Re: What exactly is our CRC algorithm?

On Thu, Apr 2, 2015 at 5:33 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> It was added in gcc 4.2. That's good enough for me.

I think it's fine to have optional optimizations that require gcc >=
4.2, as long as older platforms don't break outright.

>>> We have a buildfarm animal that still uses gcc 2.95.3, which was
>>> released in 2001. I don't have a compiler of that vintage to test
>>> with, but I assume an old enough assembler would not know about the
>>> crc32q instruction and fail to compile.
>>
>>
>> GCC from <2002 wouldn't support the symbolic operand names in inline
>> assembly. binutils from <2007 (IIRC) wouldn't support the assembler
>> instructions themselves.
>>
>> We could work around the latter by using the appropriate sequence of
>> bytes. We could work around the former by using the old syntax for
>> operands.
>
> I'm OK with not supporting the new instructions when building with an old
> compiler/assembler. But the build shouldn't fail with an old
> compiler/assembler. Using old syntax or raw bytes just to avoid failing on
> an ancient compiler seems ugly.

I dunno about old syntax, but raw bytes seems like a bad idea, for sure.

--
Robert Haas
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