Fix SCEV overly optimistic back edge taken count for multi-exit loops.
Fixes PR11375: Different results for 'clang++ huh.cpp'...
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144746 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index ac00259..77defa8 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -4153,13 +4153,19 @@
}
/// getExact - Get the exact loop backedge taken count considering all loop
-/// exits. If all exits are computable, this is the minimum computed count.
+/// exits. A computable result can only be return for loops with a single exit.
+/// Returning the minimum taken count among all exits is incorrect because one
+/// of the loop's exit limit's may have been skipped. HowFarToZero assumes that
+/// the limit of each loop test is never skipped. This is a valid assumption as
+/// long as the loop exits via that test. For precise results, it is the
+/// caller's responsibility to specify the relevant loop exit using
+/// getExact(ExitingBlock, SE).
const SCEV *
ScalarEvolution::BackedgeTakenInfo::getExact(ScalarEvolution *SE) const {
// If any exits were not computable, the loop is not computable.
if (!ExitNotTaken.isCompleteList()) return SE->getCouldNotCompute();
- // We need at least one computable exit.
+ // We need exactly one computable exit.
if (!ExitNotTaken.ExitingBlock) return SE->getCouldNotCompute();
assert(ExitNotTaken.ExactNotTaken && "uninitialized not-taken info");
@@ -4171,8 +4177,8 @@
if (!BECount)
BECount = ENT->ExactNotTaken;
- else
- BECount = SE->getUMinFromMismatchedTypes(BECount, ENT->ExactNotTaken);
+ else if (BECount != ENT->ExactNotTaken)
+ return SE->getCouldNotCompute();
}
assert(BECount && "Invalid not taken count for loop exit");
return BECount;
@@ -4253,8 +4259,15 @@
if (MaxBECount == getCouldNotCompute())
MaxBECount = EL.Max;
- else if (EL.Max != getCouldNotCompute())
- MaxBECount = getUMinFromMismatchedTypes(MaxBECount, EL.Max);
+ else if (EL.Max != getCouldNotCompute()) {
+ // We cannot take the "min" MaxBECount, because non-unit stride loops may
+ // skip some loop tests. Taking the max over the exits is sufficiently
+ // conservative. TODO: We could do better taking into consideration
+ // that (1) the loop has unit stride (2) the last loop test is
+ // less-than/greater-than (3) any loop test is less-than/greater-than AND
+ // falls-through some constant times less then the other tests.
+ MaxBECount = getUMaxFromMismatchedTypes(MaxBECount, EL.Max);
+ }
}
return BackedgeTakenInfo(ExitCounts, CouldComputeBECount, MaxBECount);
@@ -4920,7 +4933,7 @@
// the loop symbolically to determine when the condition gets a value of
// "ExitWhen".
- unsigned MaxIterations = MaxBruteForceIterations; // Limit analysis.
+ unsigned MaxIterations = MaxBruteForceIterations; // Limit analysis.
for (unsigned IterationNum = 0; IterationNum != MaxIterations;++IterationNum){
ConstantInt *CondVal =
dyn_cast_or_null<ConstantInt>(EvaluateExpression(Cond, L,
@@ -5507,10 +5520,10 @@
// behavior. Loops must exhibit defined behavior until a wrapped value is
// actually used. So the trip count computed by udiv could be smaller than the
// number of well-defined iterations.
- if (AddRec->getNoWrapFlags(SCEV::FlagNW))
+ if (AddRec->getNoWrapFlags(SCEV::FlagNW)) {
// FIXME: We really want an "isexact" bit for udiv.
return getUDivExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
-
+ }
// Then, try to solve the above equation provided that Start is constant.
if (const SCEVConstant *StartC = dyn_cast<SCEVConstant>(Start))
return SolveLinEquationWithOverflow(StepC->getValue()->getValue(),