[PATCH] D86611: GlobalISel: Combine `op undef, x` to 0

[Volkan Keles via Phabricator via llvm-commits]

volkan created this revision.
volkan added reviewers: aditya_nandakumar, dsanders, mkitzan, arsenm, paquette, aemerson.
Herald added subscribers: hiraditya, rovka.
Herald added a project: LLVM.
volkan requested review of this revision.
Herald added a subscriber: wdng.

Add a combine to transform `op undef, x` to 0. Support only G_SHL for now.


https://reviews.llvm.org/D86611

Files:
  llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
  llvm/include/llvm/Target/GlobalISel/Combine.td
  llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir


Index: llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/combine-shl.mir
@@ -0,0 +1,15 @@
+# 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_shl_undef_x
+body:             |
+  bb.1:
+  liveins: $w0
+    ; CHECK-LABEL: name: test_combine_shl_undef_x
+    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; CHECK: $w0 = COPY [[C]](s32)
+    %0:_(s32) = COPY $w0
+    %1:_(s32) = G_IMPLICIT_DEF
+    %2:_(s32) = G_SHL %1(s32), %0(s32)
+    $w0 = COPY %2(s32)
+...
Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -1853,6 +1853,12 @@
                        MRI);
 }
 
+bool CombinerHelper::matchOperandIsUndef(MachineInstr &MI, unsigned OpIdx) {
+  MachineOperand &MO = MI.getOperand(OpIdx);
+  return MO.isReg() &&
+         getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF, MO.getReg(), MRI);
+}
+
 bool CombinerHelper::replaceInstWithFConstant(MachineInstr &MI, double C) {
   assert(MI.getNumDefs() == 1 && "Expected only one def?");
   Builder.setInstr(MI);
Index: llvm/include/llvm/Target/GlobalISel/Combine.td
===================================================================
--- llvm/include/llvm/Target/GlobalISel/Combine.td
+++ llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -194,6 +194,12 @@
          [{ return Helper.matchAnyExplicitUseIsUndef(*${root}); }]),
   (apply [{ Helper.replaceInstWithConstant(*${root}, -1); }])>;
 
+def binop_left_undef_to_zero: GICombineRule<
+  (defs root:$root),
+  (match (wip_match_opcode G_SHL):$root,
+         [{ return Helper.matchOperandIsUndef(*${root}, 1); }]),
+  (apply [{ Helper.replaceInstWithConstant(*${root}, 0); }])>;
+
 // Instructions where if any source operand is undef, the instruction can be
 // replaced with undef.
 def propagate_undef_any_op: GICombineRule<
@@ -319,6 +325,7 @@
 // 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,
+                                     binop_left_undef_to_zero,
                                      propagate_undef_any_op,
                                      propagate_undef_all_ops,
                                      propagate_undef_shuffle_mask,
Index: llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
+++ llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -297,6 +297,9 @@
   /// Check if operand \p OpIdx is zero.
   bool matchOperandIsZero(MachineInstr &MI, unsigned OpIdx);
 
+  /// Check if operand \p OpIdx is undef.
+  bool matchOperandIsUndef(MachineInstr &MI, unsigned OpIdx);
+
   /// Erase \p MI
   bool eraseInst(MachineInstr &MI);
 


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