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[SCEV] Use getConstantMultiple in to get divisibility info from guards. #162617

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Merged
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Oct 9, 2025
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[SCEV] Use getConstantMultiple in to get divisibility info from guards. #162617

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46 changes: 3 additions & 43 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15749,51 +15749,11 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
return RewriteMap.lookup_or(S, S);
};

// Check for the SCEV expression (A /u B) * B while B is a constant, inside
// \p Expr. The check is done recuresively on \p Expr, which is assumed to
// be a composition of Min/Max SCEVs. Return whether the SCEV expression (A
// /u B) * B was found, and return the divisor B in \p DividesBy. For
// example, if Expr = umin (umax ((A /u 8) * 8, 16), 64), return true since
// (A /u 8) * 8 matched the pattern, and return the constant SCEV 8 in \p
// DividesBy.
std::function<bool(const SCEV *, const SCEV *&)> HasDivisibiltyInfo =
[&](const SCEV *Expr, const SCEV *&DividesBy) {
if (auto *Mul = dyn_cast<SCEVMulExpr>(Expr)) {
if (Mul->getNumOperands() != 2)
return false;
auto *MulLHS = Mul->getOperand(0);
auto *MulRHS = Mul->getOperand(1);
if (isa<SCEVConstant>(MulLHS))
std::swap(MulLHS, MulRHS);
if (auto *Div = dyn_cast<SCEVUDivExpr>(MulLHS))
if (Div->getOperand(1) == MulRHS) {
DividesBy = MulRHS;
return true;
}
}
if (auto *MinMax = dyn_cast<SCEVMinMaxExpr>(Expr))
return HasDivisibiltyInfo(MinMax->getOperand(0), DividesBy) ||
HasDivisibiltyInfo(MinMax->getOperand(1), DividesBy);
return false;
};

// Return true if Expr known to divide by \p DividesBy.
std::function<bool(const SCEV *, const SCEV *&)> IsKnownToDivideBy =
[&](const SCEV *Expr, const SCEV *DividesBy) {
if (SE.getURemExpr(Expr, DividesBy)->isZero())
return true;
if (auto *MinMax = dyn_cast<SCEVMinMaxExpr>(Expr))
return IsKnownToDivideBy(MinMax->getOperand(0), DividesBy) &&
IsKnownToDivideBy(MinMax->getOperand(1), DividesBy);
return false;
};

const SCEV *RewrittenLHS = GetMaybeRewritten(LHS);
const SCEV *DividesBy = nullptr;
if (HasDivisibiltyInfo(RewrittenLHS, DividesBy))
// Check that the whole expression is divided by DividesBy
DividesBy =
IsKnownToDivideBy(RewrittenLHS, DividesBy) ? DividesBy : nullptr;
const APInt &Multiple = SE.getConstantMultiple(RewrittenLHS);
if (!Multiple.isOne())
DividesBy = SE.getConstant(Multiple);
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As a possible followup, it would probably make sense to eliminate the DividesBy null case to avoid the DividesBy ? checks below. The existing code should already handle the 1 case. It doesn't look like the conversion from APInt back to SCEV is really needed either.


