Re: [mips] fix $gp restore bug

[Adam Nemet <anemet at caviumnetworks dot com>]

Richard Sandiford writes:
> FWIW, the idea I'm toying with at the moment is:
> 
>   - Make every branch use pic_offset_table_rtx.
> 
>   - Try to detect cases where a function only needs a global pointer
>     because of branches.  In this case, emit easily-identifiable "ghost"
>     instructions (in the .md "type" attribute sense) to set, save and
>     restore the gp.
> 
>     These ghost instructions would replace the sequences we'd normally use.
>     They would have 0 length at this stage, and in theory shouldn't interfere
>     with things like scheduling.
> 
>   - At the end of md_reorg, run shorten_branches to see whether
>     all branches are in range.  This should be safe because we run
>     dbr_schedule within mips_reorg.  Nothing after mips_reorg should
>     change the insn stream besides shorten_branches itself.
> 
>   - If some branches are not in range, go through the insn stream and
>     replace all the ghost instructions with real implementations.
>     (This will of course invalidate the results of the shorten_branches
>     call, but the "real" shorten_branches pass will then calculate new
>     lengths.)

This sounds like a good plan to me, FWIW.

> Besides the complication, the main drawback I can see is that we'd be
> unable to fill the delay slots of branches with insns that restore $gp,
> even in the usual case where no long branches are needed.
> 
> That's certainly a problem for -Os.  I'm not sure it's much of a concern
> for other options.  dbr_schedule doesn't take the pipeline into account
> at all when filling delay slots, and postponing a restoration of $gp
> isn't always a good idea performance-wise.

I guess this applies more to the old ABIs.  For the new ABIs all we lose is
the possibility to optimally schedule the pic_offset_table_rtx instructions
and that the register in pic_offset_table_rtx won't be available to the
register allocator even though it might be never used or used for a shorter
time than assumed.

Adam