[SCEV][NFC] Verify IR in isLoop[Entry,Backedge]GuardedByCond

We have a lot of various bugs that are caused by misuse of SCEV (in particular in LV),
all of them can simply be described as "we ask SCEV to prove some fact on invalid IR".
Some of examples of those are PR36311, PR37221, PR39160.

The problem is that these failues manifest differently (what we saw was failure of various
asserts across SCEV, but there can also be miscompiles). This patch adds an assert into two
SCEV methods that strongly rely on correctness of the IR and are involved in known failues.
This will at least allow us to have a clear indication of what was wrong in this case.

This patch also fixes a unit test with incorrect IR that fails this verification.

Differential Revision: https://reviews.llvm.org/D52930
Reviewed By: fhahn

llvm-svn: 346389

Author: Max Kazantsev

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -112,6 +112,7 @@
 #include "llvm/IR/Use.h"
 #include "llvm/IR/User.h"
 #include "llvm/IR/Value.h"
+#include "llvm/IR/Verifier.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
@@ -162,6 +163,11 @@ static cl::opt<bool>
                   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)"),
+    cl::init(false));
+
 static cl::opt<unsigned> MulOpsInlineThreshold(
     "scev-mulops-inline-threshold", cl::Hidden,
     cl::desc("Threshold for inlining multiplication operands into a SCEV"),
@@ -9370,6 +9376,11 @@ ScalarEvolution::isLoopBackedgeGuardedByCond(const Loop *L,
   // (interprocedural conditions notwithstanding).
   if (!L) return true;
 
+  if (VerifyIR)
+    assert(!verifyFunction(*L->getHeader()->getParent(), &dbgs()) &&
+           "This cannot be done on broken IR!");
+
+
   if (isKnownViaNonRecursiveReasoning(Pred, LHS, RHS))
     return true;
 
@@ -9475,6 +9486,10 @@ ScalarEvolution::isLoopEntryGuardedByCond(const Loop *L,
   // (interprocedural conditions notwithstanding).
   if (!L) return false;
 
+  if (VerifyIR)
+    assert(!verifyFunction(*L->getHeader()->getParent(), &dbgs()) &&
+           "This cannot be done on broken IR!");
+
   // Both LHS and RHS must be available at loop entry.
   assert(isAvailableAtLoopEntry(LHS, L) &&
          "LHS is not available at Loop Entry");

llvm/unittests/Analysis/ScalarEvolutionTest.cpp

@@ -701,7 +701,7 @@ TEST_F(ScalarEvolutionsTest, SCEVZeroExtendExpr) {
     PN->addIncoming(Dec, IncBB);
     BranchInst::Create(CondBB, IncBB);
 
-    Accum = GetElementPtrInst::Create(I8Ty, Accum, Dec, "gep", EndBB);
+    Accum = GetElementPtrInst::Create(I8Ty, Accum, PN, "gep", EndBB);
 
     PrevBB = CondBB;
     CondBB = NextBB;