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Re: negative latencies
- From: Vladimir Makarov <vmakarov at redhat dot com>
- To: shmeel gutl <shmeelgutl at shmuelhome dot mine dot nu>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Tue, 20 May 2014 11:13:37 -0400
- Subject: Re: negative latencies
- Authentication-results: sourceware.org; auth=none
- References: <5379A125 dot 5090405 at shmuelhome dot mine dot nu>
On 05/19/2014 02:13 AM, shmeel gutl wrote:
> Are there hooks in gcc to deal with negative latencies? In other
> words, an architecture that permits an instruction to use a result
> from an instruction that will be issued later.
Could you explain more on *an example* what are you trying to achieve
with the negative latency.
Scheduler is based on a critical path algorithm. Generally speaking
latency time can be negative for this algorithm. But I guess that is
not what you are asking.
> At first glance it seems that it will will break a few things.
> 1) The definition of dependencies cannot come from the simple ordering
> of rtl.
> 2) The scheduling problem starts to look like "get off the train 3
> stops before me".
> 3) The definition of live ranges needs to use actual instruction
> timing information, not just instruction sequencing.
> The hooks in the scheduler seem to be enough to stop damage but not
> enough to take advantage of this "feature".