StackThreads/MP: version 0.77 User's Guide

Chapter 8: Applying Safety Patches to GCC

Although StackThreads/MP can be practically used with unmodified GCC, there are certain cases where execution scheme of StackThreads/MP breaks the assumption that code generator of GCC exploits for optimization (See Limitations for descrpitions about such cases). We provide simple extensions to gcc that fix the problem in a form of patches to gcc (2.7.2.3 and 2.8.1). Note that this is not a bug of gcc, but just a subtle mismatch between StackThreads/MP execution scheme and the sequential exeuction scheme that is assumed by the compiler. Patches are quite small (essential modifications are just a few lines) and the patched gcc supports a couple of additional command-line options. Without these command-line options, the patched gcc functions exactly the same as the original gcc. Even when these command-line options are given, generated code is different only in cases where the original generated code is unsafe. Of course, generated code is always compatible (inter-operable) with code compiled normally (code compiled with such an option can call functions compiled normally, and vice versa). Therefore, the patched gcc can safely replace the existing gcc, if you do not like having two copies of gcc. You need not recompile any sequential library compiled by the original gcc (such as X library and Tk library).

Modifications to source code are always guarded by:

#if !defined(STHREADS_SAFE) || STHREADS_SAFE
modified source
#else /* !defined(STHREADS_SAFE) || STHREADS_SAFE */
original source
#endif /* !defined(STHREADS_SAFE) || STHREADS_SAFE */

STHREADS_SAFE to zero. Note that if STHREADS_SAFE is undefined (this is default), the modified source is used.

The patch is not a requirement for StackThreads/MP installation. In practice, you can start using the library with unmodified gcc and later apply the patch, when you like to use the library for serious purposes. If you do not want to patch gcc, you can workaround them by hand. Problematic cases are described below and you can avoid them (in particular, if StackThreads/MP is used as a compilation target, the frontend compiler are likely to be able to deal with them). If you want to worry about them and guarrantee safety, however, we recommend you to apply the patch to gcc someday. These patches are important only on SPARC and i386.