On Thu, Jan 07, 2010 at 12:20:04AM +0800, Jamie Lokier wrote:
> Roland Dreier wrote:
> >
> > > + /* please add new bits here to ensure allocation uniqueness */
> > > + BUG_ON(20 != hweight32(
> > > + O_RDONLY | O_WRONLY | O_RDWR |
> >
> > I wonder if there's a way to make this BUILD_BUG_ON(), so the problem is
> > caught at compile time (a change like adding an O_ flag is often compile
> > tested only for obscure archs). One could create a compile-time
> > macro-ized version of hweight32() and use that, I guess, although I'm
> > not sure it's worth the ugliness.
>
> Not ugly at all:
>
> #define hweight32(x) __builtin_popcount(x)
>
> Checked GCC 3.4.3 / 4.1 / 4.4: It expands as a compile-time constant
> if the argument is a compile-time constant, so can be used in
> BUILD_BUG_ON() and even for array sizes etc.
>
> If GCC's __builtin_popcount() isn't good enough for non-constant
> values, the macro would be more involved:
>
> #define hweight32(x) (__builtin_constant_p(x) ? __builtin_popcount(x) \
> : hweight32_nonconstant(x))
Jamie, Thanks for the hint! Would you sign this patch?
---
bitops: compile time optimization for hweight_long(CONSTANT)
This allows use of hweight_long() in BUILD_BUG_ON().
Suggested by Jamie.
CC: Jamie Lokier <jamie@xxxxxxxxxxxxx>
CC: Roland Dreier <rdreier@xxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
include/linux/bitops.h | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
--- linux.orig/include/linux/bitops.h 2010-01-09 12:13:00.000000000 +0800
+++ linux/include/linux/bitops.h 2010-01-09 12:21:50.000000000 +0800
@@ -40,10 +40,14 @@ static __inline__ int get_count_order(un
return order;
}
-static inline unsigned long hweight_long(unsigned long w)
-{
- return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
-}
+#define hweight_long(x) \
+( \
+ __builtin_constant_p(x) ? \
+ __builtin_popcountl(x) : \
+ (sizeof(x) <= 4 ? \
+ hweight32(x) : \
+ hweight64(x)) \
+)
/**
* rol32 - rotate a 32-bit value left
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