[SCEV] Handle more add/addrec mixed in computeConstantDifference()

computeConstantDifference() can currently look through addrecs with
identical steps, and then through adds with identical operands
(apart from constants).

However, it fails to handle minor variations, such as two nested
add recs, or an outer add with an inner addrec (rather than the
other way around).

This patch supports these cases by adding a loop over the
simplifications, limited to a small number of iterations. The
motivation is the same as in #101339, to make
computeConstantDifference() powerful enough to replace existing
uses of `dyn_cast<SCEVConstant>(getMinusSCEV())` with it. Though
as the IR test diff shows, other callers may also benefit.

Author: nikic

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -11933,56 +11933,83 @@ ScalarEvolution::computeConstantDifference(const SCEV *More, const SCEV *Less) {
   // We avoid subtracting expressions here because this function is usually
   // fairly deep in the call stack (i.e. is called many times).
 
-  // X - X = 0.
   unsigned BW = getTypeSizeInBits(More->getType());
-  if (More == Less)
-    return APInt(BW, 0);
-
-  if (isa<SCEVAddRecExpr>(Less) && isa<SCEVAddRecExpr>(More)) {
-    const auto *LAR = cast<SCEVAddRecExpr>(Less);
-    const auto *MAR = cast<SCEVAddRecExpr>(More);
-
-    if (LAR->getLoop() != MAR->getLoop())
-      return std::nullopt;
-
-    // We look at affine expressions only; not for correctness but to keep
-    // getStepRecurrence cheap.
-    if (!LAR->isAffine() || !MAR->isAffine())
-      return std::nullopt;
+  APInt Diff(BW, 0);
+  // Try various simplifications to reduce the difference to a constant. Limit
+  // the number of allowed simplifications to keep compile-time low.
+  for (unsigned I = 0; I < 4; ++I) {
+    if (More == Less)
+      return Diff;
+
+    // Reduce addrecs with identical steps to their start value.
+    if (isa<SCEVAddRecExpr>(Less) && isa<SCEVAddRecExpr>(More)) {
+      const auto *LAR = cast<SCEVAddRecExpr>(Less);
+      const auto *MAR = cast<SCEVAddRecExpr>(More);
+
+      if (LAR->getLoop() != MAR->getLoop())
+        return std::nullopt;
+
+      // We look at affine expressions only; not for correctness but to keep
+      // getStepRecurrence cheap.
+      if (!LAR->isAffine() || !MAR->isAffine())
+        return std::nullopt;
+
+      if (LAR->getStepRecurrence(*this) != MAR->getStepRecurrence(*this))
+        return std::nullopt;
+
+      Less = LAR->getStart();
+      More = MAR->getStart();
+      continue;
+    }
 
-    if (LAR->getStepRecurrence(*this) != MAR->getStepRecurrence(*this))
+    // Try to cancel out common factors in two add expressions.
+    SmallDenseMap<const SCEV *, int, 8> Multiplicity;
+    auto Add = [&](const SCEV *S, int Mul) {
+      if (auto *C = dyn_cast<SCEVConstant>(S))
+        Diff += C->getAPInt() * Mul;
+      else
+        Multiplicity[S] += Mul;
+    };
+    auto Decompose = [&](const SCEV *S, int Mul) {
+      if (isa<SCEVAddExpr>(S)) {
+        for (const SCEV *Op : S->operands())
+          Add(Op, Mul);
+      } else
+        Add(S, Mul);
+    };
+    Decompose(More, 1);
+    Decompose(Less, -1);
+
+    // Check whether all the non-constants cancel out, or reduce to new
+    // More/Less values.
+    const SCEV *NewMore = nullptr, *NewLess = nullptr;
+    for (const auto [S, Mul] : Multiplicity) {
+      if (Mul == 0)
+        continue;
+      if (Mul == 1) {
+        if (NewMore)
+          return std::nullopt;
+        NewMore = S;
+      } else if (Mul == -1) {
+        if (NewLess)
+          return std::nullopt;
+        NewLess = S;
+      } else
+        return std::nullopt;
+    }
+
+    // Values stayed the same, no point in trying further.
+    if (NewMore == More || NewLess == Less)
       return std::nullopt;
 
-    Less = LAR->getStart();
-    More = MAR->getStart();
-
-    // fall through
+    More = NewMore;
+    Less = NewLess;
+    if (!More || !Less)
+      break;
   }
 
