[PATCH] 2.4: always_inline for gcc3

[PATCH] 2.4: always_inline for gcc3

[J.A. Magallon]

Hi.

Resending for 2.4.23-pre ;)

--- 25/include/linux/compiler.h~gcc3-inline-fix	2003-03-06
03:02:43.000000000 -0800
+++ 25-akpm/include/linux/compiler.h	2003-03-06 03:11:42.000000000 -0800
@@ -1,6 +1,13 @@
 #ifndef __LINUX_COMPILER_H
 #define __LINUX_COMPILER_H
 
+#if __GNUC__ >= 3
+#define inline		__inline__ __attribute__((always_inline))
+#define inline__	__inline__ __attribute__((always_inline))
+#define __inline	__inline__ __attribute__((always_inline))
+#define __inline__	__inline__ __attribute__((always_inline))
+#endif
+
 /* Somewhere in the middle of the GCC 2.96 development cycle, we implemented
    a mechanism by which the user can annotate likely branch directions and
    expect the blocks to be reordered appropriately.  Define __builtin_expect


-- 
J.A. Magallon <jamagallon@able.es>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.22-jam1m (gcc 3.3.1 (Mandrake Linux 9.2 3.3.1-1mdk))

Re: [PATCH] 2.4: always_inline for gcc3

[Marcelo Tosatti]

Can you please explain me what are the differences when using "__inline__
__attribute__((always_inline))" and why you chose to use that?

On Tue, 26 Aug 2003, J.A. Magallon wrote:

> Hi.
>
> Resending for 2.4.23-pre ;)
>
> --- 25/include/linux/compiler.h~gcc3-inline-fix	2003-03-06
> 03:02:43.000000000 -0800
> +++ 25-akpm/include/linux/compiler.h	2003-03-06 03:11:42.000000000 -0800
> @@ -1,6 +1,13 @@
>  #ifndef __LINUX_COMPILER_H
>  #define __LINUX_COMPILER_H
>
> +#if __GNUC__ >= 3
> +#define inline		__inline__ __attribute__((always_inline))
> +#define inline__	__inline__ __attribute__((always_inline))
> +#define __inline	__inline__ __attribute__((always_inline))
> +#define __inline__	__inline__ __attribute__((always_inline))
> +#endif
> +
>  /* Somewhere in the middle of the GCC 2.96 development cycle, we implemented
>     a mechanism by which the user can annotate likely branch directions and
>     expect the blocks to be reordered appropriately.  Define __builtin_expect
>
>
>

Re: [PATCH] 2.4: always_inline for gcc3

[J.A. Magallon]

On 08.26, Marcelo Tosatti wrote:
> 
> Can you please explain me what are the differences when using "__inline__
> __attribute__((always_inline))" and why you chose to use that?
> 

gcc3 did not inline big functions, even if they were marked as inline
Thread:
http://marc.theaimsgroup.com/?t=103632325600005&r=1&w=2
Things like memcpy and copy_to/from_user were affected.
They were not inlined and you got tons of instances in vmlinux.

An initial patch was proposed by Denis Vlasenko, and refined by
akpm I think.

TIA

-- 
J.A. Magallon <jamagallon@able.es>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.22-jam1m (gcc 3.3.1 (Mandrake Linux 9.2 3.3.1-1mdk))

Re: [PATCH] 2.4: always_inline for gcc3

[J.A. Magallon]

On 08.26, J.A. Magallon wrote:
> 
> On 08.26, Marcelo Tosatti wrote:
> > 
> > Can you please explain me what are the differences when using "__inline__
> > __attribute__((always_inline))" and why you chose to use that?
> > 
> 
> gcc3 did not inline big functions, even if they were marked as inline
> Thread:
> http://marc.theaimsgroup.com/?t=103632325600005&r=1&w=2
> Things like memcpy and copy_to/from_user were affected.
> They were not inlined and you got tons of instances in vmlinux.
> 
> An initial patch was proposed by Denis Vlasenko, and refined by
> akpm I think.
> 

A comparison
run 

cat System.map | cut -d' ' -f 3 | sort | uniq -c | sort -nr | head -n 10

for my custom kernel and for a mandrake's standard one (not the late...)

System.map:
     16 __constant_c_and_count_memset
      8 .text.lock.inode
      7 parse_options
      6 __constant_memcpy
      5 devfs_handle
      4 init_once
      3 p.0
      3 debug
      2 want_value
      2 want_numeric

System.map-2.4.21-6mdksmp:
    150 __constant_c_and_count_memset
     81 __constant_memcpy
     45 __constant_copy_to_user
     45 __constant_copy_from_user
     18 level_save.1
     18 layer_save.0
     14 driver
     13 buf.0
      9 config_chipset_for_dma
      6 config_chipset_for_pio


-- 
J.A. Magallon <jamagallon@able.es>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.22-jam1m (gcc 3.3.1 (Mandrake Linux 9.2 3.3.1-1mdk))

Re: [PATCH] 2.4: always_inline for gcc3

[Alan Cox]

On Maw, 2003-08-26 at 17:24, J.A. Magallon wrote:
> gcc3 did not inline big functions, even if they were marked as inline
> Thread:
> http://marc.theaimsgroup.com/?t=103632325600005&r=1&w=2
> Things like memcpy and copy_to/from_user were affected.
> They were not inlined and you got tons of instances in vmlinux.

The more interesting question you want to answer first is - was
gcc right. Repeated code is bad for cache

Re: [PATCH] 2.4: always_inline for gcc3

[J.A. Magallon]

On 08.27, Alan Cox wrote:
> On Maw, 2003-08-26 at 17:24, J.A. Magallon wrote:
> > gcc3 did not inline big functions, even if they were marked as inline
> > Thread:
> > http://marc.theaimsgroup.com/?t=103632325600005&r=1&w=2
> > Things like memcpy and copy_to/from_user were affected.
> > They were not inlined and you got tons of instances in vmlinux.
> 
> The more interesting question you want to answer first is - was
> gcc right. Repeated code is bad for cache
> 

That would be true for a regular function, too much inlining can be bad.
But __constant_c_and_count_memset is special, you know.
It is written like a big switch:

copy( size )
	switch (size)
		case 1: ...
		case 2: ...
		...

Author and users relay on gcc's optimizer:
- gcc inlines it
- control variable in switch is constant at compile time
- switch is killed and just the suitable case stays.
(\label{1})

If gcc does not inline it, there are a ton of users of copy_to/from
and memset that execute the full switch() (w/o the patch, 150 out-of-line
instances of __constant_c_and_count_memset stay, with the patch
only 16, so it means gcc failed to inline about 140...).
Depending on which users are not inlined, this can be more or less
harmfull in terms of performance, but sure it is not the designed
behaviour.

And of course, I can be totally wrong. Comments welcome.

Ah, all this supposing that gcc still does correctly what I said
in \ref{1} ;)

-- 
J.A. Magallon <jamagallon@able.es>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.2 (Cooker) for i586
Linux 2.4.22-jam1m (gcc 3.3.1 (Mandrake Linux 9.2 3.3.1-1mdk))