[DAGCombine] fold (A+B)-(A-C) -> B+C

[nikic]

This is already handled by InstCombine: https://llvm.godbolt.org/z/dGzGzGqsc Is there legalization code that produces this pattern?

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

return DAG.getNode(ISD::ADD, DL, VT, N0.getOperand(0),
                        N0.getOperand(1).getOperand(0));
 
   // fold ((A-(B-C))-C) -> A-B
   if (N0.getOpcode() == ISD::SUB && N0.getOperand(1).getOpcode() == ISD::SUB &&
       N0.getOperand(1).getOperand(1) == N1)
     return DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0),
                        N0.getOperand(1).getOperand(0));
 
+  // fold (A+B)-(A-C) -> B+C
+  if (N0.getOpcode() == ISD::ADD && N1.getOpcode() == ISD::SUB &&
+      N0.getOperand(0) == N1.getOperand(0))
+    return DAG.getNode(ISD::ADD, DL, VT, N1.getOperand(1), N0.getOperand(1));
+
   // fold (A-(B-C)) -> A+(C-B)
   if (N1.getOpcode() == ISD::SUB && N1.hasOneUse())
     return DAG.getNode(ISD::ADD, DL, VT, N0,
                        DAG.getNode(ISD::SUB, DL, VT, N1.getOperand(1),
                                    N1.getOperand(0)));
 
   // A - (A & B)  ->  A & (~B)
   if (N1.getOpcode() == ISD::AND) {

llvm/test/CodeGen/RISCV/sub-combine.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=riscv32 -verify-machineinstrs < %s \
+; RUN:   | FileCheck -check-prefix=RV32I %s
+; RUN: llc -mtriple=riscv64 -verify-machineinstrs < %s \
+; RUN:   | FileCheck -check-prefix=RV64I %s
+
+define signext i32 @sub_combine(i32 %a, i32 %b, i32 %c) nounwind {
+; RV32I-LABEL: sub_combine:
+; RV32I:       # %bb.0:
+; RV32I-NEXT:    add a0, a2, a1
+; RV32I-NEXT:    ret
+;
+; RV64I-LABEL: sub_combine:
+; RV64I:       # %bb.0:
+; RV64I-NEXT:    addw a0, a2, a1
+; RV64I-NEXT:    ret
+  %1 = add i32 %a, %b
+  %2 = sub i32 %a, %c
+  %3 = sub i32 %1, %2
+  ret i32 %3
+}