Minor rewrite of block allocator to improve clarity.

3 files changed, 39 insertions, 15 deletions

diff --git a/ChangeLog b/ChangeLog
index 944b26e9..eb601525 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,22 @@
+2017-01-25         Andrew J. Schorr     <aschorr@telemetry-investments.com>
+
+	* awk.h (BLOCK): Remove typedef. BLOCK was used for 2 different
+	purposes: to contain a block allocation list header, and to hold
+	each individual allocated item. This was confusing, because the "size"
+	field was set only in the header, but not in each element.
+	(struct block_header): The block header contains a pointer to the first
+	element and the element size.
+	(struct block_item): Represent a single allocated item. This contains
+	only a pointer to the next element. This reduces the minimum allocated
+	item size from 2 pointers to 1 (16 bytes to 8 bytes on x86_64).
+	(nextfree): Change array item type from BLOCK to struct block_header.
+	(getblock, freeblock): Change cast from 'BLOCK' to 'struct block_item'.
+	* node.c (nextfree): Now an array of 'struct block_header' instead of
+	BLOCK. Switch the ordering to put the next pointer before the size.
+	(more_blocks): Replace 'BLOCK' with 'struct block_item', and add
+	an assert to ensure that the allocation size is at least as large
+	as 'struct block_item', i.e. 1 pointer.
+
 2017-01-22         Andrew J. Schorr     <aschorr@telemetry-investments.com>
 
 	* awk.h (numtype_choose): New backend macro used to implement
diff --git a/awk.h b/awk.h
index 34c0e07d..66163519 100644
--- a/awk.h
+++ b/awk.h
@@ -1050,10 +1050,14 @@ struct flagtab {
 };
 
 
-typedef struct block_item {
-	size_t size;
+struct block_item {
+	struct block_item *freep;
+};
+
+struct block_header {
 	struct block_item *freep;
-} BLOCK;
+	size_t size;
+};
 
 enum block_id {
 	BLOCK_INVALID = 0,	/* not legal */
@@ -1118,7 +1122,7 @@ extern afunc_t int_array_func[];
 /* special node used to indicate success in array routines (not NULL) */
 extern NODE *success_node;
 
-extern BLOCK nextfree[];
+extern struct block_header nextfree[];
 extern bool field0_valid;
 
 extern int do_flags;
@@ -1305,10 +1309,10 @@ DEREF(NODE *r)
 				&((n)->var_value) : r_get_lhs((n), (r))
 
 #define getblock(p, id, ty)  (void) ((p = (ty) nextfree[id].freep) ? \
-			(ty) (nextfree[id].freep = ((BLOCK *) p)->freep) \
+			(ty) (nextfree[id].freep = ((struct block_item *) p)->freep) \
 			: (p = (ty) more_blocks(id)))
-#define freeblock(p, id)	 (void) (((BLOCK *) p)->freep = nextfree[id].freep, \
-					nextfree[id].freep = (BLOCK *) p)
+#define freeblock(p, id)	 (void) (((struct block_item *) p)->freep = nextfree[id].freep, \
+					nextfree[id].freep = (struct block_item *) p)
 
 #define getnode(n)	getblock(n, BLOCK_NODE, NODE *)
 #define freenode(n)	freeblock(n, BLOCK_NODE)
diff --git a/node.c b/node.c
index 97f65fa3..f1f80177 100644
--- a/node.c
+++ b/node.c
@@ -1001,32 +1001,33 @@ void init_btowc_cache()
 
 #define BLOCKCHUNK 100
 
-BLOCK nextfree[BLOCK_MAX] = {
-	{ 0, NULL},	/* invalid */
-	{ sizeof(NODE), NULL },
-	{ sizeof(BUCKET), NULL },
+struct block_header nextfree[BLOCK_MAX] = {
+	{ NULL, 0},	/* invalid */
+	{ NULL, sizeof(NODE) },
+	{ NULL, sizeof(BUCKET) },
 };
 
 
 /* more_blocks --- get more blocks of memory and add to the free list;
-	size of a block must be >= sizeof(BLOCK)
+	size of a block must be >= sizeof(struct block_item)
  */
 
 void *
 more_blocks(int id)
 {
-	BLOCK *freep, *np, *next;
+	struct block_item *freep, *np, *next;
 	char *p, *endp;
 	size_t size;
 
 	size = nextfree[id].size;
 
-	emalloc(freep, BLOCK *, BLOCKCHUNK * size, "more_blocks");
+	assert(size >= sizeof(struct block_item));
+	emalloc(freep, struct block_item *, BLOCKCHUNK * size, "more_blocks");
 	p = (char *) freep;
 	endp = p + BLOCKCHUNK * size;
 
 	for (np = freep; ; np = next) {
-		next = (BLOCK *) (p += size);
+		next = (struct block_item *) (p += size);
 		if (p >= endp) {
 			np->freep = NULL;
 			break;