[ValueTracking] usub(a, b) cannot overflow if a >= b.

If we know that a >= b (unsigned), usub.with.overflow(a, b) cannot
overflow. Similarly, if b > a, the same expression overflows.

Reviewers: nikic, RKSimon, lebedev.ri, spatel

Reviewed By: nikic, Gerolf

Differential Revision: https://reviews.llvm.org/D74066

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 70d2bb6..3d3c912 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4439,6 +4439,16 @@ OverflowResult llvm::computeOverflowForUnsignedSub(const Value *LHS,
                                                    AssumptionCache *AC,
                                                    const Instruction *CxtI,
                                                    const DominatorTree *DT) {
+  // Checking for conditions implied by dominating conditions may be expensive.
+  // Limit it to usub_with_overflow calls for now.
+  if (match(CxtI,
+            m_Intrinsic<Intrinsic::usub_with_overflow>(m_Value(), m_Value())))
+    if (auto C =
+            isImpliedByDomCondition(CmpInst::ICMP_UGE, LHS, RHS, CxtI, DL)) {
+      if (*C)
+        return OverflowResult::NeverOverflows;
+      return OverflowResult::AlwaysOverflowsLow;
+    }
   ConstantRange LHSRange = computeConstantRangeIncludingKnownBits(
       LHS, /*ForSigned=*/false, DL, /*Depth=*/0, AC, CxtI, DT);
   ConstantRange RHSRange = computeConstantRangeIncludingKnownBits(