[PATCH v2 12/13] Document the x86 address spaces

[Richard Henderson <rth@redhat.com>]

---
 gcc/doc/extend.texi | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index bdbf513..677a4d4 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -1240,8 +1240,8 @@ As an extension, GNU C supports named address spaces as
 defined in the N1275 draft of ISO/IEC DTR 18037.  Support for named
 address spaces in GCC will evolve as the draft technical report
 changes.  Calling conventions for any target might also change.  At
-present, only the AVR, SPU, M32C, and RL78 targets support address
-spaces other than the generic address space.
+present, only the AVR, SPU, M32C, RL78, and i386/x86_64 targets support
+address spaces other than the generic address space.
 
 Address space identifiers may be used exactly like any other C type
 qualifier (e.g., @code{const} or @code{volatile}).  See the N1275
@@ -1430,6 +1430,48 @@ It may use runtime library
 support, or generate special machine instructions to access that address
 space.
 
+@subsection X86 Named Address Spaces
+
+On the i386/x86_64 target, variables may be declared as being relative
+to the @code{%fs} or @code{%gs} segments.
+
+@table @code
+@item __seg_fs
+@itemx __seg_gs
+@cindex @code{__seg_fs} X86 Named Address Spaces
+@cindex @code{__seg_gs} X86 Named Address Spaces
+The object is accessed with the respective segment override prefix.
+
+The respective segment base must be set via some operating-system
+specific method.  Since an operating-system specific method would
+also be required to find the segment base, these address spaces are
+not considered to be subspaces of the generic (flat) address space.
+This means that explicit casts are required to convert pointers
+between these address spaces and the generic address space.  In
+practice one will need to cast to @code{uintptr_t} and apply the
+segment base offset known to the application.
+
+The preprocessor symbols @code{__SEG_FS} and @code{__SEG_GS} are
+defined when these address spaces are supported.
+
+@item __seg_tls
+@cindex @code{__seg_tls} X86 Named Address Spaces
+Some operating systems define either the @code{%fs} or @code{%gs}
+segment as the thread-local storage base for each thread.  Objects
+within this address space are accessed with the appropriate
+segment override prefix.
+
+The pointer located at address 0 within the segment contains the
+offset of the segment within the generic address space.  Thus this
+address space is considered a subspace of the generic address space,
+and the known segment offset is applied when converting addresses
+to and from the generic address space.
+
+The preprocessor symbol @code{__SEG_TLS} is defined when this
+address space is supported.
+
+@end table
+
 @node Zero Length
 @section Arrays of Length Zero
 @cindex arrays of length zero
-- 
2.4.3