Re: [patch] spinlocks: remove 'volatile'

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Thu, 6 Jul 2006, Ingo Molnar wrote:

>
> * Linus Torvalds <torvalds@osdl.org> wrote:
>
>> I wonder if we should remove the "volatile". There really isn't
>> anything _good_ that gcc can do with it, but we've seen gcc code
>> generation do stupid things before just because "volatile" seems to
>> just disable even proper normal working.

Then GCC must be fixed. The keyword volatile is correct. It should
force the compiler to read the variable every time it's used.

>
> yeah. I tried this and it indeed slashed 42K off text size (0.2%):
>
> text            data    bss     dec             filename
> 20779489        6073834 3075176 29928499        vmlinux.volatile
> 20736884        6073834 3075176 29885894        vmlinux.non-volatile
>
> i booted the resulting allyesconfig bzImage and everything seems to be
> working fine. Find patch below.
>
> 	Ingo
>
> ------------------>
> Subject: spinlocks: remove 'volatile'
> From: Ingo Molnar <mingo@elte.hu>
>
> remove 'volatile' from the spinlock types, it causes gcc to
> generate really bad code. (and it's pointless anyway)
>

This is not pointless. If GCC generates bad code, tell the
GCC people. The volatile keyword is essential.


> this reduces the non-debug SMP kernel's size by 0.2% (!).
>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> ---
> include/asm-i386/spinlock_types.h   |    4 ++--
> include/asm-x86_64/spinlock_types.h |    4 ++--
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
> Index: linux/include/asm-i386/spinlock_types.h
> ===================================================================
> --- linux.orig/include/asm-i386/spinlock_types.h
> +++ linux/include/asm-i386/spinlock_types.h
> @@ -6,13 +6,13 @@
> #endif
>
> typedef struct {
> -	volatile unsigned int slock;
> +	unsigned int slock;
> } raw_spinlock_t;
>
> #define __RAW_SPIN_LOCK_UNLOCKED	{ 1 }
>
> typedef struct {
> -	volatile unsigned int lock;
> +	unsigned int lock;
> } raw_rwlock_t;
>
> #define __RAW_RW_LOCK_UNLOCKED		{ RW_LOCK_BIAS }
> Index: linux/include/asm-x86_64/spinlock_types.h
> ===================================================================
> --- linux.orig/include/asm-x86_64/spinlock_types.h
> +++ linux/include/asm-x86_64/spinlock_types.h
> @@ -6,13 +6,13 @@
> #endif
>
> typedef struct {
> -	volatile unsigned int slock;
> +	unsigned int slock;
> } raw_spinlock_t;
>
> #define __RAW_SPIN_LOCK_UNLOCKED	{ 1 }
>
> typedef struct {
> -	volatile unsigned int lock;
> +	unsigned int lock;
> } raw_rwlock_t;
>
> #define __RAW_RW_LOCK_UNLOCKED		{ RW_LOCK_BIAS }
> -
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.88 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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