Re: [google] remove redundant push {lr} for -mthumb (issue4441050)

[Carrot Wei <carrot at google dot com>]

On Tue, Apr 19, 2011 at 5:57 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Tue, Apr 19, 2011 at 11:41 AM, Guozhi Wei <carrot@google.com> wrote:
>> Reload pass tries to determine the stack frame, so it needs to check the
>> push/pop lr optimization opportunity. One of the criteria is if there is any
>> far jump inside the function. Unfortunately at this time gcc can't decide each
>> instruction's length and basic block layout, so it can't know the offset of
>> a jump. To be conservative it assumes every jump is a far jump. So any jump
>> in a function will prevent this push/pop lr optimization.
>>
>> To enable the push/pop lr optimization in reload pass, I compute the possible
>> maximum length of the function body. If the length is not large enough, far
>> jump is not necessary, so we can safely do push/pop lr optimization.
>
> What about hot/cold partitioning? ?That might cause jumps to different
> sections.
>
> Richard.
>

The hot/cold partitioning is disabled in arm backend.
http://gcc.gnu.org/ml/gcc-cvs/2009-10/msg00671.html.

thanks
Carrot