Add some more documentation for x86 special address spaces.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71012 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 26 insertions, 4 deletions

diff --git a/docs/CodeGenerator.html b/docs/CodeGenerator.html
index e620c1782e..dd9cd4a45b 100644
--- a/docs/CodeGenerator.html
+++ b/docs/CodeGenerator.html
@@ -1838,7 +1838,8 @@ OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg,   SignExtImm
 
 <div class="doc_text">
 
-<p>x86 has the ability to perform loads and stores to different address spaces
+<p>x86 has an experimental feature which provides
+   the ability to perform loads and stores to different address spaces
    via the x86 segment registers.  A segment override prefix byte on an
    instruction causes the instruction's memory access to go to the specified
    segment.  LLVM address space 0 is the default address space, which includes
@@ -1848,9 +1849,30 @@ OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg,   SignExtImm
    address space 257. Other x86 segments have yet to be allocated address space
    numbers.</p>
 
-<p>Some operating systems use the FS/GS-segment to implement TLS, so care 
-   should be taken when reading and writing to address space 256/257 on these
-   platforms.</p>
+<p>While these address spaces may seem similar to TLS via the
+   <tt>thread_local</tt> keyword, and often use the same underlying hardware,
+   there are some fundamental differences.</p>
+
+<p>The <tt>thread_local</tt> keyword applies to global variables and
+   specifies that they are to be allocated in thread-local memory. There are
+   no type qualifiers involved, and these variables can be pointed to with
+   normal pointers and accessed with normal loads and stores.
+   The <tt>thread_local</tt> keyword is target-independent at the LLVM IR
+   level (though LLVM doesn't yet have implementations of it for some
+   configurations).<p>
+
+<p>Special address spaces, in contrast, apply to static types. Every
+   load and store has a particular address space in its address operand type,
+   and this is what determines which address space is accessed.
+   LLVM ignores these special address space qualifiers on global variables,
+   and does not provide a way to directly allocate storage in them.
+   At the LLVM IR level, the behavior of these special address spaces depends
+   in part on the underlying OS or runtime environment, and they are specific
+   to x86 (and LLVM doesn't yet handle them correctly in some cases).</p>
+
+<p>Some operating systems and runtime environments use (or may in the future
+   use) the FS/GS-segment registers for various low-level purposes, so care
+   should be taken when considering them.</p>
 
 </div>