Thanks for the comments.
The patch checked the usage of teh trip count register, making sure
that it is not used in the loop body other than the doloop_end or
lives past the doloop_end instruction, as the following code snippet
shows:
+ /* Scan all the blocks to make sure they don't use iter_reg. */
+ if (loop->iter_reg_used || loop->iter_reg_used_outside)
+ {
+ if (dump_file)
+ fprintf (dump_file, ";; loop %d uses iterator\n",
+ loop->loop_no);
+ return false;
+ }
For the spill issue, I think we need to handle it. The reason is
that currently we are not telling GCC about the existence of the
LCOUNT register. Instead, we keep the trip count in a general register
and it's possible that this register can be spilled when register
pressure is high.
It's a good idea to post another patch to describe the LCOUNT
register in GCC in order to free this general register. But I want
this patch applied as a first step, OK?
Cheers,
Felix
On Tue, Oct 14, 2014 at 12:09 AM, augustine.sterl...@gmail.com
<augustine.sterl...@gmail.com> wrote:
> On Fri, Oct 10, 2014 at 6:59 AM, Felix Yang <fei.yang0...@gmail.com> wrote:
>> Hi Sterling,
>>
>> I made some improvement to the patch. Two changes:
>> 1. TARGET_LOOPS is now used as a condition of the doloop related
>> patterns, which is more elegant.
>
> Fine.
>
>> 2. As the trip count register of the zero-cost loop maybe
>> potentially spilled, we need to change the patterns in order to handle
>> this issue.
>
> Actually, for xtensa you don't. The trip count is copied into LCOUNT
> at the execution of the loop instruction, and therefore a spill or
> whatever doesn't matter--it won't affect the result. So as long as you
> have the trip count at the start of the loop, you are fine.
>
> This does bring up an issue of whether or not the trip count can be
> modified during the loop. (note that this is different than early
> exit.) If it can, you can't use a zero-overhead loop. Does your patch
> address this case.
>
> The solution is similar to that adapted by c6x backend.
>> Just turn the zero-cost loop into a regular loop when that happens
>> when reload is completed.
>> Attached please find version 4 of the patch. Make check regression
>> tested with xtensa-elf-gcc/simulator.
>> OK for trunk?
