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[IR] Use iteration limit in stripPointerCastsAndOffsets (#190472)

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Using a SmallPtrSet is not quite free for such a frequently called
operation. However, calls on ill-formed IR are not particularly rare, so
some iteration limit is needed. Therefore, use a simple counter.

Termination statistics on a Clang Release build for N>5:

   2448 N=6
   1295 N=7
    480 N=8
    294 N=9
    160 N=10
  14350 (endless loop)

Therefore, bound the number of iterations by 12, which should cover most
practically relevant cases.

It is worth noting that _all_ of the endless loop cases have the
following form:

  %incdec.ptr = getelementptr inbounds nuw i8, ptr %incdec.ptr, i64 <N>

I haven't investigated where exactly this comes from, though; but it
occurred frequent enough to add a special case for this.
This commit is contained in:
Alexis Engelke 2026-04-04 20:13:21 +02:00 committed by GitHub
parent ba3dbbfff3
commit b5e7dbb30a
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1 changed files with 12 additions and 7 deletions
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19
llvm/lib/IR/Value.cpp
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@ -642,11 +642,12 @@ static const Value *stripPointerCastsAndOffsets(
return V;
// Even though we don't look through PHI nodes, we could be called on an
// instruction in an unreachable block, which may be on a cycle.
SmallPtrSet<const Value *, 4> Visited;
Visited.insert(V);
do {
// instruction in an unreachable block, which may be on a cycle. E.g.:
// %gep = getelementptr inbounds nuw i8, ptr %gep, i64 32
//
// Bound against that case with a simple iteration counter. Practically, more
// than 10 iterations are almost never needed.
for (unsigned N = 0; N < 12; ++N) {
Func(V);
if (auto *GEP = dyn_cast<GEPOperator>(V)) {
switch (StripKind) {
@ -666,7 +667,11 @@ static const Value *stripPointerCastsAndOffsets(
return V;
break;
}
V = GEP->getPointerOperand();
const Value *NewV = GEP->getPointerOperand();
// Quick exit for degenerate IR, which can happen in unreachable blocks.
if (NewV == V)
return V;
V = NewV;
} else if (Operator::getOpcode(V) == Instruction::BitCast) {
Value *NewV = cast<Operator>(V)->getOperand(0);
if (!NewV->getType()->isPointerTy())
@ -701,7 +706,7 @@ static const Value *stripPointerCastsAndOffsets(
return V;
}
assert(V->getType()->isPointerTy() && "Unexpected operand type!");
} while (Visited.insert(V).second);
}
return V;
}