[SCEV] Match both (-1)b + a and a + (-1)b as a - b (#84247)

In our analysis of guarding conditions, we were converting a-b == 0 into
a == b alternate form, but we were only checking for one of the two
forms for the sub. There's no requirement that the multiply only be on
the LHS of the add.

Author: preames

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -10577,6 +10577,25 @@ static bool HasSameValue(const SCEV *A, const SCEV *B) {
   return false;
 }
 
+static bool MatchBinarySub(const SCEV *S, const SCEV *&LHS, const SCEV *&RHS) {
+  const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S);
+  if (!Add || Add->getNumOperands() != 2)
+    return false;
+  if (auto *ME = dyn_cast<SCEVMulExpr>(Add->getOperand(0));
+      ME && ME->getNumOperands() == 2 && ME->getOperand(0)->isAllOnesValue()) {
+    LHS = Add->getOperand(1);
+    RHS = ME->getOperand(1);
+    return true;
+  }
+  if (auto *ME = dyn_cast<SCEVMulExpr>(Add->getOperand(1));
+      ME && ME->getNumOperands() == 2 && ME->getOperand(0)->isAllOnesValue()) {
+    LHS = Add->getOperand(0);
+    RHS = ME->getOperand(1);
+    return true;
+  }
+  return false;
+}
+
 bool ScalarEvolution::SimplifyICmpOperands(ICmpInst::Predicate &Pred,
                                            const SCEV *&LHS, const SCEV *&RHS,
                                            unsigned Depth) {
@@ -10652,19 +10671,10 @@ bool ScalarEvolution::SimplifyICmpOperands(ICmpInst::Predicate &Pred,
       case ICmpInst::ICMP_EQ:
       case ICmpInst::ICMP_NE:
         // Fold ((-1) * %a) + %b == 0 (equivalent to %b-%a == 0) into %a == %b.
-        if (!RA)
-          if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(LHS))
-            if (const SCEVMulExpr *ME =
-                    dyn_cast<SCEVMulExpr>(AE->getOperand(0)))
-              if (AE->getNumOperands() == 2 && ME->getNumOperands() == 2 &&
-                  ME->getOperand(0)->isAllOnesValue()) {
-                RHS = AE->getOperand(1);
-                LHS = ME->getOperand(1);
-                Changed = true;
-              }
+        if (RA.isZero() && MatchBinarySub(LHS, LHS, RHS))
+          Changed = true;
         break;
 
-
         // The "Should have been caught earlier!" messages refer to the fact
         // that the ExactCR.isFullSet() or ExactCR.isEmptySet() check above
         // should have fired on the corresponding cases, and canonicalized the

llvm/test/Analysis/ScalarEvolution/trip-count.ll

@@ -145,7 +145,7 @@ define void @dual_sext_ne_with_slt_guard(i8 %s, i8 %n) {
 ; CHECK-LABEL: 'dual_sext_ne_with_slt_guard'
 ; CHECK-NEXT:  Determining loop execution counts for: @dual_sext_ne_with_slt_guard
 ; CHECK-NEXT:  Loop %for.body: backedge-taken count is (-1 + (sext i8 %n to i64) + (-1 * (sext i8 %s to i64))<nsw>)
-; CHECK-NEXT:  Loop %for.body: constant max backedge-taken count is i64 -1
+; CHECK-NEXT:  Loop %for.body: constant max backedge-taken count is i64 -2
 ; CHECK-NEXT:  Loop %for.body: symbolic max backedge-taken count is (-1 + (sext i8 %n to i64) + (-1 * (sext i8 %s to i64))<nsw>)
 ; CHECK-NEXT:  Loop %for.body: Trip multiple is 1
 ;