Index: llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
+++ llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -217,6 +217,10 @@
   /// equivalent instructions.
   bool matchEqualDefs(const MachineOperand &MOP1, const MachineOperand &MOP2);
 
+  /// Return true if \p MOP is defined by a G_CONSTANT with a value equal to
+  /// \p C.
+  bool matchConstantOp(const MachineOperand &MOP, int64_t C);
+
   /// Optimize (cond ? x : x) -> x
   bool matchSelectSameVal(MachineInstr &MI);
 
Index: llvm/include/llvm/Target/GlobalISel/Combine.td
===================================================================
--- llvm/include/llvm/Target/GlobalISel/Combine.td
+++ llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -192,6 +192,14 @@
   (apply [{ return Helper.replaceSingleDefInstWithOperand(*${root}, 2); }])
 >;
 
+// Fold x - 0 -> x
+def sub_identity: GICombineRule<
+  (defs root:$root),
+  (match (wip_match_opcode G_SUB):$root,
+    [{ return Helper.matchConstantOp(${root}->getOperand(2), 0); }]),
+  (apply [{ return Helper.replaceSingleDefInstWithOperand(*${root}, 1); }])
+>;
+
 // 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,
@@ -199,7 +207,7 @@
                                      propagate_undef_all_ops,
                                      propagate_undef_shuffle_mask]>;
 
-def identity_combines : GICombineGroup<[select_same_val]>;
+def identity_combines : GICombineGroup<[select_same_val, sub_identity]>;
 
 def trivial_combines : GICombineGroup<[copy_prop, mul_to_shl]>;
 def all_combines : GICombineGroup<[trivial_combines, ptr_add_immed_chain,
Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -1519,6 +1519,13 @@
   return Builder.getTII().produceSameValue(*I1, *I2, &MRI);
 }
 
+bool CombinerHelper::matchConstantOp(const MachineOperand &MOP, int64_t C) {
+  if (!MOP.isReg())
+    return false;
+  auto ValAndVReg = getConstantVRegValWithLookThrough(MOP.getReg(), MRI);
+  return ValAndVReg && ValAndVReg->Value == C;
+}
+
 bool CombinerHelper::replaceSingleDefInstWithOperand(MachineInstr &MI,
                                                      unsigned OpIdx) {
   assert(OpIdx < MI.getNumOperands() && "OpIdx is out of bounds");
Index: llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir
@@ -0,0 +1,22 @@
+# 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:            right_ident_sub
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    liveins: $w0
+    ; Fold (x - 0) -> x
+    ;
+    ; CHECK-LABEL: name: right_ident_sub
+    ; CHECK: liveins: $w0
+    ; CHECK: %x:_(s32) = COPY $w0
+    ; CHECK: $w0 = COPY %x(s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %x:_(s32) = COPY $w0
+    %cst:_(s32) = G_CONSTANT i32 0
+    %op:_(s32) = G_SUB %x(s32), %cst
+    $w0 = COPY %op(s32)
+    RET_ReallyLR implicit $w0