[GISel]: Add combine for G_FABS to G_FABS

https://reviews.llvm.org/D87554

Patch adds one new GICombinerRule for G_FABS. The combine rule folds G_FABS(G_FABS(X)) to G_FABS(X).
Patch additionally adds new combiner tests for the AArch64 target to test this new combiner rule.

Patch by mkitzan.

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

@@ -280,6 +280,10 @@ public:
   /// Transform fneg(fneg(x)) to x.
   bool matchCombineFNegOfFNeg(MachineInstr &MI, Register &Reg);
 
+  /// Match fabs(fabs(x)) to fabs(x).
+  bool matchCombineFAbsOfFAbs(MachineInstr &MI, Register &Src);
+  bool applyCombineFAbsOfFAbs(MachineInstr &MI, Register &Src);
+
   /// 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

@@ -403,6 +403,15 @@ def unmerge_merge : GICombineRule<
   (apply [{ return Helper.applyCombineUnmergeMergeToPlainValues(*${d}, ${info}); }])
 >;
 
+// Fold (fabs (fabs x)) -> (fabs x).
+def fabs_fabs_fold_matchinfo : GIDefMatchData<"Register">;
+def fabs_fabs_fold: GICombineRule<
+  (defs root:$root, fabs_fabs_fold_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_FABS):$root,
+         [{ return Helper.matchCombineFAbsOfFAbs(*${root}, ${matchinfo}); }]),
+  (apply [{ return Helper.applyCombineFAbsOfFAbs(*${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,
@@ -433,4 +442,5 @@ def all_combines : GICombineGroup<[trivial_combines, ptr_add_immed_chain,
     shl_ashr_to_sext_inreg, sext_inreg_of_load,
     width_reduction_combines, select_combines,
     known_bits_simplifications, ext_ext_fold,
-    not_cmp_fold, opt_brcond_by_inverting_cond, unmerge_merge]>;
+    not_cmp_fold, opt_brcond_by_inverting_cond,
+    unmerge_merge, fabs_fabs_fold]>;

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -1878,6 +1878,21 @@ bool CombinerHelper::matchCombineFNegOfFNeg(MachineInstr &MI, Register &Reg) {
   return mi_match(SrcReg, MRI, m_GFNeg(m_Reg(Reg)));
 }
 
+bool CombinerHelper::matchCombineFAbsOfFAbs(MachineInstr &MI, Register &Src) {
+  assert(MI.getOpcode() == TargetOpcode::G_FABS && "Expected a G_FABS");
+  Src = MI.getOperand(1).getReg();
+  Register AbsSrc;
+  return mi_match(Src, MRI, m_GFabs(m_Reg(AbsSrc)));
+}
+
+bool CombinerHelper::applyCombineFAbsOfFAbs(MachineInstr &MI, Register &Src) {
+  assert(MI.getOpcode() == TargetOpcode::G_FABS && "Expected a G_FABS");
+  Register Dst = MI.getOperand(0).getReg();
+  MI.eraseFromParent();
+  replaceRegWith(MRI, Dst, Src);
+  return true;
+}
+
 bool CombinerHelper::matchAnyExplicitUseIsUndef(MachineInstr &MI) {
   return any_of(MI.explicit_uses(), [this](const MachineOperand &MO) {
     return MO.isReg() &&

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

@@ -0,0 +1,32 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs -mtriple aarch64-unknown-unknown %s -o - | FileCheck %s
+# RUN: llc -debugify-and-strip-all-safe -run-pass=aarch64-prelegalizer-combiner -verify-machineinstrs -mtriple aarch64-unknown-unknown %s -o - | FileCheck %s
+
+---
+name:            test_combine_fabs_fabs
+body:             |
+  bb.1:
+  liveins: $w0
+    ; CHECK-LABEL: name: test_combine_fabs_fabs
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK: [[FABS:%[0-9]+]]:_(s32) = G_FABS [[COPY]]
+    ; CHECK: $w0 = COPY [[FABS]](s32)
+    %0:_(s32) = COPY $w0
+    %1:_(s32) = G_FABS %0(s32)
+    %2:_(s32) = G_FABS %1(s32)
+    $w0 = COPY %2(s32)
+...
+---
+name:            test_combine_fabs_fabs_vec
+body:             |
+  bb.1:
+  liveins: $x0
+    ; CHECK-LABEL: name: test_combine_fabs_fabs_vec
+    ; CHECK: [[COPY:%[0-9]+]]:_(<2 x s32>) = COPY $x0
+    ; CHECK: [[FABS:%[0-9]+]]:_(<2 x s32>) = G_FABS [[COPY]]
+    ; CHECK: $x0 = COPY [[FABS]](<2 x s32>)
+    %0:_(<2 x s32>) = COPY $x0
+    %1:_(<2 x s32>) = G_FABS %0(<2 x s32>)
+    %2:_(<2 x s32>) = G_FABS %1(<2 x s32>)
+    $x0 = COPY %2(<2 x s32>)
+...