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

Thomas Gleixner
2008-04-28T14:05:52Z

On Sun, 27 Apr 2008, Linus Torvalds wrote:
> On Sun, 27 Apr 2008, Harvey Harrison wrote:
> >
> > No need for a sentinal if we explicitly catch the no bits/all bits set
> > cases, make it clear they are special cases returning size/BITS_PER_LONG.
>
> These things need to be re-done anyway. David already complained that the
> thing inlines to something much much too big.

The delta between the inlined version including the constant
optimizations and the original non inlined version is exactly 1400
bytes on a sparc64 defconfig build.

> Let's just make it not be an inline at all.

Which makes the whole constant optimization moot.

How about making the optimization controlled by a config switch so
arch maintainers can decide whether they want to enable the constant
optimization or unconditionally call the lib function ?

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

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);
}