Re: [PATCH] bitops: simplify generic bit finding functions

Alexander van Heukelum
2008-04-28T15:10:59Z

> See patch below. It gives back the 1400 bytes on SPARC64 and other
> platforms that have no instruction for find bit.

Hi,

I, personally, see this patch as a stopgap measure. The real problem
is that the generic __ffs is inlined and ends up generating a lot
of instructions.

Until the generic implementation of __ffs is fixed not to be an
enormous inline function, this seems to be a reasonable thing
to do, though it's a shame that the optimization is now default-
off for architectures with a good implementation of __ffs.

Greetings,
Alexander

> Thanks,
> tglx
>
> ---------->
>
> Subject: bitops: optional bitops mapsize optimization on config switch
> From: Thomas Gleixner <tglx@linutronix.de>
> Date: Mon, 28 Apr 2008 00:22:06 +0200
>
> The mapsize optimizations which were moved from x86 to the generic
> code in commit 64970b68d2b3ed32b964b0b30b1b98518fde388e increased the
> binary size on non x86 architectures.
>
> Make the optimization depend on a config switch so architecture
> maintainers can decide.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Acked-by: Ingo Molnar <mingo@elte.hu>
>
> ---
> arch/x86/Kconfig.cpu | 1 +
> include/linux/bitops.h | 6 ++++--
> 2 files changed, 5 insertions(+), 2 deletions(-)
>
> Index: linux-2.6/arch/x86/Kconfig.cpu
> ===================================================================
> --- linux-2.6.orig/arch/x86/Kconfig.cpu
> +++ linux-2.6/arch/x86/Kconfig.cpu
> @@ -282,6 +282,7 @@ config X86_CPU
> def_bool y
> select GENERIC_FIND_FIRST_BIT
> select GENERIC_FIND_NEXT_BIT
> + select GENERIC_FIND_BIT_MAPSIZE_OPTIMIZE
>
> config X86_GENERIC
> bool "Generic x86 support"
> Index: linux-2.6/include/linux/bitops.h
> ===================================================================
> --- linux-2.6.orig/include/linux/bitops.h
> +++ linux-2.6/include/linux/bitops.h
> @@ -190,6 +190,7 @@ static __always_inline unsigned long
> find_next_bit(const unsigned long *addr, unsigned long size,
> unsigned long offset)
> {
> +#ifdef CONFIG_GENERIC_FIND_BIT_MAPSIZE_OPTIMIZE
> unsigned long value;
>
> /* Avoid a function call if the bitmap size is a constant */
> @@ -209,7 +210,7 @@ find_next_bit(const unsigned long *addr,
> value = (*addr) & ((~0ul) << offset);
> return (value == 0) ? BITS_PER_LONG : __ffs(value);
> }
> -
> +#endif
> /* size is not constant or too big */
> return __find_next_bit(addr, size, offset);
> }
> @@ -227,6 +228,7 @@ static __always_inline unsigned long
> find_next_zero_bit(const unsigned long *addr, unsigned long size,
> unsigned long offset)
> {
> +#ifdef CONFIG_GENERIC_FIND_BIT_MAPSIZE_OPTIMIZE
> unsigned long value;
>
> /* Avoid a function call if the bitmap size is a constant */
> @@ -246,7 +248,7 @@ find_next_zero_bit(const unsigned long *
> value = (~(*addr)) & ((~0ul) << offset);
> return (value == 0) ? BITS_PER_LONG : __ffs(value);
> }
> -
> +#endif
> /* size is not constant or too big */
> return __find_next_zero_bit(addr, size, offset);
> }
> --
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>
--
Alexander van Heukelum
heukelum@fastmail.fm

--
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