// Collect rewrites for LHS and its transitive operands based on the
// condition.
Expand Down
4 changes: 2 additions & 2 deletions llvm/test/Analysis/ScalarEvolution/trip-count-minmax.ll
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Expand Up @@ -61,7 +61,7 @@ define void @umin(i32 noundef %a, i32 noundef %b) {
; CHECK-NEXT: Loop %for.body: backedge-taken count is (-1 + ((2 * %a) umin (4 * %b)))
; 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) umin (4 * %b)))
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
; CHECK-NEXT: Loop %for.body: Trip multiple is 2
;
; void umin(unsigned a, unsigned b) {
; a *= 2;
Expand Down Expand Up @@ -157,7 +157,7 @@ define void @smin(i32 noundef %a, i32 noundef %b) {
; CHECK-NEXT: Loop %for.body: backedge-taken count is (-1 + ((2 * %a)<nsw> smin (4 * %b)<nsw>))
; 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> smin (4 * %b)<nsw>))
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
; CHECK-NEXT: Loop %for.body: Trip multiple is 2
;
; void smin(signed a, signed b) {
; a *= 2;
Expand Down
47 changes: 40 additions & 7 deletions llvm/test/Transforms/LoopVectorize/single_early_exit.ll
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Expand Up @@ -546,19 +546,50 @@ define i64 @loop_guards_needed_to_prove_deref_multiple(i32 %x, i1 %c, ptr derefe
; CHECK-NEXT: call void @llvm.assume(i1 [[PRE_2]])
; CHECK-NEXT: [[N:%.*]] = add i32 [[SEL]], -1
; CHECK-NEXT: [[N_EXT:%.*]] = zext i32 [[N]] to i64
; CHECK-NEXT: [[TMP0:%.*]] = add i32 [[SEL]], -2
; CHECK-NEXT: [[TMP1:%.*]] = zext i32 [[TMP0]] to i64
; CHECK-NEXT: [[TMP2:%.*]] = add nuw nsw i64 [[TMP1]], 2
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP2]], 4
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK: vector.ph:
; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[TMP2]], 4
; CHECK-NEXT: [[IV_NEXT:%.*]] = sub i64 [[TMP2]], [[N_MOD_VF]]
; CHECK-NEXT: br label [[LOOP_HEADER:%.*]]
; CHECK: vector.body:
; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[LOOP_HEADER]] ]
; CHECK-NEXT: [[TMP3:%.*]] = getelementptr i8, ptr [[SRC]], i64 [[INDEX]]
; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <4 x i8>, ptr [[TMP3]], align 1
; CHECK-NEXT: [[TMP4:%.*]] = icmp eq <4 x i8> [[WIDE_LOAD]], zeroinitializer
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
; CHECK-NEXT: [[TMP5:%.*]] = freeze <4 x i1> [[TMP4]]
; CHECK-NEXT: [[TMP6:%.*]] = call i1 @llvm.vector.reduce.or.v4i1(<4 x i1> [[TMP5]])
; CHECK-NEXT: [[TMP7:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[IV_NEXT]]
; CHECK-NEXT: [[TMP8:%.*]] = or i1 [[TMP6]], [[TMP7]]
; CHECK-NEXT: br i1 [[TMP8]], label [[MIDDLE_SPLIT:%.*]], label [[LOOP_HEADER]], !llvm.loop [[LOOP11:![0-9]+]]
; CHECK: middle.split:
; CHECK-NEXT: br i1 [[TMP6]], label [[VECTOR_EARLY_EXIT:%.*]], label [[LOOP_LATCH:%.*]]
; CHECK: middle.block:
; CHECK-NEXT: [[CMP_N:%.*]] = icmp eq i64 [[TMP2]], [[IV_NEXT]]
; CHECK-NEXT: br i1 [[CMP_N]], label [[EXIT_LOOPEXIT:%.*]], label [[SCALAR_PH]]
; CHECK: vector.early.exit:
; CHECK-NEXT: [[TMP9:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.v4i1(<4 x i1> [[TMP4]], i1 true)
; CHECK-NEXT: [[TMP10:%.*]] = add i64 [[INDEX]], [[TMP9]]
; CHECK-NEXT: br label [[EXIT_LOOPEXIT]]
; CHECK: scalar.ph:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[IV_NEXT]], [[LOOP_LATCH]] ], [ 0, [[PH]] ]
; CHECK-NEXT: br label [[LOOP_HEADER1:%.*]]
; CHECK: loop.header:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ], [ 0, [[PH]] ]
; CHECK-NEXT: [[GEP_SRC_I:%.*]] = getelementptr i8, ptr [[SRC]], i64 [[IV]]
; CHECK-NEXT: [[IV1:%.*]] = phi i64 [ [[IV_NEXT1:%.*]], [[LOOP_LATCH1:%.*]] ], [ [[IV]], [[SCALAR_PH]] ]
; CHECK-NEXT: [[GEP_SRC_I:%.*]] = getelementptr i8, ptr [[SRC]], i64 [[IV1]]
; CHECK-NEXT: [[L:%.*]] = load i8, ptr [[GEP_SRC_I]], align 1
; CHECK-NEXT: [[C_1:%.*]] = icmp eq i8 [[L]], 0
; CHECK-NEXT: br i1 [[C_1]], label [[EXIT_LOOPEXIT:%.*]], label [[LOOP_LATCH]]
; CHECK-NEXT: br i1 [[C_1]], label [[EXIT_LOOPEXIT]], label [[LOOP_LATCH1]]
; CHECK: loop.latch:
; CHECK-NEXT: [[IV_NEXT]] = add i64 [[IV]], 1
; CHECK-NEXT: [[EC:%.*]] = icmp eq i64 [[IV]], [[N_EXT]]
; CHECK-NEXT: br i1 [[EC]], label [[EXIT_LOOPEXIT]], label [[LOOP_HEADER]]
; CHECK-NEXT: [[IV_NEXT1]] = add i64 [[IV1]], 1
; CHECK-NEXT: [[EC:%.*]] = icmp eq i64 [[IV1]], [[N_EXT]]
; CHECK-NEXT: br i1 [[EC]], label [[EXIT_LOOPEXIT]], label [[LOOP_HEADER1]], !llvm.loop [[LOOP12:![0-9]+]]
; CHECK: exit.loopexit:
; CHECK-NEXT: [[RES_PH:%.*]] = phi i64 [ [[IV]], [[LOOP_HEADER]] ], [ 0, [[LOOP_LATCH]] ]
; CHECK-NEXT: [[RES_PH:%.*]] = phi i64 [ [[IV1]], [[LOOP_HEADER1]] ], [ 0, [[LOOP_LATCH1]] ], [ 0, [[LOOP_LATCH]] ], [ [[TMP10]], [[VECTOR_EARLY_EXIT]] ]
; CHECK-NEXT: br label [[EXIT]]
; CHECK: exit:
; CHECK-NEXT: [[RES:%.*]] = phi i64 [ -1, [[ENTRY:%.*]] ], [ -2, [[THEN]] ], [ [[RES_PH]], [[EXIT_LOOPEXIT]] ]
Expand Down Expand Up @@ -609,4 +640,6 @@ exit:
; CHECK: [[LOOP8]] = distinct !{[[LOOP8]], [[META2]], [[META1]]}
; CHECK: [[LOOP9]] = distinct !{[[LOOP9]], [[META1]], [[META2]]}
; CHECK: [[LOOP10]] = distinct !{[[LOOP10]], [[META2]], [[META1]]}
; CHECK: [[LOOP11]] = distinct !{[[LOOP11]], [[META1]], [[META2]]}
; CHECK: [[LOOP12]] = distinct !{[[LOOP12]], [[META2]], [[META1]]}
;.