[LTO] Don't create a new common unless merged has different size

[tejohnson]

Summary

This addresses a regression in common handling from the new LTO
API in r278338. Only create a new common if the size is different.
The type comparison against an array type fails when the size is
different but not an array. GlobalMerge does not handle the
array types as well and we lose some global merging opportunities.

[mehdi_amini]

LGTM.

llvm/trunk/lib/LTO/LTO.cpp

if (RegularLTO.CombinedModule)
     if (auto E = runRegularLTO(AddOutput))
       return E;
   return runThinLTO(AddOutput);
 }
 
 Error LTO::runRegularLTO(AddOutputFn AddOutput) {
   // Make sure commons have the right size/alignment: we kept the largest from
   // all the prevailing when adding the inputs, and we apply it here.
+  const DataLayout &DL = RegularLTO.CombinedModule->getDataLayout();
   for (auto &I : RegularLTO.Commons) {
-    ArrayType *Ty =
-        ArrayType::get(Type::getInt8Ty(RegularLTO.Ctx), I.second.Size);
     GlobalVariable *OldGV = RegularLTO.CombinedModule->getNamedGlobal(I.first);
-    if (OldGV && OldGV->getType()->getElementType() == Ty) {
+    if (OldGV && DL.getTypeAllocSize(OldGV->getValueType()) == I.second.Size) {
       // Don't create a new global if the type is already correct, just make
       // sure the alignment is correct.
       OldGV->setAlignment(I.second.Align);
       continue;
     }
+    ArrayType *Ty =
+        ArrayType::get(Type::getInt8Ty(RegularLTO.Ctx), I.second.Size);
     auto *GV = new GlobalVariable(*RegularLTO.CombinedModule, Ty, false,
                                   GlobalValue::CommonLinkage,
                                   ConstantAggregateZero::get(Ty), "");
     GV->setAlignment(I.second.Align);
     if (OldGV) {
       OldGV->replaceAllUsesWith(ConstantExpr::getBitCast(GV, OldGV->getType()));
       GV->takeName(OldGV);
       OldGV->eraseFromParent();

llvm/trunk/test/tools/gold/X86/common.ll

 ; A: @a = common global [4 x i8] zeroinitializer, align 8
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
 ; RUN:    -shared %t1.o %t2b.o -o %t3.o
 ; RUN: llvm-dis %t3.o -o - | FileCheck %s --check-prefix=B
 
 ; (i16 align 8) + (i8 align 16) = i16 align 16
-; B: @a = common global [2 x i8] zeroinitializer, align 16
+; B: @a = common global i16 0, align 16
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
 ; RUN:    -shared %t1.o %t2c.o -o %t3.o
 ; RUN: llvm-dis %t3.o -o - | FileCheck %s --check-prefix=C
 
 ; (i16 align 8) + (i8 align 1) = i16 align 8.
-; C: @a = common global [2 x i8] zeroinitializer, align 8
+; C: @a = common global i16 0, align 8
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
 ; RUN:    %t1.o %t2.o -o %t3.o
 ; RUN: llvm-dis %t3.o -o - | FileCheck --check-prefix=EXEC %s
 
 ; All IR case, we internalize a after merging.
 ; EXEC: @a = internal global [4 x i8] zeroinitializer, align 8
 
 ; RUN: llc %p/Inputs/common.ll -o %t2native.o -filetype=obj
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
 ; RUN:    %t1.o %t2native.o -o %t3.o
 ; RUN: llvm-dis %t3.o -o - | FileCheck --check-prefix=MIXED %s
 
 ; Mixed ELF and IR. We keep ours as common so the linker will finish the merge.
-; MIXED: @a = common global [2 x i8] zeroinitializer, align 8
+; MIXED: @a = common global i16 0, align 8

llvm/trunk/test/tools/gold/X86/start-lib-common.ll

 ; ToT gold (as of 03/2016) honors --start-lib/--end-lib, drops %t2.o and ends up
 ; with (i32 align 4) symbol.
 ; Older gold does not drop %t2.o and ends up with (i32 align 8) symbol. This is
 ; incorrect behavior, but this test does not verify this in order to support
 ; both old and new gold.
 
 ; Check that the common symbol is not dropped completely, which was a regression
 ; in r262676.
-; CHECK: @x = common global [4 x i8] zeroinitializer
+; CHECK: @x = common global i32 0, align 8