On Sat, 11 May 2019, Doug Moore wrote:
Log:
When bitpos can't be implemented with an inline ffs* instruction,
change the binary search so that it does not depend on a single bit
only being set in the bitmask. Use bitpos more generally, and avoid
some clearing of bits to accommodate its current behavior.
Inline ffs*() is badly implemented in general (it is mostly not implemented
in the kernel). This makes it almost useless here, and (non-inline) ffs*()
(not used here) often slower than direct methods.
Modified: head/sys/kern/subr_blist.c
==============================================================================
--- head/sys/kern/subr_blist.c Sat May 11 04:18:06 2019 (r347483)
+++ head/sys/kern/subr_blist.c Sat May 11 09:09:10 2019 (r347484)
...
+static inline int
+bitpos(u_daddr_t mask)
+{
+
switch (sizeof(mask)) {
#ifdef HAVE_INLINE_FFSLL
case sizeof(long long):
return (ffsll(mask) - 1);
#endif
This uses the long long abomination. Only amd64 has inline ffsll().
+#ifdef HAVE_INLINE_FFS
+ case sizeof(int):
+ return (ffs(mask) - 1);
+#endif
This is unreachable, since sizeof(int) is 4 on all supported arches, and
sizeof(mask) is 8 on all arches.
default:
- lo = 0;
- hi = BLIST_BMAP_RADIX;
- while (lo + 1 < hi) {
- mid = (lo + hi) >> 1;
- if ((mask >> mid) != 0)
- lo = mid;
- else
- hi = mid;
- }
- return (lo);
+ return (generic_bitpos(mask));
}
}
So the default cause is used except on amd64. I think the direct code
for it is not as slow as the generic (libkern) ffs*(), except the generic
ffs*() is simpler so the compiler might inline it internals to
__builtin_ffs*(). Not that -ffreestanding prevents inlining extern ffs*().
Since 64/8 is 2 times 4 and sizeof(int) >= 4 in POSIX, it is trivial to
implement ffs64() using 2 inline ffs()'s, and that is exactly what clang
does on i386 to implement __builtin_ffsl(). Unfortunately, most arches
also don't implement inline ffs(). Only amd64, i386 and sparc64 implement
it.
subr_blist.c is also complilable outside of the kernel. I don't like that.
The kernel macros HAVE_INLINE_* don't exist outside of the kernel, but
ffs*() would work better outside of the kernel if it is used blindly since
inlining of it isn't prevented by -ffreestanding.
Bruce
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