patch to fix PR83712

[Vladimir Makarov]

A few people reported that the patch broke i686.  I am going to work on 
the patch more.  Meanwhile I've reverted the patch.

On 03/09/2018 11:16 AM, Vladimir Makarov wrote:
> The following patch fixes
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83712
>
> It is another "cannot find a spill reg for reload" problem.  LRA has 
> already a code splitting hard reg live ranges to avoid such problem.  
> This code is in LRA inheritance pass.  Unfortunately, the code does 
> splitting for small class pseudos only.  This PR is a more complicated 
> code and it is hard to adapt the inheritance sub-pass to reliably 
> solve such problems.
>
> To fix the PR, I added a sub-pass which works in very rare cases after 
> we already found that we have no hard regs for a reload pseudo.  It 
> tries to split a hard reg live range for the pseudo. After that it 
> tries again to assign a hard reg to the pseudo.  The patch changes 
> LRA-subpass flow for this.  I hope that the patch will finally solved 
> all such problems but I am not sure to be completely certain.
>
> The patch was bootstrapped and tested on x86-64 and ppc64.
>
> Committed as rev. 258390.
>