[Scalarizer] Avoid changing name of non-instructions

[bjope]

Summary

The "takeName" logic in ScalarizerVisitor::gather did not consider
that the value vector could refer to non-instructions, such as
global variables. This patch make sure that we avoid changing the
name of a value if it isn't an instruction.

[lebedev.ri]

SGTM, sorry about that.

llvm/lib/Transforms/Scalar/Scalarizer.cpp

401

This could use a comment.

llvm/lib/Transforms/Scalar/Scalarizer.cpp

 //===- Scalarizer.cpp - Scalarize vector operations -----------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 //
void ScalarizerVisitor::gather(Instruction *Op, const ValueVector &CV) {
   ValueVector &SV = Scattered[Op];
   if (!SV.empty()) {
     for (unsigned I = 0, E = SV.size(); I != E; ++I) {
       Value *V = SV[I];
       if (V == nullptr || SV[I] == CV[I])
         continue;
 
       Instruction *Old = cast<Instruction>(V);
+      if (isa<Instruction>(CV[I]))
         CV[I]->takeName(Old);
       Old->replaceAllUsesWith(CV[I]);
       PotentiallyDeadInstrs.emplace_back(Old);
     }
   }
   SV = CV;
   Gathered.push_back(GatherList::value_type(Op, &SV));
 }
 

llvm/test/Transforms/Scalarizer/global-bug-2.ll

+; RUN: opt < %s -scalarizer -S -o - | FileCheck %s
+; RUN: opt < %s -passes='function(scalarizer)' -S | FileCheck %s
+
+; The scalarizer used to change the name of the global variable
+; Check that the we don't do that any longer.
+;
+; CHECK: @c.a = global i16 0, align 1
+
+@c.a = global i16 0, align 1
+
+define void @c() {
+entry:
+  br label %for.cond1
+
+for.cond1:                                        ; preds = %for.cond1, %entry
+  %d.sroa.0.0 = phi <4 x i16*> [ <i16* @c.a, i16* @c.a, i16* @c.a, i16* @c.a>, %entry ], [ %d.sroa.0.1.vec.insert, %for.cond1 ]
+  %d.sroa.0.0.vec.extract = extractelement <4 x i16*> %d.sroa.0.0, i32 0
+  %d.sroa.0.1.vec.insert = shufflevector <4 x i16*> <i16* @c.a, i16* null, i16* undef, i16* undef>, <4 x i16*> %d.sroa.0.0, <4 x i32> <i32 0, i32 1, i32 6, i32 7>
+  br label %for.cond1
+}