[SCEV] Fold (C1 * A /u C2) -> A /u (C2 /u C1), if C2 > C1. (#157656)

If C2 >u C1 and C1 >u 1, fold to A /u (C2 /u C1).

Depends on https://github.com/llvm/llvm-project/pull/157555.

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

PR: https://github.com/llvm/llvm-project/pull/157656

1 files changed, 13 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 51caffc..ebb8630 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -3217,18 +3217,26 @@ const SCEV *ScalarEvolution::getMulExpr(SmallVectorImpl<const SCEV *> &Ops,
       }
 
       // Try to fold (C1 * D /u C2) -> C1/C2 * D, if C1 and C2 are powers-of-2,
-      // D is a multiple of C2, and C1 is a multiple of C2.
+      // D is a multiple of C2, and C1 is a multiple of C2. If C2 is a multiple
+      // of C1, fold to (D /u (C2 /u C1)).
       const SCEV *D;
       APInt C1V = LHSC->getAPInt();
-      // (C1 * D /u C2) == -1 * -C1 * D /u C2 when C1 != INT_MIN.
-      if (C1V.isNegative() && !C1V.isMinSignedValue())
+      // (C1 * D /u C2) == -1 * -C1 * D /u C2 when C1 != INT_MIN. Don't treat -1
+      // as -1 * 1, as it won't enable additional folds.
+      if (C1V.isNegative() && !C1V.isMinSignedValue() && !C1V.isAllOnes())
         C1V = C1V.abs();
       const SCEVConstant *C2;
       if (C1V.isPowerOf2() &&
           match(Ops[1], m_scev_UDiv(m_SCEV(D), m_SCEVConstant(C2))) &&
-          C2->getAPInt().isPowerOf2() && C1V.uge(C2->getAPInt()) &&
+          C2->getAPInt().isPowerOf2() &&
           C1V.logBase2() <= getMinTrailingZeros(D)) {
-        const SCEV *NewMul = getMulExpr(getUDivExpr(getConstant(C1V), C2), D);
+        const SCEV *NewMul;
+        if (C1V.uge(C2->getAPInt())) {
+          NewMul = getMulExpr(getUDivExpr(getConstant(C1V), C2), D);
+        } else {
+          assert(C1V.ugt(1) && "C1 <= 1 should have been folded earlier");
+          NewMul = getUDivExpr(D, getUDivExpr(C2, getConstant(C1V)));
+        }
         return C1V == LHSC->getAPInt() ? NewMul : getNegativeSCEV(NewMul);
       }
     }