Modify MR_make_string so that it first tries printing into a

Estimated hours taken: 0.5

runtime/mercury_string.c:
	Modify MR_make_string so that it first tries printing into a
	fixed size buffer.  Only if that buffer is not big enough do we
	allocate a buffer on the heap.

runtime/mercury_string.c

@@ -13,20 +13,35 @@
   #define vsnprintf	_vsnprintf
 #endif
 
+#define SIZE	4096
+
 MR_String
 MR_make_string(MR_Code *proclabel, const char *fmt, ...) {
 	va_list		ap;
 	MR_String	result;
 	int 		n;
-
-#if defined(HAVE_VSNPRINTF) || defined(HAVE__VSNPRINTF)
-	/* Guess that 100 bytes should be sufficient */
-	int 		size = 100;
 	char		*p;
 
+#if defined(HAVE_VSNPRINTF) || defined(HAVE__VSNPRINTF)
+	int 		size = 2 * SIZE;
+	char		fixed[SIZE];
+	int		dynamically_allocated = FALSE;
+	
+	va_start(ap, fmt);
+	n = vsnprintf(fixed, SIZE, fmt, ap);
+	va_end(ap);
+
+	/*
+	** If that didn't work, use a dynamically allocated array twice
+	** the size of the fixed array and keep growing the array until
+	** the string fits.
+	*/
+	if (!(n > -1 && n < size)) {
 		p = MR_NEW_ARRAY(char, size);
 
 		while (1) {
+			dynamically_allocated = TRUE;
+
 			/* Try to print in the allocated space. */
 			va_start(ap, fmt);
 			n = vsnprintf(p, size, fmt, ap);
@@ -46,16 +61,22 @@ MR_make_string(MR_Code *proclabel, const char *fmt, ...) {
 
 			MR_RESIZE_ARRAY(p, char, size);
 		}
+	} else {
+		p = fixed;
+	}
+
 #else
 		/* 
 		** It is possible for this buffer to overflow and
 		** then bad things may happen
 		*/
-	char p[40960];
+	char fixed[40960];
 
 	va_start(ap, fmt);
-	n = vsprintf(p, fmt, ap);
+	n = vsprintf(fixed, fmt, ap);
 	va_end(ap);
+
+	p = fixed;
 #endif
 	      
 	MR_allocate_aligned_string_msg(result, strlen(p),
@@ -63,7 +84,9 @@ MR_make_string(MR_Code *proclabel, const char *fmt, ...) {
 	strcpy(result, p);
 
 #ifdef HAVE_VSNPRINTF
+	if (dynamically_allocated) {
 		MR_free(p);
+	}
 #endif
 
 	return result;