[SCEV] Fold (C * A /u C) -> A, if A is a multiple of C and C a pow-of-2. (#156730)

Alive2 Proof: https://alive2.llvm.org/ce/z/JoHJE9

PR: #156730

Author: fhahn

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -3215,6 +3215,15 @@ const SCEV *ScalarEvolution::getMulExpr(SmallVectorImpl<const SCEV *> &Ops,
           return getZeroExtendExpr(Res, Ops[1]->getType(), Depth + 1);
         };
       }
+
+      // Try to fold (C * D /u C) -> D, if C is a power-of-2 and D is a multiple
+      //  of C.
+      const SCEV *D;
+      if (match(Ops[1], m_scev_UDiv(m_SCEV(D), m_scev_Specific(LHSC))) &&
+          LHSC->getAPInt().isPowerOf2() &&
+          LHSC->getAPInt().logBase2() <= getMinTrailingZeros(D)) {
+        return D;
+      }
     }
   }
 

llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info-rewrite-expressions.ll

@@ -1390,9 +1390,9 @@ define void @ptr_induction_eq_2(ptr %a, i64 %n) {
 ; CHECK-NEXT:    %b = getelementptr inbounds ptr, ptr %a, i64 %n
 ; CHECK-NEXT:    --> ((8 * %n)<nsw> + %a) U: full-set S: full-set
 ; CHECK-NEXT:    %ptr.iv = phi ptr [ %ptr.iv.next, %loop ], [ %a, %entry ]
-; CHECK-NEXT:    --> {%a,+,8}<nuw><%loop> U: full-set S: full-set Exits: ((8 * ((-8 + (8 * %n)<nsw>) /u 8))<nuw> + %a) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {%a,+,8}<nuw><%loop> U: full-set S: full-set Exits: (-8 + (8 * %n)<nsw> + %a) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %ptr.iv.next = getelementptr inbounds i8, ptr %ptr.iv, i64 8
-; CHECK-NEXT:    --> {(8 + %a),+,8}<nuw><%loop> U: full-set S: full-set Exits: (8 + (8 * ((-8 + (8 * %n)<nsw>) /u 8))<nuw> + %a) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(8 + %a),+,8}<nuw><%loop> U: full-set S: full-set Exits: ((8 * %n)<nsw> + %a) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @ptr_induction_eq_2
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-8 + (8 * %n)<nsw>) /u 8)
 ; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is i64 2305843009213693951