Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

[David Daney]

Why not also CC the PPC maintainers as well?  I am not certain, but I 
think they may be reached at:

linuxppc-dev@lists.ozlabs.org


On 01/27/2011 04:12 AM, Coly Li wrote:
> Current BUG_ON() arch/powerpc/include/asm/bug.h does not use unlikely(),
> in order to get better branch predict result, source code may have to call
> BUG_ON() with unlikely() explicitly. This is not a suggested method
> to use BUG_ON().
>
> This patch adds unlikely() inside BUG_ON implementation on PPC
> code, callers can use BUG_ON without explicit unlikely() now.
>
> I don't have any PPC hardware to compile and test this fix, any feedback
> of this patch is welcome.
>
> Signed-off-by: Coly Li<bosong.ly@taobao.com>
> Cc: Jeremy Fitzhardinge<jeremy@goop.org>
> Cc: David Daney<ddaney@caviumnetworks.com>
> Cc: Wang Cong<xiyou.wangcong@gmail.com>
> Cc: Yong Zhang<yong.zhang0@gmail.com>
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index 065c590..10889a6 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -2,6 +2,7 @@
>   #define _ASM_POWERPC_BUG_H
>   #ifdef __KERNEL__
>
> +#include<linux/compiler.h>
>   #include<asm/asm-compat.h>
>
>   /*
> @@ -71,7 +72,7 @@
>   	unreachable();						\
>   } while (0)
>
> -#define BUG_ON(x) do {						\
> +#define __BUG_ON(x) do {					\
>   	if (__builtin_constant_p(x)) {				\
>   		if (x)						\
>   			BUG();					\
> @@ -85,6 +86,8 @@
>   	}							\
>   } while (0)
>
> +#define BUG_ON(x) __BUG_ON(unlikely(x))
> +

This is the same type of frobbing you were trying to do to MIPS.

I will let the powerpc maintainers weigh in on it, but my opinion is 
that, as with MIPS, BUG_ON() is expanded to a single machine 
instruction, and this unlikely() business will not change the generated 
code in any useful way.  It is thus gratuitous code churn and 
complexification.

David Daney

>   #define __WARN_TAINT(taint) do {				\
>   	__asm__ __volatile__(					\
>   		"1:	twi 31,0,0\n"				\

Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

[Scott Wood]

On Thu, 27 Jan 2011 09:57:39 -0800
David Daney <ddaney@caviumnetworks.com> wrote:

> On 01/27/2011 04:12 AM, Coly Li wrote:
> > diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> > index 065c590..10889a6 100644
> > --- a/arch/powerpc/include/asm/bug.h
> > +++ b/arch/powerpc/include/asm/bug.h
> > @@ -2,6 +2,7 @@
> >   #define _ASM_POWERPC_BUG_H
> >   #ifdef __KERNEL__
> >
> > +#include<linux/compiler.h>
> >   #include<asm/asm-compat.h>
> >
> >   /*
> > @@ -71,7 +72,7 @@
> >   	unreachable();						\
> >   } while (0)
> >
> > -#define BUG_ON(x) do {						\
> > +#define __BUG_ON(x) do {					\
> >   	if (__builtin_constant_p(x)) {				\
> >   		if (x)						\
> >   			BUG();					\
> > @@ -85,6 +86,8 @@
> >   	}							\
> >   } while (0)
> >
> > +#define BUG_ON(x) __BUG_ON(unlikely(x))
> > +
> 
> This is the same type of frobbing you were trying to do to MIPS.
> 
> I will let the powerpc maintainers weigh in on it, but my opinion is 
> that, as with MIPS, BUG_ON() is expanded to a single machine 
> instruction, and this unlikely() business will not change the generated 
> code in any useful way.  It is thus gratuitous code churn and 
> complexification.

What about just doing this:

#define BUG() __builtin_trap()

#define BUG_ON(x) do {	\
	if (x) \
		BUG(); \
} while (0)

GCC should produce better code than forcing it into twnei.  A simple
BUG_ON(x != y) currently generates something like this (GCC 4.3):

xor     r0,r11,r0
addic   r10,r0,-1
subfe   r9,r10,r0
twnei   r9,0

Or this (GCC 4.5):

xor     r0,r11,r0
cntlzw	r0,r0
srwi	r0,r0,5
xori	r0,r0,1
twnei   r0,0

Instead of:

twne	r0,r11

And if GCC doesn't treat code paths that lead to __builtin_trap() as
unlikely (which could make a difference with complex expressions,
even with a conditional trap instruction), that's something that could
and should be fixed in GCC.

The downside is that GCC says, "The mechanism used may vary from
release to release so you should not rely on any particular
implementation."  However, some architectures (sparc, m68k, ia64)
already use __builtin_trap() for this, it seems unlikely that future GCC
versions would switch away from using the trap instruction[1], and there
doesn't seem to be a better-defined way to make GCC generate trap
instructions intelligently.

-Scott

[1] A more likely possibility is that an older compiler just generates a
call to abort() or similar, and later versions implemented trap -- need
to check what the oldest supported GCC does.

Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

[David Daney]

On 01/27/2011 12:04 PM, Scott Wood wrote:
> On Thu, 27 Jan 2011 09:57:39 -0800
> David Daney<ddaney@caviumnetworks.com>  wrote:
>
>> On 01/27/2011 04:12 AM, Coly Li wrote:
>>> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
>>> index 065c590..10889a6 100644
>>> --- a/arch/powerpc/include/asm/bug.h
>>> +++ b/arch/powerpc/include/asm/bug.h
>>> @@ -2,6 +2,7 @@
>>>    #define _ASM_POWERPC_BUG_H
>>>    #ifdef __KERNEL__
>>>
>>> +#include<linux/compiler.h>
>>>    #include<asm/asm-compat.h>
>>>
>>>    /*
>>> @@ -71,7 +72,7 @@
>>>    	unreachable();						\
>>>    } while (0)
>>>
>>> -#define BUG_ON(x) do {						\
>>> +#define __BUG_ON(x) do {					\
>>>    	if (__builtin_constant_p(x)) {				\
>>>    		if (x)						\
>>>    			BUG();					\
>>> @@ -85,6 +86,8 @@
>>>    	}							\
>>>    } while (0)
>>>
>>> +#define BUG_ON(x) __BUG_ON(unlikely(x))
>>> +
>>
>> This is the same type of frobbing you were trying to do to MIPS.
>>
>> I will let the powerpc maintainers weigh in on it, but my opinion is
>> that, as with MIPS, BUG_ON() is expanded to a single machine
>> instruction, and this unlikely() business will not change the generated
>> code in any useful way.  It is thus gratuitous code churn and
>> complexification.
>
> What about just doing this:
>
> #define BUG() __builtin_trap()
>
> #define BUG_ON(x) do {	\
> 	if (x) \
> 		BUG(); \
> } while (0)
>
> GCC should produce better code than forcing it into twnei.  A simple
> BUG_ON(x != y) currently generates something like this (GCC 4.3):
>
> xor     r0,r11,r0
> addic   r10,r0,-1
> subfe   r9,r10,r0
> twnei   r9,0
>
> Or this (GCC 4.5):
>
> xor     r0,r11,r0
> cntlzw	r0,r0
> srwi	r0,r0,5
> xori	r0,r0,1
> twnei   r0,0
>
> Instead of:
>
> twne	r0,r11
>
> And if GCC doesn't treat code paths that lead to __builtin_trap() as
> unlikely (which could make a difference with complex expressions,
> even with a conditional trap instruction), that's something that could
> and should be fixed in GCC.
>
> The downside is that GCC says, "The mechanism used may vary from
> release to release so you should not rely on any particular
> implementation."  However, some architectures (sparc, m68k, ia64)
> already use __builtin_trap() for this, it seems unlikely that future GCC
> versions would switch away from using the trap instruction[1], and there
> doesn't seem to be a better-defined way to make GCC generate trap
> instructions intelligently.
>

This is good in theory, however powerpc has this _EMIT_BUG_ENTRY 
business that wouldn't work with __builtin_trap().

David Daney

> -Scott
>
> [1] A more likely possibility is that an older compiler just generates a
> call to abort() or similar, and later versions implemented trap -- need
> to check what the oldest supported GCC does.
>

RE: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

[David Laight]

=20
> > +#define __BUG_ON(x) do {					\
> >   	if (__builtin_constant_p(x)) {				\
> >   		if (x)						\
> >   			BUG();					\
> > @@ -85,6 +86,8 @@
> >   	}							\
> >   } while (0)
> >
> > +#define BUG_ON(x) __BUG_ON(unlikely(x))
> > +

>From my experiments, adding an 'unlikely' at that point isn't
enough for non-trivial conditions - so its presence will
give a false sense the the optimisation is present!
In particular 'if (unlikely(x && y))' needs to be
'if (unlikely(x) && unlikely(y))' in order to avoid
mispredicted branches when 'x' is false.

Also, as (I think) in some of the generated code quoted,
use of __builtin_expect() with a boolean expression can
force some versions of gcc to generate the integer
value of the expression - rather than just selecting the
branch instructions that statically predict the
normal code path.

Sometimes I've also also had to add an asm() statement
that generates no code in order to actually force a
forwards branch (so it has something to place at the
target).

(I've been counting clocks ....)

	David

Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

[Andreas Schwab]

"David Laight" <David.Laight@ACULAB.COM> writes:

> Also, as (I think) in some of the generated code quoted,
> use of __builtin_expect() with a boolean expression can
> force some versions of gcc to generate the integer
> value of the expression

That's more likely a side effect of the definition of likely/unlikely:
they expand to !!(x).

Andreas.

-- 
Andreas Schwab, schwab@redhat.com
GPG Key fingerprint = D4E8 DBE3 3813 BB5D FA84  5EC7 45C6 250E 6F00 984E
"And now for something completely different."

Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON()

[Coly Li]

On 2011年01月28日 18:14, Andreas Schwab Wrote:
> "David Laight"<David.Laight@ACULAB.COM>  writes:
>
>> Also, as (I think) in some of the generated code quoted,
>> use of __builtin_expect() with a boolean expression can
>> force some versions of gcc to generate the integer
>> value of the expression
>
> That's more likely a side effect of the definition of likely/unlikely:
> they expand to !!(x).
>

It seems whether or not using unlikely() inside arch implemented BUG_ON() is arch dependent. Maybe a reasonable method 
to use BUG_ON() is,
1) do not explicitly use unlikely() when using macro BUG_ON().
2) whether or not using unlikely() inside BUG_ON(), it depends on the implementation of BUG_ON(), including arch 
implementation.

So from current feed back, doing "unlikely() optimization" here doesn't make anything better.

Thanks for all of your feed back :-)

-- 
Coly Li