[GlobalISel] Combine G_MERGE_VALUES of x and undef (#113616)

into anyext x

; CHECK-NEXT: [[MV1:%[0-9]+]]:_(s64) = G_MERGE_VALUES [[TRUNC]](s32),
[[DEF]](s32)

Please continue padding merge values.

//   %bits_8_15:_(s8) = G_IMPLICIT_DEF
//   %0:_(s16) = G_MERGE_VALUES %bits_0_7:(s8), %bits_8_15:(s8)

%bits_8_15 is defined by undef. Its value is undefined and we can pick
an arbitrary value. For optimization, we pick anyext, which plays well
with the undefinedness.

//   %0:_(s16) = G_ANYEXT %bits_0_7:(s8)

The upper bits of %0 are undefined and the lower bits come from
%bits_0_7.

Author: Thorsten Schütt

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

@@ -929,6 +929,9 @@ class CombinerHelper {
   bool matchUnmergeValuesAnyExtBuildVector(const MachineInstr &MI,
                                            BuildFnTy &MatchInfo);
 
+  // merge_values(_, undef) -> anyext
+  bool matchMergeXAndUndef(const MachineInstr &MI, BuildFnTy &MatchInfo);
+
 private:
   /// Checks for legality of an indexed variant of \p LdSt.
   bool isIndexedLoadStoreLegal(GLoadStore &LdSt) const;

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

@@ -857,14 +857,23 @@ def unmerge_zext_to_zext : GICombineRule<
   (apply [{ Helper.applyCombineUnmergeZExtToZExt(*${d}); }])
 >;
 
+/// Transform merge_x_undef -> anyext.
+def merge_of_x_and_undef : GICombineRule <
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (G_IMPLICIT_DEF $undef),
+         (G_MERGE_VALUES $root, $x, $undef):$MI,
+         [{ return Helper.matchMergeXAndUndef(*${MI}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${MI}, ${matchinfo}); }])>;
+
 def merge_combines: GICombineGroup<[
   unmerge_anyext_build_vector,
   unmerge_merge,
   merge_unmerge,
   unmerge_cst,
   unmerge_undef,
   unmerge_dead_to_trunc,
-  unmerge_zext_to_zext
+  unmerge_zext_to_zext,
+  merge_of_x_and_undef
 ]>;
 
 // Under certain conditions, transform:

llvm/lib/CodeGen/GlobalISel/CMakeLists.txt

@@ -6,6 +6,7 @@ add_llvm_component_library(LLVMGlobalISel
   GlobalISel.cpp
   Combiner.cpp
   CombinerHelper.cpp
+  CombinerHelperArtifacts.cpp
   CombinerHelperCasts.cpp
   CombinerHelperCompares.cpp
   CombinerHelperVectorOps.cpp

llvm/lib/CodeGen/GlobalISel/CombinerHelperArtifacts.cpp

@@ -0,0 +1,58 @@
+//===- CombinerHelperArtifacts.cpp-----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements CombinerHelper for legalization artifacts.
+//
+//===----------------------------------------------------------------------===//
+//
+// G_MERGE_VALUES
+//
+//===----------------------------------------------------------------------===//
+#include "llvm/CodeGen/GlobalISel/CombinerHelper.h"
+#include "llvm/CodeGen/GlobalISel/LegalizerHelper.h"
+#include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
+#include "llvm/CodeGen/GlobalISel/Utils.h"
+#include "llvm/CodeGen/LowLevelTypeUtils.h"
+#include "llvm/CodeGen/MachineOperand.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetOpcodes.h"
+#include "llvm/Support/Casting.h"
+
+#define DEBUG_TYPE "gi-combiner"
+
+using namespace llvm;
+
+bool CombinerHelper::matchMergeXAndUndef(const MachineInstr &MI,
+                                         BuildFnTy &MatchInfo) {
+  const GMerge *Merge = cast<GMerge>(&MI);
+
+  Register Dst = Merge->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Merge->getSourceReg(0));
+
+  // Otherwise, we would miscompile.
+  assert(Merge->getNumSources() == 2 && "Unexpected number of operands");
+
+  //
+  //   %bits_8_15:_(s8) = G_IMPLICIT_DEF
+  //   %0:_(s16) = G_MERGE_VALUES %bits_0_7:(s8), %bits_8_15:(s8)
+  //
+  // ->
+  //
+  //   %0:_(s16) = G_ANYEXT %bits_0_7:(s8)
+  //
+
+  if (!isLegalOrBeforeLegalizer({TargetOpcode::G_ANYEXT, {DstTy, SrcTy}}))
+    return false;
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    B.buildAnyExt(Dst, Merge->getSourceReg(0));
+  };
+  return true;
+}

