NaCl docs: update computed goto support

native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst

 ============================
 PNaCl C/C++ Language Support
 ============================
 
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 Source language support
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     PNaCl currently disallows them in the *pexe* because they aren't
     supported on all platforms and can't realistically be
     emulated. PNaCl could (but currently doesn't) only generate them in
     the backend if fast-math were specified and the hardware supports
     the operation.
   * Transcendentals aren't exposed by PNaCl's ABI; they are part of the
     math library that is included in the *pexe*. PNaCl could, but
     currently doesn't, use hardware support if fast-math were provided
     in the *pexe*.
 
+Computed ``goto``
+=================
+
+PNaCl supports computed ``goto``, a non-standard extension to C used by

[Mark Seaborn, 2014/03/03 21:23:04]

You could be more specific and say "GCC extension".

ExpandIndirectBr.cpp has this extra advice that would be useful to put in the
docs:

// The resulting use of switches might not be as fast as the original
// indirectbrs.  If you are compiling a program that has a
// compile-time option for using computed gotos, it's possible that
// the program will run faster with the option turned off than with
// using computed gotos + ExpandIndirectBr (for example, if the
// program does extra work to take advantage of computed gotos).

[JF, 2014/03/03 21:30:32]

That's helpful, I added it. Thanks!

+some interpreters, by lowering them to ``switch`` statements.
+
+NaCl supports computed ``goto``.
+
 Future Directions
 =================
 
 SIMD
 ----
 
 PNaCl currently doesn't support SIMD. We plan to add SIMD support in the
 very near future.
 
 NaCl supports SIMD.
@@ skipping 28 matching lines @@
   A similar feature is **thread suspension**: The ability to
   asynchronously suspend and resume a thread and inspect or modify its
   execution state (such as register state).
 
   Neither PNaCl nor NaCl currently support asynchronous interruption
   or suspension of threads.
 
 If PNaCl were to support either of these, the interaction of
 ``volatile`` and atomics with same-thread signal handling would need
 to be carefully detailed.
-
-Computed ``goto``
------------------
-
-PNaCl currently doesn't support computed ``goto``, a non-standard
-extension to C used by some interpreters.
-
-NaCl supports computed ``goto``.