[GlobalMerge] Update the GlobalMerge pass to merge private global variables.

This patch updates the GlobalMerge pass to be able to handle private global
variables, which is required for a follow-up PowerPC specific GlobalMerge patch
to merge internal and private globals.

A new LIT test is also added to exhibit the ability to merge private globals.

Author: amy-kwan

llvm/lib/CodeGen/GlobalMerge.cpp

@@ -664,7 +664,7 @@ bool GlobalMergeImpl::run(Module &M) {
       continue;
 
     if (!(Opt.MergeExternal && GV.hasExternalLinkage()) &&
-        !GV.hasInternalLinkage())
+        !GV.hasInternalLinkage() && !GV.hasPrivateLinkage())
       continue;
 
     PointerType *PT = dyn_cast<PointerType>(GV.getType());

llvm/test/Transforms/GlobalMerge/private-global.ll

@@ -0,0 +1,31 @@
+; RUN: opt -global-merge -global-merge-max-offset=100 -S -o - %s | FileCheck %s
+; RUN: opt -passes='global-merge<max-offset=100>' -S -o - %s | FileCheck %s
+
+; NOTE: This is a copy of the llvm/test/Transforms/GlobalMerge/used.ll test,
+; using `private` global variables instead of `internal`. This is to show that
+; that private globals can be merged in the GlobalMerge pass.
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: @_MergedGlobals = private global <{ i32, i32 }> <{ i32 3, i32 3 }>, align 4
+
+@a = private global i32 1
+@b = private global i32 2
+@c = private global i32 3
+@d = private global i32 3
+
+@llvm.used = appending global [1 x ptr] [ptr @a], section "llvm.metadata"
+@llvm.compiler.used = appending global [1 x ptr] [ptr @b], section "llvm.metadata"
+
+define void @use_private() {
+  ; CHECK: load i32, ptr @a
+  %x = load i32, ptr @a
+  ; CHECK: load i32, ptr @b
+  %y = load i32, ptr @b
+  ; CHECK: load i32, ptr @_MergedGlobals
+  %z1 = load i32, ptr @c
+  ; CHECK: load i32, ptr getelementptr inbounds (<{ i32, i32 }>, ptr @_MergedGlobals, i32 0, i32 1)
+  %z2 = load i32, ptr @d
+  ret void
+}