GlobalISel: Combine fneg(fneg x) to x

https://reviews.llvm.org/D87473

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

@@ -269,6 +269,9 @@ public:
   bool applyCombineExtOfExt(MachineInstr &MI,
                             std::tuple<Register, unsigned> &MatchInfo);
 
+  /// Transform fneg(fneg(x)) to x.
+  bool matchCombineFNegOfFNeg(MachineInstr &MI, Register &Reg);
+
   /// Return true if any explicit use operand on \p MI is defined by a
   /// G_IMPLICIT_DEF.
   bool matchAnyExplicitUseIsUndef(MachineInstr &MI);

llvm/include/llvm/Target/GlobalISel/Combine.td

@@ -385,6 +385,15 @@ def not_cmp_fold : GICombineRule<
   (apply [{ return Helper.applyNotCmp(*${d}, ${info}); }])
 >;
 
+// Fold (fneg (fneg x)) -> x.
+def fneg_fneg_fold_matchinfo : GIDefMatchData<"Register">;
+def fneg_fneg_fold: GICombineRule <
+  (defs root:$root, fneg_fneg_fold_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_FNEG):$root,
+         [{ return Helper.matchCombineFNegOfFNeg(*${root}, ${matchinfo}); }]),
+  (apply [{ return Helper.replaceSingleDefInstWithReg(*${root}, ${matchinfo}); }])
+>;
+
 // 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,
@@ -397,7 +406,8 @@ def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
 def identity_combines : GICombineGroup<[select_same_val, right_identity_zero,
                                         binop_same_val, binop_left_to_zero,
                                         binop_right_to_zero, p2i_to_i2p,
-                                        i2p_to_p2i, anyext_trunc_fold]>;
+                                        i2p_to_p2i, anyext_trunc_fold,
+                                        fneg_fneg_fold]>;
 
 def known_bits_simplifications : GICombineGroup<[
   and_trivial_mask, redundant_sext_inreg]>;

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -1813,6 +1813,12 @@ bool CombinerHelper::applyCombineExtOfExt(
   return false;
 }
 
+bool CombinerHelper::matchCombineFNegOfFNeg(MachineInstr &MI, Register &Reg) {
+  assert(MI.getOpcode() == TargetOpcode::G_FNEG && "Expected a G_FNEG");
+  Register SrcReg = MI.getOperand(1).getReg();
+  return mi_match(SrcReg, MRI, m_GFNeg(m_Reg(Reg)));
+}
+
 bool CombinerHelper::matchAnyExplicitUseIsUndef(MachineInstr &MI) {
   return any_of(MI.explicit_uses(), [this](const MachineOperand &MO) {
     return MO.isReg() &&

llvm/test/CodeGen/AArch64/GlobalISel/combine-fneg.mir

@@ -0,0 +1,28 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -o - -mtriple=aarch64-unknown-unknown -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs  %s | FileCheck %s
+---
+name:            test_combine_fneg_fneg
+body:             |
+  bb.1:
+  liveins: $w0
+    ; CHECK-LABEL: name: test_combine_fneg_fneg
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK: $w0 = COPY [[COPY]](s32)
+    %0:_(s32) = COPY $w0
+    %1:_(s32) = G_FNEG %0(s32)
+    %2:_(s32) = G_FNEG %1(s32)
+    $w0 = COPY %2(s32)
+...
+---
+name:            test_combine_fneg_fneg_vec
+body:             |
+  bb.1:
+  liveins: $x0
+    ; CHECK-LABEL: name: test_combine_fneg_fneg_vec
+    ; CHECK: [[COPY:%[0-9]+]]:_(<2 x s32>) = COPY $x0
+    ; CHECK: $x0 = COPY [[COPY]](<2 x s32>)
+    %0:_(<2 x s32>) = COPY $x0
+    %1:_(<2 x s32>) = G_FNEG %0(<2 x s32>)
+    %2:_(<2 x s32>) = G_FNEG %1(<2 x s32>)
+    $x0 = COPY %2(<2 x s32>)
+...