Re: [PATCH] x86: merge the simple bitops and move them to bitops.h

From: Jeremy Fitzhardinge
Date: Fri Mar 14 2008 - 17:17:22 EST


Alexander van Heukelum wrote:
On Fri, 14 Mar 2008 11:07:55 -0700, "Jeremy Fitzhardinge"
<jeremy@xxxxxxxx> said:
Alexander van Heukelum wrote:
x86: merge the simple bitops and move them to bitops.h

Some of those can be written in such a way that the same
inline assembly can be used to generate both 32 bit and
64 bit code.

For ffs and fls, x86_64 unconditionally used the cmov
instruction and i386 unconditionally used a conditional
branch over a mov instruction. In the current patch I
chose to select the version based on the availability
of the cmov instruction instead. A small detail here is
that x86_64 did not previously set CONFIG_X86_CMOV=y.
Looks good in general. What's left in bitops_{32,64}.h now?

Thanks for taking a look!

bitops_{32,64}.h are getting pretty empty ;)

Both contain find_first_bit/find_first_zero_bit, i386 has them inlined,
x86_64 has an ugly define to select between small bitmaps (inlined) and
an out-of-line version. I think they should be unified much like how
find_next_bit and find_next_zero_bit work now (in x86#testing).

Both define fls64(), but i386 uses a generic one and x86_64 defines
one all by itself. The generic one is currently not suitable for
use by 64-bit archs... that can change.

x86_64 defines ARCH_HAS_FAST_MULTIPLIER, i386 not. This affects a
choice of generated code in the (generic) hweight function. It would
be nice if that could move to some other file.

x86_64 has a mysterious inline function set_bit_string, which is
only used by pci-calgary_64.c and pci-gart_64.c. Not sure what to
do with it.

(Some comments below.)

--- %< ---

+#ifdef __KERNEL__
+/**
+ * ffs - find first bit set
+ * @x: the word to search
+ *
+ * This is defined the same way as
+ * the libc and compiler builtin ffs routines, therefore
+ * differs in spirit from the above ffz() (man ffs).
This comment seems wrong. My "man ffs" says that it returns 1-32 for non-zero inputs, and 0 for a zero input. This function returns 0-31, or -1 for a zero input.

Seems, indeed. You missed the "return r + 1;" ;-)

Indeed I did.

J
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