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Re: [PATCH 3/8] Remove cached_first_cycle_multipass_dfa_lookahead and cached_issue_rate
- From: Maxim Kuvyrkov <maxim dot kuvyrkov at linaro dot org>
- To: Vladimir Makarov <vmakarov at redhat dot com>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Thu, 23 Oct 2014 16:29:39 +1300
- Subject: Re: [PATCH 3/8] Remove cached_first_cycle_multipass_dfa_lookahead and cached_issue_rate
- Authentication-results: sourceware.org; auth=none
- References: <EF6BB45F-FB61-4134-A9D0-FBF0AD2E70EE at linaro dot org> <5446815C dot 6050609 at redhat dot com>
On Oct 22, 2014, at 4:53 AM, Vladimir Makarov <vmakarov@redhat.com> wrote:
> On 10/20/2014 11:16 PM, Maxim Kuvyrkov wrote:
>> Hi,
>>
>> This patch cleans up (removes) cached_first_cycle_multipass_dfa_lookahead and cached_issue_rate.
>>
>> These seem to be an artifact from the scheduler refactoring 10+ years ago. They assume that dfa_lookahead and issue_rate can change mid-way through scheduling, which is never the case. All backends currently treat dfa_lookahead and issue_rate as constants for the duration of scheduling passes.
>>
>> Bootstrapped on x86_64-linux-gnu. Regression testing is in progress. OK to commit if no regressions?
>>
> Yes. the patch for issue rate itself is ok but you should have modified
> doc/tm.texi too for dfa lookahead hook saying that it should be a
> constant (issue rate already has such clause).
>
> On the other hand I'd not assume that dfa look ahead is a constant. In
> future we could make it non-constant to differentiate non-hot and hot
> functions to speed up the scheduler as dfa look ahead scheduling is
> pretty expensive.
>
> So issue rate change is ok but I'd not rush to change dfa look ahead
> related code.
It is trivial to prove that currently cached_first_cycle_multipass_dfa_lookahead always has the same value as dfa_lookahead. And, even should targetm.sched.first_cycle_multipass_dfa_lookahead start returning different values, max_issue will happily continue to use the value that the hook returned in sched_init().
Also, your suggestion to use different dfa_lookahead values for hot/cold functions is not affected by this patch. The values of dfa_lookahead variable have scope of the scheduling pass, which is invoked separately for every function.
I guess, you could, potentially, start differentiating values of dfa_lookahead based on whether a basic_block is hot or cold, but handling this scenario would require significant changes throughout the scheduler.
To summarize, dfa_lookahead is currently a pass-time invariant, that is free to change between invocations of the scheduler pass. Cached_first_cycle_multipass_dfa_lookahead always holds the same value as dfa_lookahead, and, as such, is extraneous.
Thank you,
--
Maxim Kuvyrkov
www.linaro.org