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

[Amara Emerson via llvm-commits]

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@@ -0,0 +1,57 @@
+//===- 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);
+  Register Undef = Merge->getSourceReg(1);
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Merge->getSourceReg(0));
+
+  //
+  //   %bits_8_15:_(s8) = G_IMPLICIT_DEF
+  //   %0:_(s16) = G_MERGE_VALUES %bits_0_7:(s8), %bits_8_15:(s8)
+  //
+  // ->
+  //
+  //   %0:_(s16) = G_ZEXT %bits_0_7:(s8)
+  //
+
+  if (!MRI.hasOneNonDBGUse(Undef) ||
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aemerson wrote:

Undefs are free to generate since they're not "real" instructions, so we shouldn't need this oneuse check, unless something weird is going on.

https://github.com/llvm/llvm-project/pull/113616