Re: [PATCH, RFT] byteswap: try to avoid __builtin_constant_p gcc bug

[Quinn Tran]

-----Original Message-----
From: <linux-scsi-owner@xxxxxxxxxxxxxxx> on behalf of Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Date: Wednesday, April 27, 2016 at 3:11 PM
To: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Martin Jambor <mjambor@xxxxxxx>, "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>, linux-scsi <linux-scsi@xxxxxxxxxxxxxxx>, linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>, Denys Vlasenko <dvlasenk@xxxxxxxxxx>, Thomas Graf <tgraf@xxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, David Rientjes <rientjes@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Himanshu Madhani <himanshu.madhani@xxxxxxxxxx>, Dept-Eng QLA2xxx Upstream <qla2xxx-upstream@xxxxxxxxxx>, Jan Hubicka <hubicka@xxxxxx>
Subject: Re: [PATCH, RFT] byteswap: try to avoid __builtin_constant_p gcc bug

>On Thu, Apr 28, 2016 at 12:00:36AM +0200, Arnd Bergmann wrote:
>> This is another attempt to avoid a regression in wwn_to_u64()
>> after that started using get_unaligned_be64(), which in turn
>> ran into a bug on gcc-4.9 through 6.1.
>> 
>> As part of the problem is how __builtin_constant_p gets evaluated
>> on an argument passed by reference into an inline function, this
>> avoids the use of __builtin_constant_p() for all architectures
>> that set CONFIG_ARCH_USE_BUILTIN_BSWAP. Most architectures do not
>> set ARCH_SUPPORTS_OPTIMIZED_INLINING, which means they probably
>> do not suffer from the problem in the qla2xxx driver, but they
>> might still run into it elsewhere.
>> 
>> I have not been able to reproduce the original problem, so I don't
>> know if this patch solves it, but at least it leads to simpler
>> code doing the same thing, so at least there should be no downsides.
>> 
>> Please test.
>> 
>> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
>
>Nice patch.  I can confirm it fixes the issue with gcc 5.3.1.
>
>Tested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>
>> diff --git a/include/uapi/linux/swab.h b/include/uapi/linux/swab.h
>> index 3f10e5317b46..de56fd54428d 100644
>> --- a/include/uapi/linux/swab.h
>> +++ b/include/uapi/linux/swab.h
>> @@ -45,9 +45,7 @@
>>  
>>  static inline __attribute_const__ __u16 __fswab16(__u16 val)
>>  {
>> -#ifdef __HAVE_BUILTIN_BSWAP16__
>> -	return __builtin_bswap16(val);
>> -#elif defined (__arch_swab16)
>> +#if defined (__arch_swab16)
>>  	return __arch_swab16(val);
>>  #else
>>  	return ___constant_swab16(val);
>> @@ -56,9 +54,7 @@ static inline __attribute_const__ __u16 __fswab16(__u16 val)
>>  
>>  static inline __attribute_const__ __u32 __fswab32(__u32 val)
>>  {
>> -#ifdef __HAVE_BUILTIN_BSWAP32__
>> -	return __builtin_bswap32(val);
>> -#elif defined(__arch_swab32)
>> +#if defined(__arch_swab32)
>>  	return __arch_swab32(val);
>>  #else
>>  	return ___constant_swab32(val);
>> @@ -67,9 +63,7 @@ static inline __attribute_const__ __u32 __fswab32(__u32 val)
>>  
>>  static inline __attribute_const__ __u64 __fswab64(__u64 val)
>>  {
>> -#ifdef __HAVE_BUILTIN_BSWAP64__
>> -	return __builtin_bswap64(val);
>> -#elif defined (__arch_swab64)
>> +#if defined (__arch_swab64)
>>  	return __arch_swab64(val);
>>  #elif defined(__SWAB_64_THRU_32__)
>>  	__u32 h = val >> 32;
>> @@ -102,28 +96,40 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
>>   * __swab16 - return a byteswapped 16-bit value
>>   * @x: value to byteswap
>>   */
>> +#ifdef __HAVE_BUILTIN_BSWAP16__
>> +#define __swab16(x) __builtin_bswap16((__u16)(x))
>> +#else
>>  #define __swab16(x)				\
>>  	(__builtin_constant_p((__u16)(x)) ?	\
>>  	___constant_swab16(x) :			\
>>  	__fswab16(x))
>> +#endif
>>  
>>  /**
>>   * __swab32 - return a byteswapped 32-bit value
>>   * @x: value to byteswap
>>   */
>> +#ifdef __HAVE_BUILTIN_BSWAP32__
>> +#define __swab32(x) __builtin_bswap32((__u32)(x))
>> +#else
>>  #define __swab32(x)				\
>>  	(__builtin_constant_p((__u32)(x)) ?	\
>>  	___constant_swab32(x) :			\
>>  	__fswab32(x))
>> +#endif
>>  
>>  /**
>>   * __swab64 - return a byteswapped 64-bit value
>>   * @x: value to byteswap
>>   */
>> +#ifdef __HAVE_BUILTIN_BSWAP64__
>> +#define __swab64(x) __builtin_bswap64((__u64)(x))
>> +#else
>>  #define __swab64(x)				\
>>  	(__builtin_constant_p((__u64)(x)) ?	\
>>  	___constant_swab64(x) :			\
>>  	__fswab64(x))
>> +#endif
>>  
>>  /**
>>   * __swahw32 - return a word-swapped 32-bit value
>> 

The patch works. Thanks.

>
>-- 
>Josh
>--
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