[GlobalMerge] Preserve symbol visibility when merging globals

Symbols created for merged external global variables have default
visibility. This can break programs when compiling with -Oz
-fvisibility=hidden as symbols that should be hidden will be exported at
link time.

Differential Revision: https://reviews.llvm.org/D73235

(cherry picked from commit a2fb2c0)

llvm/lib/CodeGen/GlobalMerge.cpp

@@ -524,6 +524,7 @@ bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
     for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
       GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
       std::string Name = Globals[k]->getName();
+      GlobalValue::VisibilityTypes Visibility = Globals[k]->getVisibility();
       GlobalValue::DLLStorageClassTypes DLLStorage =
           Globals[k]->getDLLStorageClass();
 
@@ -549,6 +550,7 @@ bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
       if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
         GlobalAlias *GA = GlobalAlias::create(Tys[StructIdxs[idx]], AddrSpace,
                                               Linkage, Name, GEP, &M);
+        GA->setVisibility(Visibility);
         GA->setDLLStorageClass(DLLStorage);
       }
 

llvm/test/CodeGen/AArch64/global-merge-hidden-minsize.ll

@@ -0,0 +1,26 @@
+; RUN: llc %s -mtriple=arm-none-linux-gnu -o - | FileCheck %s
+; RUN: llc %s -mtriple=aarch64-none-linux-gnu -o - | FileCheck %s
+
+@x = hidden global i32 0, align 4
+@y = hidden global i32 0, align 4
+
+define hidden void @f() #0 {
+  store i32 0, i32* @x, align 4
+  store i32 0, i32* @y, align 4
+  ret void
+}
+
+attributes #0 = { minsize optsize }
+
+; CHECK: .local .L_MergedGlobals
+; CHECK: .comm .L_MergedGlobals,8,4
+
+; CHECK: .globl x
+; CHECK: .hidden x
+; CHECK: .set x, .L_MergedGlobals
+; CHECK: .size x, 4
+
+; CHECK: .globl y
+; CHECK: .hidden y
+; CHECK: .set y, .L_MergedGlobals+4
+; CHECK: .size y, 4