Re: RFC: PATCH: PR gas/12049: Unnecessary relaxation

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sun, Oct 17, 2010 at 8:35 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Sun, Oct 17, 2010 at 6:32 PM, Alan Modra <amodra@gmail.com> wrote:
>> On Sat, Oct 16, 2010 at 03:28:50PM -0700, H.J. Lu wrote:
>>> 1. Set address for each frag.
>>> 2. Relax and update address for each frag. ?For rs_machine_dependent
>>> frag, if its size grows with forward reference, update fr_address for
>>> all successive frags.
>>> 3. If stretched, goto 2.
>>>
>>> Does it make any senses?
>>
>> No it does not. ?The frag loop here is already two deep in loops (loop
>> until nothing changed this segment, and loop until nothing changed in
>> all segments). ?You've just made the innermost loop over frags
>> O(n**2). ?I think that is way too expensive.
>
> We can limit the new one to the first pass only.
>
>> A possible fix for your testcase is to allow relax_frag to shrink
>> rs_machine_dependent frags, rather than just grow them as it does at
>> the moment.
>>
>
> I tried it and it doesn't work. The problem is
>
> ? ? ? .fill 14, 1, 0x90
> ? ? ? jmp ? ? .LBB1_23 ? <- frag 1
> ? ? ? .fill 14, 1, 0x90
> ? ? ? jmp ? ? .LBB1_23 ?<- frag 2
> ? ? ? .fill 18, 1, 0x90
> ? ? ? .align ?16, 0x90 ? ?<- frag 3
> ? ? ? .fill 96, 1, 0x90
> .LBB1_23:
>
> After frag 1 is grown by 4 bytes, we may not
> shrink frag 2 unless the address of frag 3
> is adjusted first, which is what my patch does.
>

Here is the updated patch to do it only for the first iteration.


-- 
H.J.

Attachment: gas-pr12049-2.patch
Description: Text document