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Re: [PATCH] bb-reorder: Improve compgotos pass (PR71785)
On Wed, Nov 2, 2016 at 2:40 PM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> On Wed, Nov 02, 2016 at 11:39:20AM +0100, Steven Bosscher wrote:
>> On Wed, Nov 2, 2016 at 10:02 AM, Richard Biener
>> <richard.guenther@gmail.com> wrote:
>> > On Mon, Oct 31, 2016 at 4:35 PM, Segher Boessenkool wrote:
>> >> On Mon, Oct 31, 2016 at 04:09:48PM +0100, Steven Bosscher wrote:
>> >>> On Sun, Oct 30, 2016 at 8:10 PM, Segher Boessenkool wrote:
>> >>> > + cfg_layout_finalize ();
>> >>>
>> >>> I don't think you have to go into/out-of cfglayout mode for each iteration.
>> >>
>> >> Yeah probably. I was too lazy :-) It needs the cleanup_cfg every
>> >> iteration though, I was afraid that interacts.
>> >
>> > Ick. Why would it need a cfg-cleanup every iteration?
>>
>> I don't believe it needs a cleanup on every iteration. One cleanup at
>> the end should work fine.
>
> But as the comment there says:
>
> /* Merge the duplicated blocks into predecessors, when possible. */
> cleanup_cfg (0);
>
> (this is not a new comment), and without merging blocks this whole
> patch does zilch.
>
> There is no need to create new basic blocks at all (can replace the
> final branch in a block with a copy of the whole block it jumps to,
> instead); and then it is painfully obvious that switching to layout
> mode here is pointless, too.
>
> Do you want me to do that?
>
> Btw, this isn't quadratic anyway; it is a constant number (the maximal
> length allowed of the block to copy) linear. Unless there are unboundedly
> many forwarder blocks, which there shouldn't be, but I cannot prove that.
Well, you iterate calling functions (find candidates, merge, cleanup-cfg) that
walk the whole function. So unless the number of iterations is bound
with a constant I call this quadratic (in function size).
> And on a testcase with 2000 cases (instead of the 4 in the testcase in
> the PR) this pass takes less than 1% of the compiler runtime; and in
> the normal cases (no computed gotos to unfactor) it does the same as
> before.
>
>
> Segher