[SCEV] Enable verification in LoopPM

Currently, we hardly ever actually run SCEV verification, even in
tests with -verify-scev. This is because the NewPM LPM does not
verify SCEV. The reason for this is that SCEV verification can
actually change the result of subsequent SCEV queries, which means
that you see different transformations depending on whether
verification is enabled or not.

To allow verification in the LPM, this limits verification to
BECounts that have actually been cached. It will not calculate
new BECounts.

BackedgeTakenInfo::getExact() is still not entirely readonly,
it still calls getUMinFromMismatchedTypes(). But I hope that this
is not problematic in the same way. (This could be avoided by
performing the umin in the other SCEV instance, but this would
require duplicating some of the code.)

Differential Revision: https://reviews.llvm.org/D120551

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -67,6 +67,8 @@ class Type;
 class Value;
 enum SCEVTypes : unsigned short;
 
+extern bool VerifySCEV;
+
 /// This class represents an analyzed expression in the program.  These are
 /// opaque objects that the client is not allowed to do much with directly.
 ///

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -142,6 +142,9 @@ STATISTIC(NumTripCountsNotComputed,
 STATISTIC(NumBruteForceTripCountsComputed,
           "Number of loops with trip counts computed by force");
 
+// FIXME: Enable this with EXPENSIVE_CHECKS when the test suite is clean.
+bool llvm::VerifySCEV = false;
+
 static cl::opt<unsigned>
 MaxBruteForceIterations("scalar-evolution-max-iterations", cl::ReallyHidden,
                         cl::ZeroOrMore,
@@ -150,9 +153,8 @@ MaxBruteForceIterations("scalar-evolution-max-iterations", cl::ReallyHidden,
                                  "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,
+static cl::opt<bool, true> VerifySCEVOpt(
+    "verify-scev", cl::Hidden, cl::location(VerifySCEV),
     cl::desc("Verify ScalarEvolution's backedge taken counts (slow)"));
 static cl::opt<bool> VerifySCEVStrict(
     "verify-scev-strict", cl::Hidden,
@@ -13359,8 +13361,14 @@ void ScalarEvolution::verify() const {
     if (!ReachableBlocks.contains(L->getHeader()))
       continue;
 
-    auto *CurBECount = SCM.visit(
-        const_cast<ScalarEvolution *>(this)->getBackedgeTakenCount(L));
+    // Only verify cached BECounts. Computing new BECounts may change the
+    // results of subsequent SCEV uses.
+    auto It = BackedgeTakenCounts.find(L);
+    if (It == BackedgeTakenCounts.end())
+      continue;
+
+    auto *CurBECount =
+        SCM.visit(It->second.getExact(L, const_cast<ScalarEvolution *>(this)));
     auto *NewBECount = SE2.getBackedgeTakenCount(L);
 
     if (CurBECount == SE2.getCouldNotCompute() ||

llvm/lib/Transforms/Scalar/LoopPassManager.cpp

@@ -309,12 +309,12 @@ PreservedAnalyses FunctionToLoopPassAdaptor::run(Function &F,
 
 #ifndef NDEBUG
     // LoopAnalysisResults should always be valid.
-    // Note that we don't LAR.SE.verify() because that can change observed SE
-    // queries. See PR44815.
     if (VerifyDomInfo)
       LAR.DT.verify();
     if (VerifyLoopInfo)
       LAR.LI.verify(LAR.DT);
+    if (VerifySCEV)
+      LAR.SE.verify();
     if (LAR.MSSA && VerifyMemorySSA)
       LAR.MSSA->verifyMemorySSA();
 #endif

llvm/test/Transforms/IndVarSimplify/X86/deterministic-scev-verify.ll

@@ -1,18 +1,13 @@
-; RUN: opt -indvars -stats -disable-output < %s 2>&1 | FileCheck %s --check-prefix=STATS
-; RUN: opt -indvars -S < %s | FileCheck %s --check-prefix=IR
-; REQUIRES: asserts
+; RUN: opt -indvars -S < %s | FileCheck %s
 
 ; Check that IndVarSimplify's result is not influenced by stray calls to
 ; ScalarEvolution in debug builds. However, -verify-indvars may still do
 ; such calls.
 ; llvm.org/PR44815
 
-; STATS: 1 scalar-evolution - Number of loops with trip counts computed by force
-; STATS: 2 scalar-evolution - Number of loops with predictable loop counts
-
 ; In this test, adding -verify-indvars causes %tmp13 to not be optimized away.
-; IR-LABEL: @foo
-; IR-NOT:   phi i32
+; CHECK-LABEL: @foo
+; CHECK-NOT:   phi i32
 
 target triple = "x86_64-unknown-linux-gnu"
 

llvm/test/Transforms/IndVarSimplify/X86/pr35406.ll

@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -passes='loop(indvars),verify<scalar-evolution>' %s | FileCheck %s
+; RUN: opt -S -indvars -verify-scev %s | FileCheck %s
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
 target triple = "x86_64-unknown-linux-gnu"