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

@@ -10,9 +10,8 @@ body:             |
   bb.1:
     ; CHECK-LABEL: name: test_combine_unmerge_merge
     ; CHECK: [[DEF:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
-    ; CHECK-NEXT: [[DEF1:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
     ; CHECK-NEXT: $w0 = COPY [[DEF]](s32)
-    ; CHECK-NEXT: $w1 = COPY [[DEF1]](s32)
+    ; CHECK-NEXT: $w1 = COPY [[DEF]](s32)
     %0:_(s32) = G_IMPLICIT_DEF
     %1:_(s32) = G_IMPLICIT_DEF
     %2:_(s64) = G_MERGE_VALUES %0(s32), %1(s32)
@@ -115,9 +114,8 @@ body:             |
   bb.1:
     ; CHECK-LABEL: name: test_combine_unmerge_bitcast_merge
     ; CHECK: [[DEF:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
-    ; CHECK-NEXT: [[DEF1:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
     ; CHECK-NEXT: $w0 = COPY [[DEF]](s32)
-    ; CHECK-NEXT: $w1 = COPY [[DEF1]](s32)
+    ; CHECK-NEXT: $w1 = COPY [[DEF]](s32)
     %0:_(s32) = G_IMPLICIT_DEF
     %1:_(s32) = G_IMPLICIT_DEF
     %2:_(s64) = G_MERGE_VALUES %0(s32), %1(s32)
@@ -135,14 +133,11 @@ name:            test_combine_unmerge_merge_incompatible_types
 body:             |
   bb.1:
     ; CHECK-LABEL: name: test_combine_unmerge_merge_incompatible_types
-    ; CHECK: [[DEF:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
-    ; CHECK-NEXT: [[DEF1:%[0-9]+]]:_(s32) = G_IMPLICIT_DEF
-    ; CHECK-NEXT: [[MV:%[0-9]+]]:_(s64) = G_MERGE_VALUES [[DEF]](s32), [[DEF1]](s32)
-    ; CHECK-NEXT: [[UV:%[0-9]+]]:_(s16), [[UV1:%[0-9]+]]:_(s16), [[UV2:%[0-9]+]]:_(s16), [[UV3:%[0-9]+]]:_(s16) = G_UNMERGE_VALUES [[MV]](s64)
-    ; CHECK-NEXT: $h0 = COPY [[UV]](s16)
-    ; CHECK-NEXT: $h1 = COPY [[UV1]](s16)
-    ; CHECK-NEXT: $h2 = COPY [[UV2]](s16)
-    ; CHECK-NEXT: $h3 = COPY [[UV3]](s16)
+    ; CHECK: [[DEF:%[0-9]+]]:_(s16) = G_IMPLICIT_DEF
+    ; CHECK-NEXT: $h0 = COPY [[DEF]](s16)
+    ; CHECK-NEXT: $h1 = COPY [[DEF]](s16)
+    ; CHECK-NEXT: $h2 = COPY [[DEF]](s16)
+    ; CHECK-NEXT: $h3 = COPY [[DEF]](s16)
     %0:_(s32) = G_IMPLICIT_DEF
     %1:_(s32) = G_IMPLICIT_DEF
     %2:_(s64) = G_MERGE_VALUES %0(s32), %1(s32)
@@ -539,3 +534,36 @@ body:             |
     $q0 = COPY %un1(s128)
     $q1 = COPY %un2(s128)
 ...
+
+# Check that we anyext the merge
+---
+name:            test_merge_undef
+body:             |
+  bb.1:
+    ; CHECK-LABEL: name: test_merge_undef
+    ; CHECK: %opaque:_(s64) = COPY $x0
+    ; CHECK-NEXT: %me:_(s128) = G_ANYEXT %opaque(s64)
+    ; CHECK-NEXT: $q0 = COPY %me(s128)
+    %opaque:_(s64) = COPY $x0
+    %def:_(s64) = G_IMPLICIT_DEF
+    %me:_(s128) = G_MERGE_VALUES %opaque(s64), %def
+    $q0 = COPY %me(s128)
+...
+
+# Check that we don't anyext the merge, multi-use
+---
+name:            test_merge_undef_multi_use
+body:             |
+  bb.1:
+    ; CHECK-LABEL: name: test_merge_undef_multi_use
+    ; CHECK: %opaque:_(s64) = COPY $x0
+    ; CHECK-NEXT: %def:_(s64) = G_IMPLICIT_DEF
+    ; CHECK-NEXT: %me:_(s128) = G_ANYEXT %opaque(s64)
+    ; CHECK-NEXT: $q0 = COPY %me(s128)
+    ; CHECK-NEXT: $x0 = COPY %def(s64)
+    %opaque:_(s64) = COPY $x0
+    %def:_(s64) = G_IMPLICIT_DEF
+    %me:_(s128) = G_MERGE_VALUES %opaque(s64), %def
+    $q0 = COPY %me(s128)
+    $x0 = COPY %def(s64)
+...

llvm/test/CodeGen/AArch64/bswap.ll

@@ -60,7 +60,6 @@ define i128 @bswap_i16_to_i128_anyext(i16 %a) {
 ; CHECK-GI-NEXT:    mov x0, xzr
 ; CHECK-GI-NEXT:    rev w8, w8
 ; CHECK-GI-NEXT:    lsr w8, w8, #16
-; CHECK-GI-NEXT:    bfi x8, x8, #32, #32
 ; CHECK-GI-NEXT:    and x8, x8, #0xffff
 ; CHECK-GI-NEXT:    lsl x1, x8, #48
 ; CHECK-GI-NEXT:    ret