Hi,
This diff is based upon kshe's diff, but there's one differene: I am
using the __builtin_ffs instead of ffs(3). Looking at the assembly
generated by calling ffs(3) produces a function call, while the
__builtin_ffs produces just a few machine instructions on all the
platforms I've checked. __builtin_ffs also makes it more easy to port
over this diff to the ld.so version of malloc.
Using a standalone test program both gcc and clang generate
__builtin_ffs. I do not know why __builtin_ffs is not generated by
either gcc or clang when compiling libc. I suppose the flags given to
the compiler force it to generate a function call, but I did not find
the actual flag that's to blame.
Please test on as much platforms as possible,
-Otto
Index: malloc.c
===================================================================
RCS file: /cvs/src/lib/libc/stdlib/malloc.c,v
retrieving revision 1.239
diff -u -p -r1.239 malloc.c
--- malloc.c 8 Jan 2018 12:20:23 -0000 1.239
+++ malloc.c 9 Jan 2018 11:44:40 -0000
@@ -931,7 +931,7 @@ malloc_bytes(struct dir_info *d, size_t
u_int i, r;
int j, listnum;
size_t k;
- u_short u, b, *lp;
+ u_short *lp;
struct chunk_info *bp;
void *p;
@@ -960,15 +960,12 @@ malloc_bytes(struct dir_info *d, size_t
/* start somewhere in a short */
lp = &bp->bits[i / MALLOC_BITS];
if (*lp) {
- b = *lp;
- k = i % MALLOC_BITS;
- u = 1 << k;
- while (k < MALLOC_BITS) {
- if (b & u)
- goto found;
- k++;
- u <<= 1;
- }
+ j = i % MALLOC_BITS;
+ k = __builtin_ffs(*lp >> j);
+ if (k != 0) {
+ k += j - 1;
+ goto found;
+ }
}
/* no bit halfway, go to next full short */
i /= MALLOC_BITS;
@@ -977,14 +974,10 @@ malloc_bytes(struct dir_info *d, size_t
i = 0;
lp = &bp->bits[i];
if (*lp) {
- b = *lp;
- k = 0;
- u = 1;
- for (;;) {
- if (b & u)
- goto found;
- k++;
- u <<= 1;
+ k = __builtin_ffs(*lp);
+ if (k != 0) {
+ k--;
+ break;
}
}
}
@@ -996,7 +989,7 @@ found:
}
#endif
- *lp ^= u;
+ *lp ^= 1 << k;
/* If there are no more free, remove from free-list */
if (--bp->free == 0)
