Re: [mips] fix $gp restore bug

[Richard Sandiford <rdsandiford at googlemail dot com>]

Adam Nemet <anemet@caviumnetworks.com> writes:
> 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.

Thanks.  I tried it, and unfortunately it doesn't look like it's
going to work.  Too much rtl infrastructure expects unconditional
jumps to be simple sets.  I'm also not sure they really want a
situation where inserting a jump can suddenly change the liveness
of $gp.

Oh well.  Back to the drawing board.

Richard