Add -f[no-]direct-access-external-data to supersede -mpie-copy-relocations

GCC r218397 "x86-64: Optimize access to globals in PIE with copy reloc" made
-fpie code emit R_X86_64_PC32 to reference external data symbols by default.
Clang adopted -mpie-copy-relocations D19996 as a flexible alternative.

The name -mpie-copy-relocations can be improved [1] and does not capture the
idea that this option can apply to -fno-pic and -fpic [2], so this patch
introduces -f[no-]direct-access-external-data and makes -mpie-copy-relocations
their aliases for compatibility.

[1]
For
```
extern int var;
int get() { return var; }
```
if var is defined in another translation unit in the link unit, there is no copy
relocation.

[2]
-fno-pic -fno-direct-access-external-data is useful to avoid copy relocations.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65888
If a shared object is linked with -Bsymbolic or --dynamic-list and exports a
data symbol, normally the data symbol cannot be accessed by -fno-pic code
(because by default an absolute relocation is produced which will lead to a copy
relocation). -fno-direct-access-external-data can prevent copy relocations.

-fpic -fdirect-access-external-data can avoid GOT indirection. This is like the
undefined counterpart of -fno-semantic-interposition. However, the user should
define var in another translation unit and link with -Bsymbolic or
--dynamic-list, otherwise the linker will error in a -shared link. Generally
the user has better tools for their goal but I want to mention that this
combination is valid.

On COFF, the behavior is like always -fdirect-access-external-data.
`__declspec(dllimport)` is needed to enable indirect access.

There is currently no plan to affect non-ELF behaviors or -fpic behaviors.

-fno-pic -fno-direct-access-external-data will be implemented in the subsequent patch.

GCC feature request https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112

Reviewed By: tmsriram

Differential Revision: https://reviews.llvm.org/D92633

1 files changed, 7 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index dd66bf5..9020363 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -942,6 +942,13 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
     }
   }
 
+  // PIC defaults to -fno-direct-access-external-data while non-PIC defaults to
+  // -fdirect-access-external-data.
+  Opts.DirectAccessExternalData =
+      Args.hasArg(OPT_fdirect_access_external_data) ||
+      (!Args.hasArg(OPT_fno_direct_access_external_data) &&
+       getLastArgIntValue(Args, OPT_pic_level, 0, Diags) == 0);
+
   // If -fuse-ctor-homing is set and limited debug info is already on, then use
   // constructor homing.
   if (Args.getLastArg(OPT_fuse_ctor_homing))