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Re: negative latencies


On 19-May-14 09:39 AM, Andrew Pinski wrote:
On Sun, May 18, 2014 at 11:13 PM, shmeel gutl
<shmeelgutl@shmuelhome.mine.nu> 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.
Do you mean bypasses?  If so there is a bypass feature which you can use:
https://gcc.gnu.org/onlinedocs/gccint/Processor-pipeline-description.html#index-data-bypass-3773

Thanks,
Andrew Pinski
Unfortunately, bypasses in the pipeline description is not enough.
They only allow you to calculate the latency of true dependencies. They are also forced to be zero or greater. The real question is how the scheduler and register allocator can deal with negative latencies.

Thanks
Shmeel
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".

Thanks


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