[SCEV] Improve handling of not expressions in isImpliedCond()

SCEV currently tries to prove implications of x pred y by also
trying to imply ~y pred ~x. This is expensive in terms of
compile-time (in fact, the majority of isImpliedCond compile-time
is spent here) and generally not fruitful. The issue is that this
also swaps the operands and thus breaks canonical ordering. If
originally we were trying to prove an implication like
X > C1 -> Y > C2, then we'll now try to prove X > C1 -> C3 > ~Y,
which will not work.

The only real case where we can get some use out of this transform
is if the original conditions were in the form X > C1 -> Y < C2, were
then swapped to X > C1 -> C2 > Y and are then swapped again here to
X > C1 -> ~Y > C3.

As such, handle this at a higher level, where we are doing the
swapping in the first place. There's four different ways that we
can line up a predicate and a swapped predicate, so we use some
heuristics to pick some profitable way.

Because we now try this transform at a higher level
(isImpliedCondOperands rather than isImpliedCondOperandsHelper),
we can also prove additional facts. Of the added tests, one was
proven previously while the other wasn't.

Differential Revision: https://reviews.llvm.org/D90926

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -10341,11 +10341,26 @@ bool ScalarEvolution::isImpliedCondBalancedTypes(
   // Check whether swapping the found predicate makes it the same as the
   // desired predicate.
   if (ICmpInst::getSwappedPredicate(FoundPred) == Pred) {
-    if (isa<SCEVConstant>(RHS))
+    // We can write the implication
+    // 0.  LHS Pred      RHS  <-   FoundLHS SwapPred  FoundRHS
+    // using one of the following ways:
+    // 1.  LHS Pred      RHS  <-   FoundRHS Pred      FoundLHS
+    // 2.  RHS SwapPred  LHS  <-   FoundLHS SwapPred  FoundRHS
+    // 3.  LHS Pred      RHS  <-  ~FoundLHS Pred     ~FoundRHS
+    // 4. ~LHS SwapPred ~RHS  <-   FoundLHS SwapPred  FoundRHS
+    // Forms 1. and 2. require swapping the operands of one condition. Don't
+    // do this if it would break canonical constant/addrec ordering.
+    if (!isa<SCEVConstant>(RHS) && !isa<SCEVAddRecExpr>(LHS))
+      return isImpliedCondOperands(FoundPred, RHS, LHS, FoundLHS, FoundRHS,
+                                   Context);
+    if (!isa<SCEVConstant>(FoundRHS) && !isa<SCEVAddRecExpr>(FoundLHS))
       return isImpliedCondOperands(Pred, LHS, RHS, FoundRHS, FoundLHS, Context);
-    else
+
-      return isImpliedCondOperands(ICmpInst::getSwappedPredicate(Pred), RHS,
+    // There's no clear preference between forms 3. and 4., try both.
-                                   LHS, FoundLHS, FoundRHS, Context);
+    return isImpliedCondOperands(FoundPred, getNotSCEV(LHS), getNotSCEV(RHS),
+                                 FoundLHS, FoundRHS, Context) ||
+           isImpliedCondOperands(Pred, LHS, RHS, getNotSCEV(FoundLHS),
+                                 getNotSCEV(FoundRHS), Context);
   }
 
   // Unsigned comparison is the same as signed comparison when both the operands
@@ -10768,11 +10783,7 @@ bool ScalarEvolution::isImpliedCondOperands(ICmpInst::Predicate Pred,
     return true;
 
   return isImpliedCondOperandsHelper(Pred, LHS, RHS,
-                                     FoundLHS, FoundRHS) ||
+                                     FoundLHS, FoundRHS);
-         // ~x < ~y --> x > y
-         isImpliedCondOperandsHelper(Pred, LHS, RHS,
-                                     getNotSCEV(FoundRHS),
-                                     getNotSCEV(FoundLHS));
 }
 
