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Subject: Re: [OMPI devel] Bug in openmpi-1.5/opal/config/opal_config_asm.m4
From: George Bosilca (bosilca_at_[hidden])
Date: 2011-02-24 01:56:09


Going on the Redhat thread regarding this issue (https://bugzilla.redhat.com/show_bug.cgi?id=679489), one can find the following comment

> Jakub Jelinek 2011-02-22 19:53:22 EST
> Could be "+m" (ret) too, but I think the "=m" + "m" variant should cover even
> prehistoric buggy gccs while "+m" might not work well there.

Apparently, for historical reasons the code should have =m on the output and m on the input. I could live with that. I'll take a look at our assembly to fix this.

  george.

On Feb 23, 2011, at 17:03 , George Bosilca wrote:

> Or how about this version ? Here I use the + modifier and I don't put any constraints on the input line.
>
> static inline int32_t opal_atomic_add_32(volatile int32_t* v, int i)
> {
> int ret = i;
> __asm__ __volatile__(
> SMPLOCK "xaddl %1,%0"
> : "+m" (*v), "+r" (ret)
> :
> : "memory", "cc"
> );
> return (ret+i);
> }
>
> george.
>
> On Feb 23, 2011, at 16:59 , George Bosilca wrote:
>
>> Jay,
>>
>> Thanks for the code. The code you pointed out is only used during configure, so I don't think is that critical. However, we use similar code deep into our voodoo assembly generation, for opal_atomic_add_32 and opal_atomic_sub_32.
>>
>> So if I understand your statement the correct version of the code should be
>>
>> static inline int32_t opal_atomic_add_32(volatile int32_t* v, int i)
>> {
>> int ret = i;
>> __asm__ __volatile__(
>> SMPLOCK "xaddl %1,%0"
>> :"=m" (*v), "+r" (ret)
>>>> new >> :"m" (*v)
>> );
>> return (ret+i);
>> }
>>
>> On the GCC extended ASM documentation (http://gcc.gnu.org/onlinedocs/gcc/Modifiers.html#Modifiers), it is specified:
>>
>> `=' Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data.
>>
>> `+' Means that this operand is both read and written by the instruction.
>>
>> Based on this info, I would rather rewrite this function like this:
>>
>> static inline int32_t opal_atomic_add_32(volatile int32_t* v, int i)
>> {
>> int ret = i;
>> __asm__ __volatile__(
>> SMPLOCK "xaddl %1,%0"
>> : "=m" (*v), "=r" (ret)
>> : "m" (*v), "1" (ret)
>> : "memory", "cc"
>> );
>> return (ret+i);
>> }
>>
>> Can you ask the kindly GCC expert which version is "correct" (whatever the definition of correct might means related to GCC extended assembly). Do I have to specify = for the output if I put the register on the input? Aren't this conflicting?
>>
>> george.
>>
>> On Feb 23, 2011, at 13:04 , Jay Fenlason wrote:
>>
>>> I was recently handed
>>> https://bugzilla.redhat.com/attachment.cgi?id=480307
>>> for which a kindly GCC expert attached the enclosed patch. Apparently
>>> this only causes problems on 32-bit i686 machines, which could by why
>>> it has gone undetected until now.
>>>
>>> -- JF
>>>
>>> --- openmpi-1.5/opal/config/opal_config_asm.m4.jj 2010-09-28 23:33:51.000000000 +0200
>>> +++ openmpi-1.5/opal/config/opal_config_asm.m4 2011-02-23 01:39:21.191433509 +0100
>>> @@ -885,7 +885,7 @@ AC_DEFUN([OMPI_CONFIG_ASM],[
>>> ompi_cv_asm_arch="AMD64"
>>> fi
>>> OPAL_ASM_SUPPORT_64BIT=1
>>> - OMPI_GCC_INLINE_ASSIGN='"xaddl %1,%0" : "=m"(ret), "+r"(negone)'
>>> + OMPI_GCC_INLINE_ASSIGN='"xaddl %1,%0" : "=m"(ret), "+r"(negone) : "m"(ret)'
>>> ;;
>>>
>>> ia64-*)
>>> _______________________________________________
>>> devel mailing list
>>> devel_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>>
>> "I disapprove of what you say, but I will defend to the death your right to say it"
>> -- Evelyn Beatrice Hall
>>
>>
>> _______________________________________________
>> devel mailing list
>> devel_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>
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>
>
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"To preserve the freedom of the human mind then and freedom of the press, every spirit should be ready to devote itself to martyrdom; for as long as we may think as we will, and speak as we think, the condition of man will proceed in improvement."
  -- Thomas Jefferson, 1799