[ConstraintElimination] Support add with precondition.

If we can prove that an addition without wrap flags won't wrap, decompse
the operation.

Issue #48253

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -229,6 +229,12 @@ decompose(Value *V, SmallVector<PreconditionTy, 4> &Preconditions,
   ConstantInt *CI;
   if (match(V, m_NUWAdd(m_Value(Op0), m_ConstantInt(CI))))
     return {{CI->getSExtValue(), nullptr}, {1, Op0}};
+  if (match(V, m_Add(m_Value(Op0), m_ConstantInt(CI))) && CI->isNegative()) {
+    Preconditions.emplace_back(
+        CmpInst::ICMP_UGE, Op0,
+        ConstantInt::get(Op0->getType(), CI->getSExtValue() * -1));
+    return {{CI->getSExtValue(), nullptr}, {1, Op0}};
+  }
   if (match(V, m_NUWAdd(m_Value(Op0), m_Value(Op1))))
     return {{0, nullptr}, {1, Op0}, {1, Op1}};
 

llvm/test/Transforms/ConstraintElimination/wrapping-math.ll

@@ -22,7 +22,7 @@ define i1 @wrapping_add_known_1(i8 %a) {
 ; CHECK:       then:
 ; CHECK-NEXT:    [[SUB_1:%.*]] = add i8 [[A]], -1
 ; CHECK-NEXT:    [[C_1:%.*]] = icmp eq i8 [[SUB_1]], 0
-; CHECK-NEXT:    ret i1 [[C_1]]
+; CHECK-NEXT:    ret i1 true
 ; CHECK:       else:
 ; CHECK-NEXT:    [[SUB_2:%.*]] = add i8 [[A]], -1
 ; CHECK-NEXT:    [[C_2:%.*]] = icmp eq i8 [[SUB_2]], 0
@@ -85,7 +85,7 @@ define i1 @test_48253_eq_ne(i8 %a, i8 %b) {
 ; CHECK-NEXT:    [[T_1:%.*]] = icmp ult i8 [[SUB_1]], [[A]]
 ; CHECK-NEXT:    [[SUB_2:%.*]] = add i8 [[B]], -2
 ; CHECK-NEXT:    [[C_2:%.*]] = icmp ult i8 [[SUB_2]], [[A]]
-; CHECK-NEXT:    [[XOR_1:%.*]] = xor i1 [[T_1]], [[C_2]]
+; CHECK-NEXT:    [[XOR_1:%.*]] = xor i1 true, [[C_2]]
 ; CHECK-NEXT:    ret i1 [[XOR_1]]
 ; CHECK:       exit.1:
 ; CHECK-NEXT:    [[SUB_3:%.*]] = add i8 [[B]], -1
@@ -133,9 +133,9 @@ define i1 @test_ult(i8 %a, i8 %b) {
 ; CHECK:       if.end:
 ; CHECK-NEXT:    [[T_1:%.*]] = icmp ult i8 [[SUB_1]], [[B]]
 ; CHECK-NEXT:    [[T_2:%.*]] = icmp ult i8 [[SUB_2]], [[B]]
-; CHECK-NEXT:    [[XOR_1:%.*]] = xor i1 [[T_1]], [[T_2]]
+; CHECK-NEXT:    [[XOR_1:%.*]] = xor i1 true, true
 ; CHECK-NEXT:    [[T_3:%.*]] = icmp ult i8 [[SUB_3]], [[B]]
-; CHECK-NEXT:    [[XOR_2:%.*]] = xor i1 [[XOR_1]], [[T_3]]
+; CHECK-NEXT:    [[XOR_2:%.*]] = xor i1 [[XOR_1]], true
 ; CHECK-NEXT:    [[C_1:%.*]] = icmp ult i8 [[SUB_4]], [[B]]
 ; CHECK-NEXT:    [[XOR_3:%.*]] = xor i1 [[XOR_2]], [[C_1]]
 ; CHECK-NEXT:    [[C_2:%.*]] = icmp ult i8 [[ADD_1]], [[B]]