[InstSimplify] Drop leftover "division-by-zero guard" around `@llvm.umul.with.overflow` overflow bit
Summary:
Now that with D65143/D65144 we've produce `@llvm.umul.with.overflow`,
and with D65147 we've flattened the CFG, we now can see that
the guard may have been there to prevent division by zero is redundant.
We can simply drop it:
```
----------------------------------------
Name: no overflow and not zero
%iszero = icmp ne i4 %y, 0
%umul = umul_overflow i4 %x, %y
%umul.ov = extractvalue {i4, i1} %umul, 1
%retval.0 = and i1 %iszero, %umul.ov
ret i1 %retval.0
=>
%iszero = icmp ne i4 %y, 0
%umul = umul_overflow i4 %x, %y
%umul.ov = extractvalue {i4, i1} %umul, 1
%retval.0 = and i1 %iszero, %umul.ov
ret %umul.ov
Done: 1
Optimization is correct!
```
Reviewers: nikic, spatel, xbolva00
Reviewed By: spatel
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65150
llvm-svn: 370350
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 85002aa..d320333 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1759,6 +1759,38 @@
return nullptr;
}
+/// The @llvm.[us]mul.with.overflow intrinsic could have been folded from some
+/// other form of check, e.g. one that was using division; it may have been
+/// guarded against division-by-zero. We can drop that check now.
+/// Look for:
+/// %Op0 = icmp ne i4 %X, 0
+/// %Agg = tail call { i4, i1 } @llvm.[us]mul.with.overflow.i4(i4 %X, i4 %???)
+/// %Op1 = extractvalue { i4, i1 } %Agg, 1
+/// %??? = and i1 %Op0, %Op1
+/// We can just return %Op1
+static Value *omitCheckForZeroBeforeMulWithOverflow(Value *Op0, Value *Op1) {
+ ICmpInst::Predicate Pred;
+ Value *X;
+ if (!match(Op0, m_ICmp(Pred, m_Value(X), m_Zero())) ||
+ Pred != ICmpInst::Predicate::ICMP_NE)
+ return nullptr;
+ auto *Extract = dyn_cast<ExtractValueInst>(Op1);
+ // We should only be extracting the overflow bit.
+ if (!Extract || !Extract->getIndices().equals(1))
+ return nullptr;
+ Value *Agg = Extract->getAggregateOperand();
+ // This should be a multiplication-with-overflow intrinsic.
+ if (!match(Agg, m_CombineOr(m_Intrinsic<Intrinsic::umul_with_overflow>(),
+ m_Intrinsic<Intrinsic::smul_with_overflow>())))
+ return nullptr;
+ // One of its multipliers should be the value we checked for zero before.
+ if (!match(Agg, m_CombineOr(m_Argument<0>(m_Specific(X)),
+ m_Argument<1>(m_Specific(X)))))
+ return nullptr;
+ // Can omit 'and', and just return the overflow bit.
+ return Op1;
+}
+
/// Given operands for an And, see if we can fold the result.
/// If not, this returns null.
static Value *SimplifyAndInst(Value *Op0, Value *Op1, const SimplifyQuery &Q,
@@ -1813,6 +1845,14 @@
return Op0;
}
+ // If we have a multiplication overflow check that is being 'and'ed with a
+ // check that one of the multipliers is not zero, we can omit the 'and', and
+ // only keep the overflow check.
+ if (Value *V = omitCheckForZeroBeforeMulWithOverflow(Op0, Op1))
+ return V;
+ if (Value *V = omitCheckForZeroBeforeMulWithOverflow(Op1, Op0))
+ return V;
+
// A & (-A) = A if A is a power of two or zero.
if (match(Op0, m_Neg(m_Specific(Op1))) ||
match(Op1, m_Neg(m_Specific(Op0)))) {