Index: llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
+++ llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -231,6 +231,12 @@
   /// Optimize (x op x) -> x
   bool matchBinOpSameVal(MachineInstr &MI);
 
+  /// Return true if MI is a G_ADD which can be simplified to a G_SUB.
+  bool matchSimplifyAddToSub(MachineInstr &MI,
+                             std::tuple<Register, Register> &MatchInfo);
+  bool applySimplifyAddToSub(MachineInstr &MI,
+                             std::tuple<Register, Register> &MatchInfo);
+
   /// Try to transform \p MI by using all of the above
   /// combine functions. Returns true if changed.
   bool tryCombine(MachineInstr &MI);
Index: llvm/include/llvm/Target/GlobalISel/Combine.td
===================================================================
--- llvm/include/llvm/Target/GlobalISel/Combine.td
+++ llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -208,6 +208,14 @@
   (apply [{ return Helper.replaceSingleDefInstWithOperand(*${root}, 1); }])
 >;
 
+def simplify_add_to_sub_matchinfo: GIDefMatchData<"std::tuple<Register, Register>">;
+def simplify_add_to_sub: GICombineRule <
+  (defs root:$root, simplify_add_to_sub_matchinfo:$info),
+  (match (wip_match_opcode G_ADD):$root,
+    [{ return Helper.matchSimplifyAddToSub(*${root}, ${info}); }]),
+  (apply [{ return Helper.applySimplifyAddToSub(*${root}, ${info});}])
+>;
+
 // FIXME: These should use the custom predicate feature once it lands.
 def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
                                      undef_to_negative_one,
@@ -221,4 +229,4 @@
 def trivial_combines : GICombineGroup<[copy_prop, mul_to_shl]>;
 def all_combines : GICombineGroup<[trivial_combines, ptr_add_immed_chain,
     combines_for_extload, combine_indexed_load_store, undef_combines,
-    identity_combines]>;
+    identity_combines, simplify_add_to_sub]>;
Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -1604,6 +1604,38 @@
   return true;
 }
 
+bool CombinerHelper::matchSimplifyAddToSub(
+    MachineInstr &MI, std::tuple<Register, Register> &MatchInfo) {
+  Register LHS = MI.getOperand(1).getReg();
+  Register RHS = MI.getOperand(2).getReg();
+  Register &NewLHS = std::get<0>(MatchInfo);
+  Register &NewRHS = std::get<1>(MatchInfo);
+
+  // Helper lambda to check for opportunities for
+  // ((0-A) + B) -> B - A
+  // (A + (0-B)) -> A - B
+  auto CheckFold = [&](Register &MaybeSub, Register &MaybeNewLHS) {
+    int64_t Cst;
+    if (!mi_match(MaybeSub, MRI, m_GSub(m_ICst(Cst), m_Reg(NewRHS))) ||
+        Cst != 0)
+      return false;
+    NewLHS = MaybeNewLHS;
+    return true;
+  };
+
+  return CheckFold(LHS, RHS) || CheckFold(RHS, LHS);
+}
+
+bool CombinerHelper::applySimplifyAddToSub(
+    MachineInstr &MI, std::tuple<Register, Register> &MatchInfo) {
+  Builder.setInstr(MI);
+  Register SubLHS, SubRHS;
+  std::tie(SubLHS, SubRHS) = MatchInfo;
+  Builder.buildSub(MI.getOperand(0).getReg(), SubLHS, SubRHS);
+  MI.eraseFromParent();
+  return true;
+}
+
 bool CombinerHelper::tryCombine(MachineInstr &MI) {
   if (tryCombineCopy(MI))
     return true;
Index: llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-simplify-add.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-simplify-add.mir
@@ -0,0 +1,48 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
+
+name:            pat1
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x0, $x1
+    ; Fold ((0-A) + B) -> B - A
+    ; CHECK-LABEL: name: pat1
+    ; CHECK: liveins: $x0, $x1
+    ; CHECK: %copy1:_(s64) = COPY $x0
+    ; CHECK: %copy2:_(s64) = COPY $x1
+    ; CHECK: %add:_(s64) = G_SUB %copy2, %copy1
+    ; CHECK: $x0 = COPY %add(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %copy1:_(s64) = COPY $x0
+    %copy2:_(s64) = COPY $x1
+    %zero:_(s64) = G_CONSTANT i64 0
+    %sub:_(s64) = G_SUB %zero, %copy1
+    %add:_(s64) = G_ADD %sub, %copy2
+    $x0 = COPY %add(s64)
+    RET_ReallyLR implicit $x0
+
+...
+---
+name:            pat2
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x0, $x1
+    ; Fold (A + (0-B)) -> A - B
+    ; CHECK-LABEL: name: pat2
+    ; CHECK: liveins: $x0, $x1
+    ; CHECK: %copy1:_(s64) = COPY $x0
+    ; CHECK: %copy2:_(s64) = COPY $x1
+    ; CHECK: %add:_(s64) = G_SUB %copy1, %copy2
+    ; CHECK: $x0 = COPY %add(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %copy1:_(s64) = COPY $x0
+    %copy2:_(s64) = COPY $x1
+    %zero:_(s64) = G_CONSTANT i64 0
+    %sub:_(s64) = G_SUB %zero, %copy2
+    %add:_(s64) = G_ADD %copy1, %sub
+    $x0 = COPY %add(s64)
+    RET_ReallyLR implicit $x0
+
+...