Change x86 default arch for 4.5?

[Jason Merrill]

On 02/18/2010 07:46 PM, Joseph S. Myers wrote:
> On Thu, 18 Feb 2010, Jason Merrill wrote:
>
>> I periodically get bitten by bug 34115: a compiler configured without
>> --with-arch on i686-pc-linux-gnu doesn't support atomics.  I think we would
>> only need to bump the default to i486 to get atomic support.  Can we
>> reconsider the default for 4.5?
>
> My position remains that configuring for i686-* should default to
> -march=i686 rather than -mtune=i686.

That makes sense to me.

> Perhaps someone would like to review HJ's patch to that effect?
>
> http://gcc.gnu.org/ml/gcc-patches/2008-08/msg02078.html

Using all the noncanonical variants as arch defaults makes me a bit 
nervous, but I suppose if someone actually writes $GCC/configure 
nocona-pc-linux-gnu that's what they would expect to get...

> This will probably break building glibc, as problems building when __i686
> is a predefined macro have been known since at least 2002 but none of the
> many patches proposed since then have been accepted.

I imagine changing the default would help with that...and packagers can 
work around it.

Jason