Make -fno-pic respect -fno-direct-access-external-data

[MaskRay]

Summary

D92633 added -f[no-]direct-access-external-data to supersede -m[no-]pie-copy-relocations.
(The option works for -fpie but is a no-op for -fno-pic and -fpic.)

This patch makes -fno-pic -fno-direct-access-external-data drop dso_local from
global variable declarations. This usually causes the backend to emit a GOT
indirection for external data access. With a GOT relocation, the subsequent
-no-pie link will not have copy relocation even if the data symbol turns out to
be defined by a shared object.

// a.c
extern int var;
int foo() { return var; }

clang  a.c -S -o - => movl    var, %eax

clang -fno-direct-access-external-data a.c -S -o - => movq    var@GOTPCREL, %rax; movl    (%rax), %eax
// The same as -fpie or -fpic

[MaskRay]

This is straightforward, too :)

[MaskRay]

Improve tests

clang/lib/CodeGen/CodeGenModule.cpp

static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
   // PPC has no copy relocations and cannot use a plt entry as a symbol address.
   llvm::Triple::ArchType Arch = TT.getArch();
   if (Arch == llvm::Triple::ppc || Arch == llvm::Triple::ppc64 ||
       Arch == llvm::Triple::ppc64le)
     return false;
 
   // If we can use copy relocations we can assume it is local.
   if (auto *Var = dyn_cast<llvm::GlobalVariable>(GV))
-    if (!Var->isThreadLocal() &&
-        (RM == llvm::Reloc::Static || CGOpts.DirectAccessExternalData))
+    if (!Var->isThreadLocal() && CGOpts.DirectAccessExternalData)
       return true;
 
   // If we can use a plt entry as the symbol address we can assume it
   // is local.
   // FIXME: This should work for PIE, but the gold linker doesn't support it.
   if (isa<llvm::Function>(GV) && !CGOpts.NoPLT && RM == llvm::Reloc::Static)
     return true;
 

clang/test/CodeGen/dso-local-executable.c

 // MINGW-NEXT: @weak_bar = extern_weak global i32
 // MINGW-NEXT: @bar = external global i32
 // MINGW-NEXT: @local_thread_var = dso_local thread_local global i32 42
 // MINGW-NEXT: @thread_var = external dso_local thread_local global i32
 // MINGW-DAG: declare dso_local void @foo()
 // MINGW-DAG: define dso_local i32* @zed()
 // MINGW-DAG: declare dllimport void @import_func()
 
+/// Static relocation model defaults to -fdirect-access-external-data and sets
+/// dso_local on most global objects.
 // RUN: %clang_cc1 -triple x86_64 -emit-llvm -mrelocation-model static %s -o - | FileCheck --check-prefix=STATIC %s
 // STATIC:      @baz = dso_local global i32 42
 // STATIC-NEXT: @import_var = external dso_local global i32
 // STATIC-NEXT: @weak_bar = extern_weak dso_local global i32
 // STATIC-NEXT: @bar = external dso_local global i32
 // STATIC-NEXT: @local_thread_var = dso_local thread_local global i32 42
 // STATIC-NEXT: @thread_var = external thread_local global i32
 // STATIC-DAG: declare dso_local void @foo()
 // STATIC-DAG: define dso_local i32* @zed()
 // STATIC-DAG: declare dso_local void @import_func()
 
+/// If -fno-direct-access-external-data is set, drop dso_local from global variable
+/// declarations.
+// RUN: %clang_cc1 -triple x86_64 -emit-llvm %s -mrelocation-model static -fno-direct-access-external-data -o - | FileCheck --check-prefix=STATIC-INDIRECT %s
+// STATIC-INDIRECT:      @baz = dso_local global i32 42
+// STATIC-INDIRECT-NEXT: @import_var = external global i32
+// STATIC-INDIRECT-NEXT: @weak_bar = extern_weak global i32
+// STATIC-INDIRECT-NEXT: @bar = external global i32
+// STATIC-INDIRECT-NEXT: @local_thread_var = dso_local thread_local global i32 42
+// STATIC-INDIRECT-NEXT: @thread_var = external thread_local global i32
+// STATIC-INDIRECT-DAG: declare dso_local void @import_func()
+// STATIC-INDIRECT-DAG: define dso_local i32* @zed()
+// STATIC-INDIRECT-DAG: declare dso_local void @foo()
+
 // RUN: %clang_cc1 -triple x86_64 -emit-llvm -pic-level 1 -pic-is-pie %s -o - | FileCheck --check-prefix=PIE %s
 // PIE:      @baz = dso_local global i32 42
 // PIE-NEXT: @import_var = external global i32
 // PIE-NEXT: @weak_bar = extern_weak global i32
 // PIE-NEXT: @bar = external global i32
 // PIE-NEXT: @local_thread_var = dso_local thread_local global i32 42
 // PIE-NEXT: @thread_var = external thread_local global i32
 // PIE-DAG: declare void @foo()