-  // Try to cancel out common factors in two add expressions.
-  SmallDenseMap<const SCEV *, int, 8> Multiplicity;
-  APInt Diff(BW, 0);
-  auto Add = [&](const SCEV *S, int Mul) {
-    if (auto *C = dyn_cast<SCEVConstant>(S))
-      Diff += C->getAPInt() * Mul;
-    else
-      Multiplicity[S] += Mul;
-  };
-  auto Decompose = [&](const SCEV *S, int Mul) {
-    if (isa<SCEVAddExpr>(S)) {
-      for (const SCEV *Op : S->operands())
-        Add(Op, Mul);
-    } else
-      Add(S, Mul);
-  };
-  Decompose(More, 1);
-  Decompose(Less, -1);
-
-  // Check whether all the non-constants cancel out.
-  for (const auto &[_, Mul] : Multiplicity)
-    if (Mul != 0)
-      return std::nullopt;
-
+  if (More || Less)
+    return std::nullopt; // Did not reduce to constant.
   return Diff;
 }
 

llvm/test/Transforms/LoopVectorize/skeleton-lcssa-crash.ll

@@ -29,13 +29,13 @@ define i16 @test(ptr %arg, i64 %N) {
 ; CHECK-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP0]], 2
 ; CHECK-NEXT:    br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_MEMCHECK:%.*]]
 ; CHECK:       vector.memcheck:
-; CHECK-NEXT:    [[UGLYGEP:%.*]] = getelementptr i8, ptr [[L_2_LCSSA]], i64 2
-; CHECK-NEXT:    [[UGLYGEP5:%.*]] = getelementptr i8, ptr [[L_1_LCSSA]], i64 2
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, ptr [[L_2_LCSSA]], i64 2
+; CHECK-NEXT:    [[SCEVGEP5:%.*]] = getelementptr i8, ptr [[L_1_LCSSA]], i64 2
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl i64 [[N]], 1
 ; CHECK-NEXT:    [[TMP2:%.*]] = add i64 [[TMP1]], 4
-; CHECK-NEXT:    [[UGLYGEP6:%.*]] = getelementptr i8, ptr [[L_1_LCSSA]], i64 [[TMP2]]
-; CHECK-NEXT:    [[BOUND0:%.*]] = icmp ult ptr [[L_2_LCSSA]], [[UGLYGEP6]]
-; CHECK-NEXT:    [[BOUND1:%.*]] = icmp ult ptr [[UGLYGEP5]], [[UGLYGEP]]
+; CHECK-NEXT:    [[SCEVGEP6:%.*]] = getelementptr i8, ptr [[L_1_LCSSA]], i64 [[TMP2]]
+; CHECK-NEXT:    [[BOUND0:%.*]] = icmp ult ptr [[L_2_LCSSA]], [[SCEVGEP6]]
+; CHECK-NEXT:    [[BOUND1:%.*]] = icmp ult ptr [[SCEVGEP5]], [[SCEVGEP]]
 ; CHECK-NEXT:    [[FOUND_CONFLICT:%.*]] = and i1 [[BOUND0]], [[BOUND1]]
 ; CHECK-NEXT:    br i1 [[FOUND_CONFLICT]], label [[SCALAR_PH]], label [[VECTOR_PH:%.*]]
 ; CHECK:       vector.ph:
@@ -48,10 +48,10 @@ define i16 @test(ptr %arg, i64 %N) {
 ; CHECK-NEXT:    [[TMP4:%.*]] = add nuw nsw i64 [[TMP3]], 1
 ; CHECK-NEXT:    [[TMP5:%.*]] = getelementptr inbounds i16, ptr [[L_1]], i64 [[TMP4]]
 ; CHECK-NEXT:    [[TMP6:%.*]] = getelementptr inbounds i16, ptr [[TMP5]], i32 0
-; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <2 x i16>, ptr [[TMP6]], align 2, !alias.scope !0
+; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <2 x i16>, ptr [[TMP6]], align 2, !alias.scope [[META0:![0-9]+]]
 ; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr inbounds i16, ptr [[L_2]], i64 0
 ; CHECK-NEXT:    [[TMP8:%.*]] = extractelement <2 x i16> [[WIDE_LOAD]], i32 1
-; CHECK-NEXT:    store i16 [[TMP8]], ptr [[TMP7]], align 2, !alias.scope !3, !noalias !0
+; CHECK-NEXT:    store i16 [[TMP8]], ptr [[TMP7]], align 2, !alias.scope [[META3:![0-9]+]], !noalias [[META0]]
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
 ; CHECK-NEXT:    [[TMP9:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[TMP9]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
