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[SCEV] Return nullopt from CompareValueComplexity() if depth limit reached #101022

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[SCEV] Return nullopt from CompareValueComplexity() if depth limit reached #101022

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[SCEV] Return nullopt from CompareValueComplexity() if depth limit re…
nikic Jul 29, 2024
f562b26
Update polly test
nikic Jul 30, 2024
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13 changes: 8 additions & 5 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -597,11 +597,14 @@ void SCEVUnknown::allUsesReplacedWith(Value *New) {
///
/// Since we do not continue running this routine on expression trees once we
/// have seen unequal values, there is no need to track them in the cache.
static int
static std::optional<int>
CompareValueComplexity(EquivalenceClasses<const Value *> &EqCacheValue,
const LoopInfo *const LI, Value *LV, Value *RV,
unsigned Depth) {
if (Depth > MaxValueCompareDepth || EqCacheValue.isEquivalent(LV, RV))
if (Depth > MaxValueCompareDepth)
return std::nullopt;

if (EqCacheValue.isEquivalent(LV, RV))
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This fixes the bug with inconsistent results, but did you check if the cache actually does anything useful in the average case? In the (admittedly limited) examples I looked at, it was almost pure overhead (i.e., almost no hits, but lots of writes and checks).

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I do think that we should also drop the cache, see #100721 (comment).

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Oops, missed that. SGTM.

return 0;

// Order pointer values after integer values. This helps SCEVExpander form
Expand Down Expand Up @@ -660,7 +663,7 @@ CompareValueComplexity(EquivalenceClasses<const Value *> &EqCacheValue,
return (int)LNumOps - (int)RNumOps;

for (unsigned Idx : seq(LNumOps)) {
int Result =
std::optional<int> Result =
CompareValueComplexity(EqCacheValue, LI, LInst->getOperand(Idx),
RInst->getOperand(Idx), Depth + 1);
if (Result != 0)
Expand Down Expand Up @@ -705,8 +708,8 @@ CompareSCEVComplexity(EquivalenceClasses<const SCEV *> &EqCacheSCEV,
const SCEVUnknown *LU = cast<SCEVUnknown>(LHS);
const SCEVUnknown *RU = cast<SCEVUnknown>(RHS);

int X = CompareValueComplexity(EqCacheValue, LI, LU->getValue(),
RU->getValue(), Depth + 1);
std::optional<int> X = CompareValueComplexity(
EqCacheValue, LI, LU->getValue(), RU->getValue(), Depth + 1);
if (X == 0)
EqCacheSCEV.unionSets(LHS, RHS);
return X;
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36 changes: 36 additions & 0 deletions llvm/unittests/Analysis/ScalarEvolutionTest.cpp
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Expand Up @@ -1625,4 +1625,40 @@ TEST_F(ScalarEvolutionsTest, ForgetValueWithOverflowInst) {
});
}

TEST_F(ScalarEvolutionsTest, ComplexityComparatorIsStrictWeakOrdering) {
// Regression test for a case where caching of equivalent values caused the
// comparator to get inconsistent.
LLVMContext C;
SMDiagnostic Err;
std::unique_ptr<Module> M = parseAssemblyString(R"(
define i32 @foo(i32 %arg0) {
%1 = add i32 %arg0, 1
%2 = add i32 %arg0, 1
%3 = xor i32 %2, %1
%4 = add i32 %3, %2
%5 = add i32 %arg0, 1
%6 = xor i32 %5, %arg0
%7 = add i32 %arg0, %6
%8 = add i32 %5, %7
%9 = xor i32 %8, %7
%10 = add i32 %9, %8
%11 = xor i32 %10, %9
%12 = add i32 %11, %10
%13 = xor i32 %12, %11
%14 = add i32 %12, %13
%15 = add i32 %14, %4
ret i32 %15
})",
Err, C);

ASSERT_TRUE(M && "Could not parse module?");
ASSERT_TRUE(!verifyModule(*M) && "Must have been well formed!");

runWithSE(*M, "foo", [](Function &F, LoopInfo &LI, ScalarEvolution &SE) {
// When _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG, this will
// crash if the comparator has the specific caching bug.
SE.getSCEV(F.getEntryBlock().getTerminator()->getOperand(0));
});
}

} // end namespace llvm
4 changes: 2 additions & 2 deletions polly/test/ScopInfo/invariant_load_zext_parameter-2.ll
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Expand Up @@ -25,8 +25,8 @@
; CHECK: p0: ((sext i32 %tmp6 to i64) * %p1)
; CHECK: p1: ((sext i32 %tmp3 to i64) * (sext i32 %tmp8 to i64) * (%p0 + %p1) * %p3)
; CHECK: p2: ((sext i32 %tmp3 to i64) * (%p0 + %p1))
; CHECK: p3: ((sext i32 %tmp5 to i64) * %p0)
; CHECK: p4: ((sext i32 %tmp7 to i64) * %p2)
; CHECK: p3: ((sext i32 %tmp7 to i64) * %p2)
; CHECK: p4: ((sext i32 %tmp5 to i64) * %p0)
;
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"

Expand Down
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