gcc/boehm-gc/README.sgi

Performance of the incremental collector can be greatly enhanced with
-DNO_EXECUTE_PERMISSION.

The collector should run with all of the -32, -n32 and -64 ABIs.  Remember to
define the AS macro in the Makefile to be "as -64", or "as -n32".

If you use -DREDIRECT_MALLOC=GC_malloc with C++ code, your code should make
at least one explicit call to malloc instead of new to ensure that the proper
version of malloc is linked in.

Sproc threads are not supported in this version, though there may exist other
ports.

Pthreads support is provided.  This requires that:

1) You compile the collector with -DIRIX_THREADS specified in the Makefile.

2) You have the latest pthreads patches installed.  

(Though the collector makes only documented pthread calls,
it relies on signal/threads interactions working just right in ways
that are not required by the standard.  It is unlikely that this code
will run on other pthreads platforms.  But please tell me if it does.)

3) Every file that makes thread calls should define IRIX_THREADS and then
include gc.h.  Gc.h redefines some of the pthread primitives as macros which
also provide the collector with information it requires.

4) pthread_cond_wait and pthread_cond_timed_wait should be prepared for
premature wakeups.  (I believe the pthreads and realted standards require this
anyway.  Irix pthreads often terminate a wait if a signal arrives.
The garbage collector uses signals to stop threads.)

5) It is expensive to stop a thread waiting in IO at the time the request is
initiated.  Applications with many such threads may not exhibit acceptable
performance with the collector.  (Increasing the heap size may help.)

6) The collector should not be compiled with -DREDIRECT_MALLOC.  This
confuses some library calls made by the pthreads implementation, which
expect the standard malloc.