scm_mark_dependencies & guardians

[Han-Wen <hanwen@cs.uu.nl>]

hi all,

Strolling through gc.c, I found a construct worthy of
Spaghetti-coding 101, namely


	#define MARK scm_gc_mark
	#define FNAME "scm_gc_mark"

	#endif /*!MARK_DEPENDENCIES*/

	/* Mark an object precisely.
	 */
	void
	MARK (SCM p)
	#define FUNC_NAME FNAME
	{
		:
	#ifndef MARK_DEPENDENCIES
		:

	/* And here we define `scm_gc_mark_dependencies', by including this
	 * same file in itself.
	 */
	#define MARK scm_gc_mark_dependencies
	#define FNAME "scm_gc_mark_dependencies"
	#define MARK_DEPENDENCIES
	#include "gc-mark.c"
	#undef MARK_DEPENDENCIES
	#undef MARK
	#undef FNAME

The scm_gc_mark_dependencies function is used once, to detect if a
guardian object points to itself in any way. I would like to change
this code to be like

       inline
       scm_mark (SCM p)
       {
         if (marked_p(p))
	    return
	 mark_cell (p)
         scm_mark_dependencies (p)
       }      

       /* mark all objects pointed to by P */
       inline scm_mark_dependencies (SCM p) 
       {
	..

       }

however, the current code is so obtuse (goto considered harmful) that
it is difficult to make out what the exact semantics of
scm_mark_dependencies () should be. For example, a self-reference to
the argument of scm_mark_dependencies() is not followed or marked, but
only in when such a mark happens in scm_mark_dependencies(), not if it
happens in subsequent recursive invocation of scm_gc_mark().

Any comments?  Any objections if I change the code as I proposed?  I
am a little worried that the above code may be less efficient due to
less use of goto, but that is something that can be measured.

(Some minutes later: I implemented the change, and measured a 0.4 %
speed decrease in one particular GC benchmark. I'd say that 0.4% is
not worth the spaghetti, if it were statistically significant. There
are no errors from the test suite.)


-- 
Han-Wen Nienhuys   |   hanwen@cs.uu.nl   |   http://www.cs.uu.nl/~hanwen 

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