@@ -74,7 +74,7 @@ define i16 @test(ptr %arg, i64 %N) {
 ; CHECK-NEXT:    [[LOOP_L_1:%.*]] = load i16, ptr [[GEP_1]], align 2
 ; CHECK-NEXT:    [[GEP_2:%.*]] = getelementptr inbounds i16, ptr [[L_2_LCSSA]], i64 0
 ; CHECK-NEXT:    store i16 [[LOOP_L_1]], ptr [[GEP_2]], align 2
-; CHECK-NEXT:    br i1 [[C_5]], label [[LOOP_3]], label [[EXIT_LOOPEXIT]], !llvm.loop [[LOOP7:![0-9]+]]
+; CHECK-NEXT:    br i1 [[C_5]], label [[LOOP_3]], label [[EXIT_LOOPEXIT]], !llvm.loop [[LOOP8:![0-9]+]]
 ; CHECK:       exit.loopexit:
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit.loopexit1:
@@ -138,31 +138,17 @@ define void @test2(ptr %dst) {
 ; CHECK-NEXT:    [[INDVAR_NEXT]] = add i32 [[INDVAR]], 1
 ; CHECK-NEXT:    br i1 [[C_1]], label [[LOOP_2]], label [[LOOP_3_PH:%.*]]
 ; CHECK:       loop.3.ph:
-; CHECK-NEXT:    [[INDVAR_LCSSA1:%.*]] = phi i32 [ [[INDVAR]], [[LOOP_2]] ]
 ; CHECK-NEXT:    [[INDVAR_LCSSA:%.*]] = phi i32 [ [[INDVAR]], [[LOOP_2]] ]
 ; CHECK-NEXT:    [[IV_1_LCSSA:%.*]] = phi i64 [ [[IV_1]], [[LOOP_2]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = and i64 [[IV_1_LCSSA]], 4294967295
-; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[INDVAR_LCSSA1]], -1
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[INDVAR_LCSSA]], -1
 ; CHECK-NEXT:    [[TMP2:%.*]] = add i32 [[TMP1]], 1000
-; CHECK-NEXT:    [[SMIN2:%.*]] = call i32 @llvm.smin.i32(i32 [[TMP2]], i32 1)
-; CHECK-NEXT:    [[TMP3:%.*]] = sub i32 [[TMP2]], [[SMIN2]]
+; CHECK-NEXT:    [[SMIN:%.*]] = call i32 @llvm.smin.i32(i32 [[TMP2]], i32 1)
+; CHECK-NEXT:    [[TMP3:%.*]] = sub i32 [[TMP2]], [[SMIN]]
 ; CHECK-NEXT:    [[TMP4:%.*]] = zext i32 [[TMP3]] to i64
 ; CHECK-NEXT:    [[TMP5:%.*]] = add nuw nsw i64 [[TMP4]], 1
 ; CHECK-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP5]], 2
-; CHECK-NEXT:    br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_SCEVCHECK:%.*]]
-; CHECK:       vector.scevcheck:
-; CHECK-NEXT:    [[TMP6:%.*]] = mul i32 [[INDVAR_LCSSA]], -1
-; CHECK-NEXT:    [[TMP7:%.*]] = add i32 [[TMP6]], 1000
-; CHECK-NEXT:    [[SMIN:%.*]] = call i32 @llvm.smin.i32(i32 [[TMP7]], i32 1)
-; CHECK-NEXT:    [[TMP8:%.*]] = sub i32 [[TMP7]], [[SMIN]]
-; CHECK-NEXT:    [[TMP9:%.*]] = add i32 [[TMP6]], 999
-; CHECK-NEXT:    [[MUL:%.*]] = call { i32, i1 } @llvm.umul.with.overflow.i32(i32 1, i32 [[TMP8]])
-; CHECK-NEXT:    [[MUL_RESULT:%.*]] = extractvalue { i32, i1 } [[MUL]], 0
-; CHECK-NEXT:    [[MUL_OVERFLOW:%.*]] = extractvalue { i32, i1 } [[MUL]], 1
-; CHECK-NEXT:    [[TMP10:%.*]] = sub i32 [[TMP9]], [[MUL_RESULT]]
-; CHECK-NEXT:    [[TMP11:%.*]] = icmp ugt i32 [[TMP10]], [[TMP9]]
-; CHECK-NEXT:    [[TMP12:%.*]] = or i1 [[TMP11]], [[MUL_OVERFLOW]]
-; CHECK-NEXT:    br i1 [[TMP12]], label [[SCALAR_PH]], label [[VECTOR_PH:%.*]]
+; CHECK-NEXT:    br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
 ; CHECK:       vector.ph:
 ; CHECK-NEXT:    [[N_MOD_VF:%.*]] = urem i64 [[TMP5]], 2
 ; CHECK-NEXT:    [[N_VEC:%.*]] = sub i64 [[TMP5]], [[N_MOD_VF]]
@@ -171,21 +157,21 @@ define void @test2(ptr %dst) {
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[OFFSET_IDX:%.*]] = sub i64 [[TMP0]], [[INDEX]]
-; CHECK-NEXT:    [[TMP13:%.*]] = add i64 [[OFFSET_IDX]], 0
-; CHECK-NEXT:    [[TMP14:%.*]] = add nsw i64 [[TMP13]], -1
