Automerge: Revert "[SCEV] Fold (C1 * A /u C2) -> A /u (C2 /u C1), if C2 > C1." (#158328)

Reverts llvm/llvm-project#157656

There are multiple reports that this is causing miscompiles in the MSan
test suite after bootstrapping and that this is causing miscompiles in
rustc. Let's revert for now, and work to capture a reproducer next week.

Author: rnk

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -3217,26 +3217,18 @@ 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. If C2 is a multiple
-      // of C1, fold to (D /u (C2 /u C1)).
+      // D is a multiple of C2, and C1 is a multiple of C2.
       const SCEV *D;
       APInt C1V = LHSC->getAPInt();
-      // (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())
+      // (C1 * D /u C2) == -1 * -C1 * D /u C2 when C1 != INT_MIN.
+      if (C1V.isNegative() && !C1V.isMinSignedValue())
         C1V = C1V.abs();
       const SCEVConstant *C2;
       if (C1V.isPowerOf2() &&
           match(Ops[1], m_scev_UDiv(m_SCEV(D), m_SCEVConstant(C2))) &&
-          C2->getAPInt().isPowerOf2() &&
+          C2->getAPInt().isPowerOf2() && C1V.uge(C2->getAPInt()) &&
           C1V.logBase2() <= getMinTrailingZeros(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)));
-        }
+        const SCEV *NewMul = getMulExpr(getUDivExpr(getConstant(C1V), C2), D);
         return C1V == LHSC->getAPInt() ? NewMul : getNegativeSCEV(NewMul);
       }
     }

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:    --> {(((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:    --> {((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:    %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

llvm/test/Transforms/LoopStrengthReduce/duplicated-phis.ll

@@ -18,7 +18,8 @@ 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:    [[TMP3:%.*]] = lshr i64 [[TMP4]], 1
+; CHECK-NEXT:    [[TMP5:%.*]] = lshr i64 [[TMP4]], 2
+; CHECK-NEXT:    [[TMP3:%.*]] = shl nuw nsw i64 [[TMP5]], 1
 ; CHECK-NEXT:    [[LSR_IV_NEXT:%.*]] = sub i64 -3, [[TMP3]]
 ; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
 ; CHECK:       [[FOR_BODY]]: