From c4e63c6e3635d6bff650eafea958dce91042ded8 Mon Sep 17 00:00:00 2001
From: Volkan Keles <vkeles@apple.com>
Date: Tue, 8 Sep 2020 09:46:38 -0700
Subject: [PATCH] GlobalISel: Combine `op undef, x` to 0

https://reviews.llvm.org/D86611
---
 .../llvm/CodeGen/GlobalISel/CombinerHelper.h  |  3 ++
 include/llvm/Target/GlobalISel/Combine.td     |  7 +++++
 lib/CodeGen/GlobalISel/CombinerHelper.cpp     |  6 ++++
 .../AArch64/GlobalISel/combine-shl.mir        | 29 +++++++++++++++++++
 4 files changed, 45 insertions(+)
 create mode 100644 test/CodeGen/AArch64/GlobalISel/combine-shl.mir

diff --git a/include/llvm/CodeGen/GlobalISel/CombinerHelper.h b/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
index 8607ad02d50..cff6b496cca 100644
--- a/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
+++ b/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
@@ -321,6 +321,9 @@ public:
   /// 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);
 
diff --git a/include/llvm/Target/GlobalISel/Combine.td b/include/llvm/Target/GlobalISel/Combine.td
index 6a6f97ae78b..5b940551dad 100644
--- a/include/llvm/Target/GlobalISel/Combine.td
+++ b/include/llvm/Target/GlobalISel/Combine.td
@@ -194,6 +194,12 @@ def undef_to_negative_one: GICombineRule<
          [{ 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<
@@ -384,6 +390,7 @@ def not_cmp_fold : GICombineRule<
 // 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,
diff --git a/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/lib/CodeGen/GlobalISel/CombinerHelper.cpp
index 10cd58f17e9..d58ba7cf5a8 100644
--- a/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -1989,6 +1989,12 @@ bool CombinerHelper::matchOperandIsZero(MachineInstr &MI, unsigned OpIdx) {
                        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);
diff --git a/test/CodeGen/AArch64/GlobalISel/combine-shl.mir b/test/CodeGen/AArch64/GlobalISel/combine-shl.mir
new file mode 100644
index 00000000000..fe75f9965bc
--- /dev/null
+++ b/test/CodeGen/AArch64/GlobalISel/combine-shl.mir
@@ -0,0 +1,29 @@
+# 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_s32
+body:             |
+  bb.1:
+  liveins: $w0
+    ; CHECK-LABEL: name: test_combine_shl_undef_x_s32
+    ; 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)
+...
+---
+name:            test_combine_shl_undef_x_v2s32
+body:             |
+  bb.1:
+  liveins: $d0
+    ; CHECK-LABEL: name: test_combine_shl_undef_x_v2s32
+    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; CHECK: [[BUILD_VECTOR:%[0-9]+]]:_(<2 x s32>) = G_BUILD_VECTOR [[C]](s32), [[C]](s32)
+    ; CHECK: $d0 = COPY [[BUILD_VECTOR]](<2 x s32>)
+    %0:_(<2 x s32>) = COPY $d0
+    %1:_(<2 x s32>) = G_IMPLICIT_DEF
+    %2:_(<2 x s32>) = G_SHL %1(<2 x s32>), %0(<2 x s32>)
+    $d0 = COPY %2(<2 x s32>)
+...