[SCEV] Add stricter verification option.

[fhahn]

Summary

Currently -verify-scev only fails if there is a constant difference
between two BE counts. This misses a lot of cases.

This patch adds a -verify-scev-strict options, which fails for any
non-zero differences, if used together with -verify-scev.

With the stricter checking, some unit tests fail because
of mis-matches, especially around IndVarSimplify.

If there is no reason I am missing for just checking constant deltas, I
am planning on looking into the various failures.

[sanjoy.google]

lgtm

Last time I looked at this I concluded it would be difficult to make enable something like this consistently, but no harm in trying.

llvm/lib/Analysis/ScalarEvolution.cpp

MaxBruteForceIterations("scalar-evolution-max-iterations", cl::ReallyHidden,
                                  "symbolically execute a constant "
                                  "derived loop"),
                         cl::init(100));
 
 // FIXME: Enable this with EXPENSIVE_CHECKS when the test suite is clean.
 static cl::opt<bool> VerifySCEV(
     "verify-scev", cl::Hidden,
     cl::desc("Verify ScalarEvolution's backedge taken counts (slow)"));
+static cl::opt<bool> VerifySCEVStrict(
+    "verify-scev-strict", cl::Hidden,
+    cl::desc("Enable stricter verification with -verify-scev is passed"));
 static cl::opt<bool>
     VerifySCEVMap("verify-scev-maps", cl::Hidden,
                   cl::desc("Verify no dangling value in ScalarEvolution's "
                            "ExprValueMap (slow)"));
 
 static cl::opt<bool> VerifyIR(
     "scev-verify-ir", cl::Hidden,
     cl::desc("Verify IR correctness when making sensitive SCEV queries (slow)"),
while (!LoopStack.empty()) {
 
     if (SE.getTypeSizeInBits(CurBECount->getType()) >
         SE.getTypeSizeInBits(NewBECount->getType()))
       NewBECount = SE2.getZeroExtendExpr(NewBECount, CurBECount->getType());
     else if (SE.getTypeSizeInBits(CurBECount->getType()) <
              SE.getTypeSizeInBits(NewBECount->getType()))
       CurBECount = SE2.getZeroExtendExpr(CurBECount, NewBECount->getType());
 
-    auto *ConstantDelta =
-        dyn_cast<SCEVConstant>(SE2.getMinusSCEV(CurBECount, NewBECount));
+    const SCEV *Delta = SE2.getMinusSCEV(CurBECount, NewBECount);
 
-    if (ConstantDelta && ConstantDelta->getAPInt() != 0) {
-      dbgs() << "Trip Count Changed!\n";
+    // Unless VerifySCEVStrict is set, we only compare constant deltas.
+    if ((VerifySCEVStrict || isa<SCEVConstant>(Delta)) && !Delta->isZero()) {
+      dbgs() << "Trip Count for " << *L << " Changed!\n";
       dbgs() << "Old: " << *CurBECount << "\n";
       dbgs() << "New: " << *NewBECount << "\n";
-      dbgs() << "Delta: " << *ConstantDelta << "\n";
+      dbgs() << "Delta: " << *Delta << "\n";
       std::abort();
     }
   }
 }
 
 bool ScalarEvolution::invalidate(
     Function &F, const PreservedAnalyses &PA,
     FunctionAnalysisManager::Invalidator &Inv) {