[PATCH] D126646: [DAGCombine] fold (A+B)-(A-C) -> B+C

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Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D126646

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/test/CodeGen/RISCV/sub-combine.ll


Index: llvm/test/CodeGen/RISCV/sub-combine.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/RISCV/sub-combine.ll
@@ -0,0 +1,21 @@
+; 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
+}
Index: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3516,6 +3516,11 @@
     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,


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