[IRCE] Fix corner case with Start = INT_MAX
When iterating through loop
for (int i = INT_MAX; i > 0; i--)
We fail to generate the pre-loop for it. It happens because we use the
overflown value in a comparison predicate when identifying whether or not
we need it.
In old logic, we used SLE predicate against Greatest value which exceeds all
seen values of the IV and might be overflown. Now we use the GreatestSeen
value of this IV with SLT predicate.
Also added a test that ensures that a pre-loop is generated for such loops.
Differential Revision: https://reviews.llvm.org/D35347
llvm-svn: 308001
diff --git a/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp b/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
index a40c22c..36d5010 100644
--- a/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
@@ -922,14 +922,18 @@
bool Increasing = MainLoopStructure.IndVarIncreasing;
- // We compute `Smallest` and `Greatest` such that [Smallest, Greatest) is the
- // range of values the induction variable takes.
+ // We compute `Smallest` and `Greatest` such that [Smallest, Greatest), or
+ // [Smallest, GreatestSeen] is the range of values the induction variable
+ // takes.
- const SCEV *Smallest = nullptr, *Greatest = nullptr;
+ const SCEV *Smallest = nullptr, *Greatest = nullptr, *GreatestSeen = nullptr;
+ const SCEV *One = SE.getOne(Ty);
if (Increasing) {
Smallest = Start;
Greatest = End;
+ // No overflow, because the range [Smallest, GreatestSeen] is not empty.
+ GreatestSeen = SE.getMinusSCEV(End, One);
} else {
// These two computations may sign-overflow. Here is why that is okay:
//
@@ -947,9 +951,9 @@
// will be an empty range. Returning an empty range is always safe.
//
- const SCEV *One = SE.getOne(Ty);
Smallest = SE.getAddExpr(End, One);
Greatest = SE.getAddExpr(Start, One);
+ GreatestSeen = Start;
}
auto Clamp = [this, Smallest, Greatest](const SCEV *S) {
@@ -964,7 +968,7 @@
Result.LowLimit = Clamp(Range.getBegin());
bool ProvablyNoPostLoop =
- SE.isKnownPredicate(ICmpInst::ICMP_SLE, Greatest, Range.getEnd());
+ SE.isKnownPredicate(ICmpInst::ICMP_SLT, GreatestSeen, Range.getEnd());
if (!ProvablyNoPostLoop)
Result.HighLimit = Clamp(Range.getEnd());