[IndVars][NFCI] Avoid supposedly redundant query to save some CT

[mkazantsev]

Summary

I ran a big corps of Fuzz tests and could not find a case when predicate
evaluated to 'false' would fire here, seems that they all get optimized away
by other means. We also don't have a unit test that would show it's useful.

I am not 100% sure this is an NFC, but if it really causes any troubles, we
should add a test demonstrating it's useful. Otherwise, we save some CT
by reducing number of SCEV queries.

[nikic]

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

1387–1388

Was this supposed to be isKnownPredicateAt?

[mkazantsev]

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

1387–1388

Yes... Haven't committed the last revision of patch :(

[mkazantsev]

Fixed

[nikic]

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

1388

If I put an assert(*EV) here, I get an assertion failure in llvm/test/Transforms/IndVarSimplify/2014-06-21-congruent-constant.ll. So it looks like this can trigger. Unfortunately this is a test chock full of UB, but maybe it's possible to extract a more meaningful test from it?

[mkazantsev]

Fixed comment

[mkazantsev]

I'll study this more, maybe it's not dead indeed.

[mkazantsev]

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

1388

Failing assertion doesn't mean it can't be optimized by other means, but need more study.

[lebedev.ri]

This review may be stuck/dead, consider abandoning if no longer relevant.
Removing myself as reviewer in attempt to clean dashboard.

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

static bool optimizeLoopExitWithUnknownExitCount(
 
   // 'LHS pred RHS' should now mean that we stay in loop.
   if (L->contains(FalseSucc))
     Pred = CmpInst::getInversePredicate(Pred);
 
   const SCEV *LHSS = SE->getSCEVAtScope(LHS, L);
   const SCEV *RHSS = SE->getSCEVAtScope(RHS, L);
   // Can we prove it to be trivially true or false?
-  if (auto EV = SE->evaluatePredicateAt(Pred, LHSS, RHSS, BI)) {
-    foldExit(L, ExitingBB, /*IsTaken*/ !*EV, DeadInsts);
+  if (SE->isKnownPredicateAt(Pred, LHSS, RHSS, BI)) {
+    foldExit(L, ExitingBB, /*IsTaken*/ false, DeadInsts);
     return true;
   }
 
   auto *ARTy = LHSS->getType();
   auto *MaxIterTy = MaxIter->getType();
   // If possible, adjust types.
   if (SE->getTypeSizeInBits(ARTy) > SE->getTypeSizeInBits(MaxIterTy))
     MaxIter = SE->getZeroExtendExpr(MaxIter, ARTy);