Support __seg_fs and __seg_gs on x86

[jfb]

Summary

GCC supports named address spaces macros:

https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html

clang does as well with address spaces:

https://clang.llvm.org/docs/LanguageExtensions.html#memory-references-to-specified-segments

Add the __seg_fs and __seg_gs macros for compatibility with GCC.

rdar://problem/52944935

[jfb]

• Fix test order

[craig.topper]

LGTM

clang/docs/LanguageExtensions.rst

 
 .. code-block:: gas
 
   _foo:
           movl    4(%esp), %eax
           movl    %gs:(%eax), %eax
           ret
 
+You can also use the GCC compatibility macros ``__seg_fs`` and ``__seg_gs`` for
+the same purpose. The preprocessor symbols ``__SEG_FS`` and ``__SEG_GS``
+indicate their support.
+
 PowerPC Language Extensions
 ------------------------------
 
 Set the Floating Point Rounding Mode
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 PowerPC64/PowerPC64le supports the builtin function ``__builtin_setrnd`` to set
 the floating point rounding mode. This function will use the least significant
 two bits of integer argument to set the floating point rounding mode.

clang/lib/Basic/Targets/X86.cpp

if (getTriple().getArch() == llvm::Triple::x86_64) {
     if (getTriple().getArchName() == "x86_64h") {
       Builder.defineMacro("__x86_64h");
       Builder.defineMacro("__x86_64h__");
     }
   } else {
     DefineStd(Builder, "i386", Opts);
   }
 
+  Builder.defineMacro("__SEG_GS");
+  Builder.defineMacro("__SEG_FS");
+  Builder.defineMacro("__seg_gs", "__attribute__((address_space(256)))");
+  Builder.defineMacro("__seg_fs", "__attribute__((address_space(257)))");
+
   // Subtarget options.
   // FIXME: We are hard-coding the tune parameters based on the CPU, but they
   // truly should be based on -mtune options.
   switch (CPU) {
   case CK_Generic:
     break;
   case CK_i386:
     // The rest are coming from the i386 define above.

clang/test/Preprocessor/x86_seg_fs_gs.c

+// RUN: %clang -target i386-unknown-unknown -x c -E -dM -o - %s | FileCheck -match-full-lines %s
+// RUN: %clang -target x86_64-unknown-unknown -x c -E -dM -o - %s | FileCheck -match-full-lines %s
+
+// CHECK: #define __SEG_GS 1
+// CHECK: #define __SEG_FS 1
+// CHECK: #define __seg_gs __attribute__((address_space(256)))
+// CHECK: #define __seg_fs __attribute__((address_space(257)))