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Use thread-local storage for Newlib reentrancy objects

Original author: sebastian.huber

Problem

The state of the art architectures supported by RTEMS have all efficient support for thread-local storage (MIPS has issues with thread-local storage, however, is MIPS state of the art?).

Newlib currently uses a huge object of type struct _reent to store thread-specific data. This object is returned by __getreent(). It is related to the __DYNAMIC_REENT__ Newlib configuration option which is always defined for RTEMS.

The reentrancy structure contains errno and also the standard input, output, and error file streams. This means that if an application only uses errno it has a dependency on the file stream support event if it does not use it. This is an issue for lower end targets and the pre-qualification of RTEMS.

Solution

One approach to disentangle the dependencies introduced by struct _reent is to get rid of this structure and replace the individual members of the structure with thread-local objects. For example, instead of

struct _reent {
  int _errno;
  __FILE *_stdin;
  __FILE *_stdout;
  __FILE *_stderr;
};

use

_Thread_local int _errno;
_Thread_local __FILE *_stdin;
_Thread_local __FILE *_stdout;
_Thread_local __FILE *_stderr;

Newlib already has access macros for the struct _reent members, for example:

#define _REENT_SIGNGAM(ptr)	((ptr)->_new._reent._gamma_signgam)
#define _REENT_RAND_NEXT(ptr)	((ptr)->_new._reent._rand_next)
#define _REENT_RAND48_SEED(ptr)	((ptr)->_new._reent._r48._seed)
#define _REENT_RAND48_MULT(ptr)	((ptr)->_new._reent._r48._mult)
#define _REENT_RAND48_ADD(ptr)	((ptr)->_new._reent._r48._add)

How-to Implement

The member access macros are incomplete. The first step is to use the Newlib configuration for RTEMS as is and rename all struct _reent members, for example add an TEMPORARY prefix to all member names, _errno to TEMPORARY_errno. Then add member access macros until Newlib builds again. Install this Newlib and check that RTEMS and libbsd compiles. Run the RTEMS and libbsd test suites to check for regressions.

In a second step to this for the _REENT_SMALL configuration of Newlib.

The third step is to add a new Newlib configuration option, for example _REENT_THREAD_LOCAL which turns the struct _reent members into thread-local objects with corresponding "member" access macros. Define _REENT to NULL.

Skills

C and assembly

Difficulty

This is a large (350-hour) project of hard difficulty.

Edited by Amar Takhar