alloca() portability

[Chris Torek]

In article <9122 at ncar.ucar.edu> steve at groucho.ucar.edu (Steve Emmerson) asks:
>[Why not use alloca?  Doug Gywn] admitted that systems on which [his]
>alloca() couldn't work are few and far between.

There are two reasons people use alloca() instead of malloc().  One is
that alloca()'ed space goes away automatically.  Doug Gwyn's
implementation sometimes manages to achieve this, and even if not, it
at least gets the code to run until all memory has been consumed.
The other reason people use alloca() instead of malloc() is that it
is more efficient.  Since the `semi-portable' alloca() calls malloc(),
the `it can be done' argument destroys the `it is more efficient'
argument for these machines.

The efficiency argument still holds on other machines.  Unfortunately,
on these machines, the source code

	foo(int n) {
		char *p = alloca(n);
		munge(p, n);	/* munge needs n bytes of temporary space */
	}

(which appears likely to work) is quite likely to be compiled as

	foo(int n) {
		munge(alloca(n), n);
	}

which is in fact fairly unlikely to work (it fails on, for instance, a
VAX).  To make it work, either we have to either produce suboptimal
code for

	bar(int n) {
		char *p = jiggle(n);
		munge(p, n);
	}

(which should in fact be compiled without the temporary) or else we
have to gimmick the compiler to know about alloca().  (GCC 1.37.1 on
a VAX produces the suboptimal code.  The version without `p' uses r0
to hold n; the one with p uses r6, necessitating a push and pop during
entry to and exit from bar().)

The latter is medium-difficult but is in fact the only approach that
is guaranteed always to work (provided, of course, that the compiler-
gimmicking is done correctly).  Since the ANSI standard does not
*require* this, and since alloca() is little-used, it is not all that
likely that most compiler vendors will do this.

So we can say that, where alloca is more efficient than malloc, it is
not safe (because optimising compilers may `move' calls to alloca so
that they are made when the stack is not in its canonical format).
Where alloca is not more efficient (because we have to use Doug Gywn's
version or something equivalent), it is slightly more convenient.
It is also at least slightly less portable.

All in all, I would say that the threat of failure (from optimizing
compilers) plus that of nonportability (to environments where not even
a mallocking alloca can be used) far outweigh the convenience of
not having to call free().  All that is left is the difference in
speed, and there are simple ways around that problem as well.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750)
Domain:	chris at cs.umd.edu	Path:	uunet!mimsy!chris