-; CHECK-NEXT:    [[TMP15:%.*]] = and i64 [[TMP14]], 4294967295
-; CHECK-NEXT:    [[TMP16:%.*]] = getelementptr inbounds i32, ptr [[DST:%.*]], i64 [[TMP15]]
-; CHECK-NEXT:    [[TMP17:%.*]] = getelementptr inbounds i32, ptr [[TMP16]], i32 0
-; CHECK-NEXT:    [[TMP18:%.*]] = getelementptr inbounds i32, ptr [[TMP17]], i32 -1
-; CHECK-NEXT:    store <2 x i32> zeroinitializer, ptr [[TMP18]], align 4
+; CHECK-NEXT:    [[TMP6:%.*]] = add i64 [[OFFSET_IDX]], 0
+; CHECK-NEXT:    [[TMP7:%.*]] = add nsw i64 [[TMP6]], -1
+; CHECK-NEXT:    [[TMP8:%.*]] = and i64 [[TMP7]], 4294967295
+; CHECK-NEXT:    [[TMP9:%.*]] = getelementptr inbounds i32, ptr [[DST:%.*]], i64 [[TMP8]]
+; CHECK-NEXT:    [[TMP10:%.*]] = getelementptr inbounds i32, ptr [[TMP9]], i32 0
+; CHECK-NEXT:    [[TMP11:%.*]] = getelementptr inbounds i32, ptr [[TMP10]], i32 -1
+; CHECK-NEXT:    store <2 x i32> zeroinitializer, ptr [[TMP11]], align 4
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
-; CHECK-NEXT:    [[TMP19:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
-; CHECK-NEXT:    br i1 [[TMP19]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP8:![0-9]+]]
+; CHECK-NEXT:    [[TMP12:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
+; CHECK-NEXT:    br i1 [[TMP12]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP9:![0-9]+]]
 ; CHECK:       middle.block:
 ; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 [[TMP5]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[CMP_N]], label [[LOOP_1_LATCH:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
-; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ [[IND_END]], [[MIDDLE_BLOCK]] ], [ [[TMP0]], [[LOOP_3_PH]] ], [ [[TMP0]], [[VECTOR_SCEVCHECK]] ]
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ [[IND_END]], [[MIDDLE_BLOCK]] ], [ [[TMP0]], [[LOOP_3_PH]] ]
 ; CHECK-NEXT:    br label [[LOOP_3:%.*]]
 ; CHECK:       loop.3:
 ; CHECK-NEXT:    [[IV_2:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_2_NEXT:%.*]], [[LOOP_3]] ]
@@ -195,7 +181,7 @@ define void @test2(ptr %dst) {
 ; CHECK-NEXT:    store i32 0, ptr [[GEP_DST]], align 4
 ; CHECK-NEXT:    [[IV_2_TRUNC:%.*]] = trunc i64 [[IV_2]] to i32
 ; CHECK-NEXT:    [[EC:%.*]] = icmp sgt i32 [[IV_2_TRUNC]], 1
-; CHECK-NEXT:    br i1 [[EC]], label [[LOOP_3]], label [[LOOP_1_LATCH]], !llvm.loop [[LOOP9:![0-9]+]]
+; CHECK-NEXT:    br i1 [[EC]], label [[LOOP_3]], label [[LOOP_1_LATCH]], !llvm.loop [[LOOP10:![0-9]+]]
 ; CHECK:       loop.1.latch:
 ; CHECK-NEXT:    [[C_2:%.*]] = call i1 @cond()
 ; CHECK-NEXT:    br i1 [[C_2]], label [[EXIT:%.*]], label [[LOOP_1_HEADER]]

llvm/unittests/Analysis/ScalarEvolutionTest.cpp

@@ -1199,8 +1199,8 @@ TEST_F(ScalarEvolutionsTest, SCEVComputeConstantDifference) {
     EXPECT_EQ(diff(ScevIV, ScevIVNext), -1);
     EXPECT_EQ(diff(ScevIVNext, ScevIV), 1);
     EXPECT_EQ(diff(ScevIVNext, ScevIVNext), 0);
-    EXPECT_EQ(diff(ScevIV2P3, ScevIV2), std::nullopt); // TODO
-    EXPECT_EQ(diff(ScevIV2PVar, ScevIV2PVarP3), std::nullopt); // TODO
+    EXPECT_EQ(diff(ScevIV2P3, ScevIV2), 3);
+    EXPECT_EQ(diff(ScevIV2PVar, ScevIV2PVarP3), -3);
     EXPECT_EQ(diff(ScevV0, ScevIV), std::nullopt);
     EXPECT_EQ(diff(ScevIVNext, ScevV3), std::nullopt);
     EXPECT_EQ(diff(ScevYY, ScevV3), std::nullopt);