[llvm] 70012fd - [SCEV] Fold (C1 * A /u C2) -> A /u (C2 /u C1), if C2 > C1. (#157656)

[via llvm-commits]

Author: Florian Hahn
Date: 2025-09-11T08:08:47Z
New Revision: 70012fda6312ba87bc0bf9009402e0869a816d1f

URL: https://github.com/llvm/llvm-project/commit/70012fda6312ba87bc0bf9009402e0869a816d1f
DIFF: https://github.com/llvm/llvm-project/commit/70012fda6312ba87bc0bf9009402e0869a816d1f.diff

LOG: [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

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Analysis/ScalarEvolution/mul-udiv-folds.ll
    llvm/test/Transforms/LoopStrengthReduce/duplicated-phis.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 51caffc41002b..ebb863076d2c5 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);
       }
     }

diff  --git a/llvm/test/Analysis/ScalarEvolution/mul-udiv-folds.ll b/llvm/test/Analysis/ScalarEvolution/mul-udiv-folds.ll
index 8dd8ec47e7090..1d34706baadeb 100644
--- a/llvm/test/Analysis/ScalarEvolution/mul-udiv-folds.ll
+++ b/llvm/test/Analysis/ScalarEvolution/mul-udiv-folds.ll
@@ -21,7 +21,7 @@ define void @udiv4_and_udiv2(i1 %c, ptr %A) {
 ; CHECK-NEXT:    %gep.8 = getelementptr i8, ptr %A, i64 %iv
 ; CHECK-NEXT:    --> {(((zext i32 %start to i64) /u 4) + %A),+,1}<%loop> U: full-set S: full-set Exits: (((zext i32 %start to i64) /u 2) + %A) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %gep.16 = getelementptr i16, ptr %A, i64 %iv
-; CHECK-NEXT:    --> {((2 * ((zext i32 %start to i64) /u 4))<nuw><nsw> + %A),+,2}<%loop> U: full-set S: full-set Exits: ((zext i32 %start to i64) + %A) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(((zext i32 %start to i64) /u 2) + %A),+,2}<%loop> U: full-set S: full-set Exits: ((zext i32 %start to i64) + %A) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %gep.32 = getelementptr i32, ptr %A, i64 %iv
 ; CHECK-NEXT:    --> {((zext i32 %start to i64) + %A),+,4}<%loop> U: full-set S: full-set Exits: ((2 * (zext i32 %start to i64))<nuw><nsw> + %A) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %gep.40 = getelementptr <{ i32, i8 }>, ptr %A, i64 %iv

diff  --git a/llvm/test/Transforms/LoopStrengthReduce/duplicated-phis.ll b/llvm/test/Transforms/LoopStrengthReduce/duplicated-phis.ll
index cee8c8abdb450..c59f7d9c2a41a 100644
--- a/llvm/test/Transforms/LoopStrengthReduce/duplicated-phis.ll
+++ b/llvm/test/Transforms/LoopStrengthReduce/duplicated-phis.ll
@@ -18,8 +18,7 @@ define i64 @test_duplicated_phis(i64 noundef %N) {
 ; CHECK:       [[FOR_BODY_PREHEADER_NEW]]:
 ; CHECK-NEXT:    [[UNROLL_ITER:%.*]] = and i64 [[MUL]], -4
 ; CHECK-NEXT:    [[TMP4:%.*]] = add i64 [[UNROLL_ITER]], -4
-; CHECK-NEXT:    [[TMP5:%.*]] = lshr i64 [[TMP4]], 2
-; CHECK-NEXT:    [[TMP3:%.*]] = shl nuw nsw i64 [[TMP5]], 1
+; CHECK-NEXT:    [[TMP3:%.*]] = lshr i64 [[TMP4]], 1
 ; CHECK-NEXT:    [[LSR_IV_NEXT:%.*]] = sub i64 -3, [[TMP3]]
 ; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
 ; CHECK:       [[FOR_BODY]]: