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

From: David Daney <[email-masked]>
Date: Thu, 27 Jan 2011 12:32:02 -0800

On 01/27/2011 12:04 PM, Scott Wood wrote:
> On Thu, 27 Jan 2011 09:57:39 -0800
> David Daney<[email-masked]>  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.
>