Fix issues with GlobalMerge on Mach-O.

llvm/lib/CodeGen/GlobalMerge.cpp

@@ -578,12 +578,18 @@ bool GlobalMergeImpl::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
       Globals[k]->replaceAllUsesWith(GEP);
       Globals[k]->eraseFromParent();
 
-      // When the linkage is not internal we must emit an alias for the original
-      // variable name as it may be accessed from another object. On non-Mach-O
-      // we can also emit an alias for internal linkage as it's safe to do so.
-      // It's not safe on Mach-O as the alias (and thus the portion of the
-      // MergedGlobals variable) may be dead stripped at link time.
-      if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
+      // Emit an alias for the original variable name. This is necessary for an
+      // external symbol, as it may be accessed from another object. For
+      // internal symbols, it's not strictly required, but it's useful.
+      //
+      // This _should_ also work on Mach-O ever since '.alt_entry' support was
+      // added in 2016. Unfortunately, there's a bug in ld-prime (present at
+      // least from Xcode 15.0 through Xcode 16.0), in which -dead_strip doesn't
+      // always honor alt_entry. To workaround this issue, we don't emit aliases
+      // on Mach-O. Except, we _must_ do so for external symbols. That means
+      // MergeExternal is broken with that linker. (That option is currently off
+      // by default on MachO).
+      if (!IsMachO || Linkage == GlobalValue::ExternalLinkage) {
         GlobalAlias *GA = GlobalAlias::create(Tys[StructIdxs[idx]], AddrSpace,
                                               Linkage, Name, GEP, &M);
         GA->setVisibility(Visibility);
@@ -640,6 +646,18 @@ void GlobalMergeImpl::setMustKeepGlobalVariables(Module &M) {
   }
 }
 
+// This function returns true if the given data Section name has custom
+// subsection-splitting semantics in Mach-O (such as splitting by a fixed size)
+//
+// See also ObjFile::parseSections and getRecordSize in lld/MachO/InputFiles.cpp
+static bool isSpecialMachOSection(StringRef Section) {
+  // Uses starts_with, since section attributes can appear at the end of the
+  // name.
+  return Section.starts_with("__DATA,__cfstring") ||
+         Section.starts_with("__DATA,__objc_classrefs") ||
+         Section.starts_with("__DATA,__objc_selrefs");
+}
+
 bool GlobalMergeImpl::run(Module &M) {
   if (!EnableGlobalMerge)
     return false;
@@ -678,6 +696,10 @@ bool GlobalMergeImpl::run(Module &M) {
     unsigned AddressSpace = PT->getAddressSpace();
     StringRef Section = GV.getSection();
 
+    // On Mach-O, some section names have special semantics. Don't merge these.
+    if (IsMachO && isSpecialMachOSection(Section))
+      continue;
+
     // Ignore all 'special' globals.
     if (GV.getName().starts_with("llvm.") || GV.getName().starts_with(".llvm."))
       continue;

llvm/test/Transforms/GlobalMerge/macho-sections.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
+
+;; Check that we do _not_ merge globals which are in certain special
+;; sections under Mach-O.
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-apple-macos11"
+
+; CHECK: @cfstring1 = private global i32 1, section "__DATA,__cfstring"
+@cfstring1 = private global i32 1, section "__DATA,__cfstring"
+; CHECK: @cfstring2 = private global i32 2, section "__DATA,__cfstring"
+@cfstring2 = private global i32 2, section "__DATA,__cfstring"
+; CHECK: @objcclassrefs1 = private global i32 3, section "__DATA,__objc_classrefs,regular,no_dead_strip"
+@objcclassrefs1 = private global i32 3, section "__DATA,__objc_classrefs,regular,no_dead_strip"
+; CHECK: @objcclassrefs2 = private global i32 4, section "__DATA,__objc_classrefs,regular,no_dead_strip"
+@objcclassrefs2 = private global i32 4, section "__DATA,__objc_classrefs,regular,no_dead_strip"
+; CHECK: @objcselrefs1 = private global i32 5, section "__DATA,__objc_selrefs,literal_pointers,no_dead_strip"
+@objcselrefs1 = private global i32 5, section "__DATA,__objc_selrefs,literal_pointers,no_dead_strip"
+; CHECK: @objcselrefs2 = private global i32 6, section "__DATA,__objc_selrefs,literal_pointers,no_dead_strip"
+@objcselrefs2 = private global i32 6, section "__DATA,__objc_selrefs,literal_pointers,no_dead_strip"
+
+define void @use() {
+  load ptr, ptr @cfstring1
+  load ptr, ptr @cfstring2
+  load ptr, ptr @objcclassrefs1
+  load ptr, ptr @objcclassrefs2
+  load ptr, ptr @objcselrefs1
+  load ptr, ptr @objcselrefs2
+  ret void
+}

llvm/test/Transforms/GlobalMerge/macho-symbols.ll

@@ -0,0 +1,28 @@
+; 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
+
+;; For Mach-O, we do not expect any alias symbols to be created for
+;; internal/private symbols by GlobalMerge.
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-apple-macos11"
+
+@a = private global i32 1
+@b = private global i32 2
+@c = internal global i32 3
+@d = internal global i32 4
+
+; CHECK: @_MergedGlobals = internal global <{ i32, i32, i32, i32 }> <{ i32 1, i32 2, i32 3, i32 4 }>, align 4
+; CHECK-NOT: alias
+
+define void @use() {
+  ; CHECK: load i32, ptr @_MergedGlobals,
+  %x = load i32, ptr @a
+  ; CHECK: load i32, ptr getelementptr inbounds (<{ i32, i32, i32, i32 }>, ptr @_MergedGlobals, i32 0, i32 1)
+  %y = load i32, ptr @b
+  ; CHECK: load i32, ptr getelementptr inbounds (<{ i32, i32, i32, i32 }>, ptr @_MergedGlobals, i32 0, i32 2)
+  %z1 = load i32, ptr @c
+  ; CHECK: load i32, ptr getelementptr inbounds (<{ i32, i32, i32, i32 }>, ptr @_MergedGlobals, i32 0, i32 3)
+  %z2 = load i32, ptr @d
+  ret void
+}