[ConstraintElim] Replace NUWSub decomp with recursive decomp of ops.

The current patterns for NUWSub decompositions do not handle negative
constants correctly at the moment (causing llvm#76713).

Replace the incorrect pattern by more general code that recursively
decomposes the operands and then combines the results. This is already
done in most other places that handle operators like add/mul.

This means we fall back to the general constant handling code (fixes the
mis-compile) while also being able to support reasoning about
decomposable expressions in the SUB operands.

Fixes llvm#76713.

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -366,6 +366,13 @@ struct Decomposition {
     append_range(Vars, Other.Vars);
   }
 
+  void sub(const Decomposition &Other) {
+    Decomposition Tmp = Other;
+    Tmp.mul(-1);
+    add(Tmp.Offset);
+    append_range(Vars, Tmp.Vars);
+  }
+
   void mul(int64_t Factor) {
     Offset = multiplyWithOverflow(Offset, Factor);
     for (auto &Var : Vars)
@@ -569,10 +576,12 @@ static Decomposition decompose(Value *V,
     return Result;
   }
 
-  if (match(V, m_NUWSub(m_Value(Op0), m_ConstantInt(CI))) && canUseSExt(CI))
-    return {-1 * CI->getSExtValue(), {{1, Op0}}};
-  if (match(V, m_NUWSub(m_Value(Op0), m_Value(Op1))))
-    return {0, {{1, Op0}, {-1, Op1}}};
+  if (match(V, m_NUWSub(m_Value(Op0), m_Value(Op1)))) {
+    auto ResA = decompose(Op0, Preconditions, IsSigned, DL);
+    auto ResB = decompose(Op1, Preconditions, IsSigned, DL);
+    ResA.sub(ResB);
+    return ResA;
+  }
 
   return {V, IsKnownNonNegative};
 }

llvm/test/Transforms/ConstraintElimination/sub-nuw.ll

@@ -316,12 +316,13 @@ define i1 @sub_nuw_neg_i16(i16 %a) {
 ; CHECK-LABEL: @sub_nuw_neg_i16(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[NEG2:%.*]] = sub nuw i16 [[A:%.*]], -305
-; CHECK-NEXT:    br i1 false, label [[EXIT_1:%.*]], label [[EXIT_2:%.*]]
+; CHECK-NEXT:    [[C_1:%.*]] = icmp ugt i16 0, [[NEG2]]
+; CHECK-NEXT:    br i1 [[C_1]], label [[EXIT_1:%.*]], label [[EXIT_2:%.*]]
 ; CHECK:       exit.1:
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[C_2:%.*]] = icmp ugt i16 [[A]], 0
+; CHECK-NEXT:    ret i1 [[C_2]]
 ; CHECK:       exit.2:
-; CHECK-NEXT:    [[C_3:%.*]] = icmp ugt i16 [[A]], 0
-; CHECK-NEXT:    ret i1 [[C_3]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %neg2 = sub nuw i16 %a, -305
@@ -380,7 +381,6 @@ entry:
   ret i1 %c
 }
 
-; FIXME: currently this incorrectly simplifies %c4 to true.
 define i1 @pr76713(i16 %i1, i16 %i3) {
 ; CHECK-LABEL: @pr76713(
 ; CHECK-NEXT:  entry:
@@ -394,7 +394,8 @@ define i1 @pr76713(i16 %i1, i16 %i3) {
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nuw nsw i16 [[I1]], -3
 ; CHECK-NEXT:    [[ARRAYIDX_IDX:%.*]] = mul nuw nsw i16 [[I3]], 4
 ; CHECK-NEXT:    [[I6:%.*]] = add nuw nsw i16 [[ARRAYIDX_IDX]], [[SUB]]
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[C4:%.*]] = icmp ult i16 12, [[I6]]
+; CHECK-NEXT:    ret i1 [[C4]]
 ; CHECK:       else:
 ; CHECK-NEXT:    ret i1 false
 ;
@@ -417,7 +418,6 @@ else:
   ret i1 0
 }
 
-; FIXME: Currently gets mis-compiled.
 define void @sub_nuw_chained_positive_constants(i16 %a) {
 ; CHECK-LABEL: @sub_nuw_chained_positive_constants(
 ; CHECK-NEXT:  entry:
@@ -426,16 +426,13 @@ define void @sub_nuw_chained_positive_constants(i16 %a) {
 ; CHECK-NEXT:    [[C_1:%.*]] = icmp ugt i16 [[SUB2]], 90
 ; CHECK-NEXT:    br i1 [[C_1]], label [[EXIT_1:%.*]], label [[EXIT_2:%.*]]
 ; CHECK:       exit.1:
-; CHECK-NEXT:    [[C_2:%.*]] = icmp ugt i16 [[A]], 120
-; CHECK-NEXT:    call void @use(i1 [[C_2]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[C_3:%.*]] = icmp ugt i16 [[A]], 121
 ; CHECK-NEXT:    call void @use(i1 [[C_3]])
 ; CHECK-NEXT:    ret void
 ; CHECK:       exit.2:
-; CHECK-NEXT:    [[C_4:%.*]] = icmp ugt i16 [[A]], 120
-; CHECK-NEXT:    call void @use(i1 [[C_4]])
-; CHECK-NEXT:    [[C_5:%.*]] = icmp ugt i16 [[A]], 121
-; CHECK-NEXT:    call void @use(i1 [[C_5]])
+; CHECK-NEXT:    call void @use(i1 false)
+; CHECK-NEXT:    call void @use(i1 false)
 ; CHECK-NEXT:    ret void
 ;
 entry:
@@ -459,7 +456,6 @@ exit.2:
   ret void
 }
 
-; FIXME: Currently gets mis-compiled.
 define void @sub_nuw_chained_negative_constants(i8 %a) {
 ; CHECK-LABEL: @sub_nuw_chained_negative_constants(
 ; CHECK-NEXT:  entry:
@@ -468,14 +464,13 @@ define void @sub_nuw_chained_negative_constants(i8 %a) {
 ; CHECK-NEXT:    [[C_1:%.*]] = icmp ugt i8 [[SUB2]], 20
 ; CHECK-NEXT:    br i1 [[C_1]], label [[EXIT_1:%.*]], label [[EXIT_2:%.*]]
 ; CHECK:       exit.1:
-; CHECK-NEXT:    [[C_2:%.*]] = icmp ugt i8 [[A]], -96
-; CHECK-NEXT:    call void @use(i1 [[C_2]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[C_3:%.*]] = icmp ugt i8 [[A]], -95
 ; CHECK-NEXT:    call void @use(i1 [[C_3]])
 ; CHECK-NEXT:    ret void
 ; CHECK:       exit.2:
-; CHECK-NEXT:    call void @use(i1 true)
-; CHECK-NEXT:    call void @use(i1 true)
+; CHECK-NEXT:    call void @use(i1 false)
+; CHECK-NEXT:    call void @use(i1 false)
 ; CHECK-NEXT:    ret void
 ;
 entry: