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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 4/4] bitmaps/bitops: Clarify tests for small constant size.
>>> On 27.02.14 at 17:38, Tim Deegan <tim@xxxxxxx> wrote:
> From dba941064825d723b79f866d16bb0f07585de320 Mon Sep 17 00:00:00 2001
> From: Tim Deegan <tim@xxxxxxx>
> Date: Thu, 28 Nov 2013 15:40:48 +0000
> Subject: [PATCH] bitmaps/bitops: Clarify tests for small constant size.
>
> No semantic changes, just makes the control flow a bit clearer.
>
> I was looking at this bcause the (-!__builtin_constant_p(x) | x__)
> formula is too clever for Coverity, but in fact it always takes me a
> minute or two to understand it too. :)
>
> Signed-off-by: Tim Deegan <tim@xxxxxxx>
A little reluctantly
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
>
> v3: Consistenly use '!foo' rather than 'foo == 0'
> v2: fix find_next_bit macros to evaluate 'addr' exactly once.
> ---
> xen/include/asm-x86/bitops.h | 62
> ++++++++++++++++++++------------------------
> xen/include/xen/bitmap.h | 30 ++++++++++++---------
> 2 files changed, 46 insertions(+), 46 deletions(-)
>
> diff --git a/xen/include/asm-x86/bitops.h b/xen/include/asm-x86/bitops.h
> index ab21d92..82a08ee 100644
> --- a/xen/include/asm-x86/bitops.h
> +++ b/xen/include/asm-x86/bitops.h
> @@ -335,23 +335,20 @@ static inline unsigned int __scanbit(unsigned long val,
> unsigned long max)
> * @offset: The bitnumber to start searching at
> * @size: The maximum size to search
> */
> -#define find_next_bit(addr, size, off) ({ \
> - unsigned int r__ = (size); \
> - unsigned int o__ = (off); \
> - switch ( -!__builtin_constant_p(size) | r__ ) \
> - { \
> - case 0: (void)(addr); break; \
> - case 1 ... BITS_PER_LONG: \
> - r__ = o__ + __scanbit(*(const unsigned long *)(addr) >> o__, r__); \
> - break; \
> - default: \
> - if ( __builtin_constant_p(off) && !o__ ) \
> - r__ = __find_first_bit(addr, r__); \
> - else \
> - r__ = __find_next_bit(addr, r__, o__); \
> - break; \
> - } \
> - r__; \
> +#define find_next_bit(addr, size, off) ({
> \
> + unsigned int r__;
> \
> + const unsigned long *a__ = (addr);
> \
> + unsigned int s__ = (size);
> \
> + unsigned int o__ = (off);
> \
> + if ( __builtin_constant_p(size) && !s__ ) \
> + r__ = s__;
> \
> + else if ( __builtin_constant_p(size) && s__ <= BITS_PER_LONG ) \
> + r__ = o__ + __scanbit(*(const unsigned long *)(a__) >> o__, s__); \
> + else if ( __builtin_constant_p(off) && !o__ ) \
> + r__ = __find_first_bit(a__, s__);
> \
> + else
> \
> + r__ = __find_next_bit(a__, s__, o__);
> \
> + r__;
> \
> })
>
> /**
> @@ -370,23 +367,20 @@ static inline unsigned int __scanbit(unsigned long val,
> unsigned long max)
> * @offset: The bitnumber to start searching at
> * @size: The maximum size to search
> */
> -#define find_next_zero_bit(addr, size, off) ({ \
> - unsigned int r__ = (size); \
> - unsigned int o__ = (off); \
> - switch ( -!__builtin_constant_p(size) | r__ ) \
> - { \
> - case 0: (void)(addr); break; \
> - case 1 ... BITS_PER_LONG: \
> - r__ = o__ + __scanbit(~*(const unsigned long *)(addr) >> o__, r__); \
> - break; \
> - default: \
> - if ( __builtin_constant_p(off) && !o__ ) \
> - r__ = __find_first_zero_bit(addr, r__); \
> - else \
> - r__ = __find_next_zero_bit(addr, r__, o__); \
> - break; \
> - } \
> - r__; \
> +#define find_next_zero_bit(addr, size, off) ({
> \
> + unsigned int r__;
> \
> + const unsigned long *a__ = (addr);
> \
> + unsigned int s__ = (size);
> \
> + unsigned int o__ = (off);
> \
> + if ( __builtin_constant_p(size) && !s__ ) \
> + r__ = s__;
> \
> + else if ( __builtin_constant_p(size) && s__ <= BITS_PER_LONG ) \
> + r__ = o__ + __scanbit(~*(const unsigned long *)(a__) >> o__, s__); \
> + else if ( __builtin_constant_p(off) && !o__ ) \
> + r__ = __find_first_zero_bit(a__, s__);
> \
> + else
> \
> + r__ = __find_next_zero_bit(a__, s__, o__);
> \
> + r__;
> \
> })
>
> /**
> diff --git a/xen/include/xen/bitmap.h b/xen/include/xen/bitmap.h
> index b5ec455..e2a3686 100644
> --- a/xen/include/xen/bitmap.h
> +++ b/xen/include/xen/bitmap.h
> @@ -110,13 +110,14 @@ extern int bitmap_allocate_region(unsigned long
> *bitmap, int pos, int order);
>
> #define bitmap_bytes(nbits) (BITS_TO_LONGS(nbits) * sizeof(unsigned long))
>
> -#define bitmap_switch(nbits, zero_ret, small, large) \
> - switch (-!__builtin_constant_p(nbits) | (nbits)) { \
> - case 0: return zero_ret; \
> - case 1 ... BITS_PER_LONG: \
> - small; break; \
> - default: \
> - large; break; \
> +#define bitmap_switch(nbits, zero, small, large) \
> + unsigned int n__ = (nbits); \
> + if (__builtin_constant_p(nbits) && !n__) { \
> + zero; \
> + } else if (__builtin_constant_p(nbits) && n__ <= BITS_PER_LONG) { \
> + small; \
> + } else { \
> + large; \
> }
>
> static inline void bitmap_zero(unsigned long *dst, int nbits)
> @@ -191,7 +192,8 @@ static inline void bitmap_complement(unsigned long *dst,
> const unsigned long *sr
> static inline int bitmap_equal(const unsigned long *src1,
> const unsigned long *src2, int nbits)
> {
> - bitmap_switch(nbits, -1,
> + bitmap_switch(nbits,
> + return -1,
> return !((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits)),
> return __bitmap_equal(src1, src2, nbits));
> }
> @@ -199,7 +201,8 @@ static inline int bitmap_equal(const unsigned long *src1,
> static inline int bitmap_intersects(const unsigned long *src1,
> const unsigned long *src2, int nbits)
> {
> - bitmap_switch(nbits, -1,
> + bitmap_switch(nbits,
> + return -1,
> return ((*src1 & *src2) & BITMAP_LAST_WORD_MASK(nbits)) != 0,
> return __bitmap_intersects(src1, src2, nbits));
> }
> @@ -207,21 +210,24 @@ static inline int bitmap_intersects(const unsigned long
> *src1,
> static inline int bitmap_subset(const unsigned long *src1,
> const unsigned long *src2, int nbits)
> {
> - bitmap_switch(nbits, -1,
> + bitmap_switch(nbits,
> + return -1,
> return !((*src1 & ~*src2) & BITMAP_LAST_WORD_MASK(nbits)),
> return __bitmap_subset(src1, src2, nbits));
> }
>
> static inline int bitmap_empty(const unsigned long *src, int nbits)
> {
> - bitmap_switch(nbits, -1,
> + bitmap_switch(nbits,
> + return -1,
> return !(*src & BITMAP_LAST_WORD_MASK(nbits)),
> return __bitmap_empty(src, nbits));
> }
>
> static inline int bitmap_full(const unsigned long *src, int nbits)
> {
> - bitmap_switch(nbits, -1,
> + bitmap_switch(nbits,
> + return -1,
> return !(~*src & BITMAP_LAST_WORD_MASK(nbits)),
> return __bitmap_full(src, nbits));
> }
> --
> 1.8.5.2
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