Re: Devs and rice flags (Was: Re: [gentoo-user] emerge -j, make -j and make -l )

[Michael Mol]

On Mon, Nov 28, 2011 at 1:56 PM, Florian Philipp <lists@×××××××××××.net> wrote:
> Am 28.11.2011 18:56, schrieb Michael Mol:
>> On Mon, Nov 28, 2011 at 11:46 AM, Pandu Poluan <pandu@××××××.info> wrote:
>> No, you've got some ugly flags in there. -fexcess-precision and
>> -funsafe-math-optimizations, in particular. (I must have been talking
>> to someone else last week; sorry, I'm terrible with names.)
>>
>
> I doubt -fexcess-precision=fast does anything at all. Pandu uses an
> AMD64 system, right? Then you have -mfpmath=sse set per default and SSE
> does not have excess precision issues (that's just for the old x87 FPU).
> Even if you used that, is redundant because of your other flags. To
> quote `man gcc`:
> "-fexcess-precision=standard is not implemented for languages other than
> C, and has no effect if -funsafe-math-optimizations or -ffast-math is
> specified." <-- Therefore you always have ..=fast anyway.
>
> -funsafe-math-optimizations is really terrible. Either you us floating
> point arithmetic, then you have to rely on it because it is hard enough
> already to gain necessary precision with it, or you don't, then you
> don't need that flag because it doesn't improve performance.

I didn't know (or forgot) what arch he was using.

>> -fomit-frame-pointer shouldn't cause any headaches unless you're
>> feeding a gdb stack trace, and you're not adding any debugging data,
>> so your stack traces would be pretty useless, anyway.
>>
>
> If you are on an AMD64 system, this flag is implied because it doesn't
> affect stack traces on x86_64 anymore.

AMD64 puts the requisite data in its own register, right?

Yeah, it sounds like Pandu's setup CFLAGS can use some cleanup.

>> I don't know about -floop-interchange, -floop-strip-mine or
>> -floop-block. I recognize at least one of them from the discussion of
>> graphite the other day.
>>
>
> These definitely need graphite to have any effect. Then they should be
> reasonably safe (as far as anything relying on experimental compiler
> frameworks can be considered safe).

Upstream devs might take issue with them, but I'm still not sure they
should affect bug reports of build-time failures. I would *hope*
upstream gcc is doing tests on its own build tools compiled with its
graphite optimizations. I don't know about make and autotools, though.

-- 
:wq