 /// Is MaybeMinMaxExpr an (U|S)(Min|Max) of Candidate and some other values?

llvm/test/Analysis/ScalarEvolution/zext-wrap.ll

@@ -15,7 +15,7 @@ bb.i:           ; preds = %bb1.i, %bb.nph
 
 ; This cast shouldn't be folded into the addrec.
 ; CHECK: %tmp = zext i8 %l_95.0.i1 to i16
-; CHECK: -->  (zext i8 {0,+,-1}<nw><%bb.i> to i16){{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: 2
+; CHECK: -->  (zext i8 {0,+,-1}<%bb.i> to i16){{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: 2
 
         %tmp = zext i8 %l_95.0.i1 to i16
 

llvm/unittests/Analysis/ScalarEvolutionTest.cpp

@@ -68,6 +68,13 @@ protected:
                         const SCEV *&RHS) {
     return SE.matchURem(Expr, LHS, RHS);
   }
+
+  static bool isImpliedCond(
+      ScalarEvolution &SE, ICmpInst::Predicate Pred, const SCEV *LHS,
+      const SCEV *RHS, ICmpInst::Predicate FoundPred, const SCEV *FoundLHS,
+      const SCEV *FoundRHS) {
+    return SE.isImpliedCond(Pred, LHS, RHS, FoundPred, FoundLHS, FoundRHS);
+  }
 };
 
 TEST_F(ScalarEvolutionsTest, SCEVUnknownRAUW) {
@@ -1368,6 +1375,45 @@ TEST_F(ScalarEvolutionsTest, ProveImplicationViaNarrowing) {
   });
 }
 
+TEST_F(ScalarEvolutionsTest, ImpliedCond) {
+  LLVMContext C;
+  SMDiagnostic Err;
+  std::unique_ptr<Module> M = parseAssemblyString(
+      "define void @foo(i32 %len) { "
+      "entry: "
+      "  br label %loop "
+      "loop: "
+      "  %iv = phi i32 [ 0, %entry], [%iv.next, %loop] "
+      "  %iv.next = add nsw i32 %iv, 1 "
+      "  %cmp = icmp slt i32 %iv, %len "
+      "  br i1 %cmp, label %loop, label %exit "
+      "exit:"
+      "  ret void "
+      "}",
+      Err, C);
+
+  ASSERT_TRUE(M && "Could not parse module?");
+  ASSERT_TRUE(!verifyModule(*M) && "Must have been well formed!");
+
+  runWithSE(*M, "foo", [](Function &F, LoopInfo &LI, ScalarEvolution &SE) {
+    Instruction *IV = getInstructionByName(F, "iv");
+    Type *Ty = IV->getType();
+    const SCEV *Zero = SE.getZero(Ty);
+    const SCEV *MinusOne = SE.getMinusOne(Ty);
+    // {0,+,1}<nuw><nsw>
+    const SCEV *AddRec_0_1 = SE.getSCEV(IV);
+    // {0,+,-1}<nw>
+    const SCEV *AddRec_0_N1 = SE.getNegativeSCEV(AddRec_0_1);
+
+    // {0,+,1}<nuw><nsw> > 0  ->  {0,+,-1}<nw> < 0
+    EXPECT_TRUE(isImpliedCond(SE, ICmpInst::ICMP_SLT, AddRec_0_N1, Zero,
+                                  ICmpInst::ICMP_SGT, AddRec_0_1, Zero));
+    // {0,+,-1}<nw> < -1  ->  {0,+,1}<nuw><nsw> > 0
+    EXPECT_TRUE(isImpliedCond(SE, ICmpInst::ICMP_SGT, AddRec_0_1, Zero,
+                                  ICmpInst::ICMP_SLT, AddRec_0_N1, MinusOne));
+  });
+}
+
 TEST_F(ScalarEvolutionsTest, MatchURem) {
   LLVMContext C;
   SMDiagnostic Err;