64 [[TMP1]], 1
Comment on lines +64 to +66
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@fhahn fhahn Sep 22, 2025

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This is missing a fold end-to-end from https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2845/files/ed058c0c689f0dadf267033c74e0960c3593baff#r2367574398

https://alive2.llvm.org/ce/z/BuHtNm

; CHECK-NEXT: call void @llvm.memset.p0.i64(ptr align 1 [[DST]], i8 0, i64 [[UMAX]], i1 false)
; CHECK-NEXT: br label %[[LOOP2:.*]]
; CHECK: [[LOOP2]]:
Expand Down
2 changes: 1 addition & 1 deletion llvm/test/Transforms/LoopVectorize/dont-fold-tail-for-divisible-TC.ll
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Expand Up @@ -193,7 +193,7 @@ define dso_local void @cannotProveAlignedTC(ptr noalias nocapture %A, i32 %p, i3
; CHECK-NEXT: store i32 13, ptr [[TMP12]], align 1
; CHECK-NEXT: br label [[PRED_STORE_CONTINUE6]]
; CHECK: pred.store.continue6:
; CHECK-NEXT: [[INDEX_NEXT]] = add i32 [[INDEX]], 4
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 4
; CHECK-NEXT: [[VEC_IND_NEXT]] = add <4 x i32> [[VEC_IND]], splat (i32 4)
; CHECK-NEXT: [[TMP13:%.*]] = icmp eq i32 [[INDEX_NEXT]], [[N_VEC]]
; CHECK-NEXT: br i1 [[TMP13]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
Expand Down
7 changes: 1 addition & 6 deletions llvm/test/Transforms/LoopVectorize/runtime-checks-difference.ll
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Expand Up @@ -465,12 +465,7 @@ define void @remove_diff_checks_via_guards(i32 %x, i32 %y, ptr %A) {
; CHECK-NEXT: [[TMP13:%.*]] = icmp ugt i64 [[SMAX]], 4294967295
; CHECK-NEXT: [[TMP14:%.*]] = or i1 [[TMP12]], [[TMP13]]
; CHECK-NEXT: [[TMP15:%.*]] = or i1 [[TMP9]], [[TMP14]]
; CHECK-NEXT: br i1 [[TMP15]], [[SCALAR_PH]], label %[[VECTOR_MEMCHECK:.*]]
; CHECK: [[VECTOR_MEMCHECK]]:
; CHECK-NEXT: [[TMP16:%.*]] = sext i32 [[OFFSET]] to i64
; CHECK-NEXT: [[TMP17:%.*]] = shl nsw i64 [[TMP16]], 2
; CHECK-NEXT: [[DIFF_CHECK:%.*]] = icmp ult i64 [[TMP17]], 16
; CHECK-NEXT: br i1 [[DIFF_CHECK]], [[SCALAR_PH]], [[VECTOR_PH1:label %.*]]
; CHECK-NEXT: br i1 [[TMP15]], [[SCALAR_PH]], [[VECTOR_PH:label %.*]]
;
entry:
%offset = sub i32 %x, %y
Expand Down