[mercury-users] Premature exit from pragma c_code pred/func?

[Fergus Henderson]

On 30-Nov-1999, Warwick Harvey <wharvey at cs.monash.edu.au> wrote:
> Is it possible (in a det/semidet context) to exit prematurely from a `pragma 
> c_code' definition of a predicate/function?  That is, to do something 
> analogous to inserting a `return' in the middle of a normal C function?
> 
> I'm trying to convert a C function into a `pragma c_code' implementation of 
> a Mercury function.  The basic structure of the body of the function is:
> 
> 	...1...
> 	while (...2...) {
> 		...3...
> 		if (...4...) {
> 			...5...
> 			return ...6...
> 		}
> 		...7...
> 	}
> 	...8...
> 	return ...9...

[I added the numbers to the code quoted above.]

You could rewrite it as

	{
		RetType ret_val;

		...1...
		for(;;) {
			if (!(...2...)) {
				...8...
				ret_val = ...9...
				break;
			}
			...3...
			if (...4..) {
				...5...
				ret_val = ...6...
				break;
			}
			...7...
		}
		...
	}

> Of course I can rewrite the code to only have one exit point, but it would 
> be convenient if I didn't have to do that, and the resulting code would 
> probably be more efficient (modulo my limited understanding of how this 
> stuff works).

The above rewrite is not quite as nice as the original code, it's true,
but it will be just as efficient.

If you want a more elegant solution, I suggest defining the code in
a separate C function, declared inline:

	:- pragma c_header_code("

	#ifdef __GNUC__
	  #define INLINE __inline__
	#elif __STDC_VERSION__ >= 199901 /* Jan 1999 */
	  #define INLINE inline
	#else
	  #define INLINE
	#endif

	INLINE RetType foo(...) {
		...1...
		while (...2...) {
			...3...
			if (...4...) {
				...5...
				return ...6...
			}
			...7...
		}
		...8...
		return ...9...
	}

	").

Then make your `pragma c_code' procedure just call this function.

For maximum efficiency you might have to compile with MGNUCFLAGS=-O3,
but I _think_ gcc will inline it even at -O2 (which is what mgnuc uses
by default) if you explicitly declare it inline.

Cheers,
	Fergus.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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