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[SCEV] Handle more adds in computeConstantDifference() #101339

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Merged
merged 1 commit into from
Aug 2, 2024
Merged

[SCEV] Handle more adds in computeConstantDifference() #101339

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51 changes: 25 additions & 26 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11923,8 +11923,9 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
// fairly deep in the call stack (i.e. is called many times).

// X - X = 0.
unsigned BW = getTypeSizeInBits(More->getType());
if (More == Less)
return APInt(getTypeSizeInBits(More->getType()), 0);
return APInt(BW, 0);

if (isa<SCEVAddRecExpr>(Less) && isa<SCEVAddRecExpr>(More)) {
const auto *LAR = cast<SCEVAddRecExpr>(Less);
Expand All @@ -11947,33 +11948,31 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
// fall through
}

if (isa<SCEVConstant>(Less) && isa<SCEVConstant>(More)) {
const auto &M = cast<SCEVConstant>(More)->getAPInt();
const auto &L = cast<SCEVConstant>(Less)->getAPInt();
return M - L;
}

SCEV::NoWrapFlags Flags;
const SCEV *LLess = nullptr, *RLess = nullptr;
const SCEV *LMore = nullptr, *RMore = nullptr;
const SCEVConstant *C1 = nullptr, *C2 = nullptr;
// Compare (X + C1) vs X.
if (splitBinaryAdd(Less, LLess, RLess, Flags))
if ((C1 = dyn_cast<SCEVConstant>(LLess)))
if (RLess == More)
return -(C1->getAPInt());

// Compare X vs (X + C2).
if (splitBinaryAdd(More, LMore, RMore, Flags))
if ((C2 = dyn_cast<SCEVConstant>(LMore)))
if (RMore == Less)
return C2->getAPInt();
// Try to cancel out common factors in two add expressions.
SmallDenseMap<const SCEV *, int, 8> Multiplicity;
APInt Diff(BW, 0);
auto Add = [&](const SCEV *S, int Mul) {
if (auto *C = dyn_cast<SCEVConstant>(S))
Diff += C->getAPInt() * Mul;
else
Multiplicity[S] += Mul;
};
auto Decompose = [&](const SCEV *S, int Mul) {
if (isa<SCEVAddExpr>(S)) {
for (const SCEV *Op : S->operands())
Add(Op, Mul);
} else
Add(S, Mul);
};
Decompose(More, 1);
Decompose(Less, -1);

// Compare (X + C1) vs (X + C2).
if (C1 && C2 && RLess == RMore)
return C2->getAPInt() - C1->getAPInt();
// Check whether all the non-constants cancel out.
for (const auto [_, Mul] : Multiplicity)
if (Mul != 0)
return std::nullopt;

return std::nullopt;
return Diff;
}

bool ScalarEvolution::isImpliedCondOperandsViaAddRecStart(
Expand Down
8 changes: 7 additions & 1 deletion llvm/unittests/Analysis/ScalarEvolutionTest.cpp
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Expand Up @@ -1117,10 +1117,12 @@ TEST_F(ScalarEvolutionsTest, SCEVComputeConstantDifference) {
LLVMContext C;
SMDiagnostic Err;
std::unique_ptr<Module> M = parseAssemblyString(
"define void @foo(i32 %sz, i32 %pp) { "
"define void @foo(i32 %sz, i32 %pp, i32 %x) { "
"entry: "
" %v0 = add i32 %pp, 0 "
" %v3 = add i32 %pp, 3 "
" %vx = add i32 %pp, %x "
" %vx3 = add i32 %vx, 3 "
" br label %loop.body "
"loop.body: "
" %iv = phi i32 [ %iv.next, %loop.body ], [ 0, %entry ] "
Expand All @@ -1141,6 +1143,9 @@ TEST_F(ScalarEvolutionsTest, SCEVComputeConstantDifference) {
runWithSE(*M, "foo", [](Function &F, LoopInfo &LI, ScalarEvolution &SE) {
auto *ScevV0 = SE.getSCEV(getInstructionByName(F, "v0")); // %pp
auto *ScevV3 = SE.getSCEV(getInstructionByName(F, "v3")); // (3 + %pp)
auto *ScevVX = SE.getSCEV(getInstructionByName(F, "vx")); // (%pp + %x)
// (%pp + %x + 3)
auto *ScevVX3 = SE.getSCEV(getInstructionByName(F, "vx3"));
auto *ScevIV = SE.getSCEV(getInstructionByName(F, "iv")); // {0,+,1}
auto *ScevXA = SE.getSCEV(getInstructionByName(F, "xa")); // {%pp,+,1}
auto *ScevYY = SE.getSCEV(getInstructionByName(F, "yy")); // {(3 + %pp),+,1}
Expand All @@ -1162,6 +1167,7 @@ TEST_F(ScalarEvolutionsTest, SCEVComputeConstantDifference) {
EXPECT_EQ(diff(ScevV0, ScevV3), -3);
EXPECT_EQ(diff(ScevV0, ScevV0), 0);
EXPECT_EQ(diff(ScevV3, ScevV3), 0);
EXPECT_EQ(diff(ScevVX3, ScevVX), 3);
EXPECT_EQ(diff(ScevIV, ScevIV), 0);
EXPECT_EQ(diff(ScevXA, ScevXB), 0);
EXPECT_EQ(diff(ScevXA, ScevYY), -3);
Expand Down
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