Re: [RFC] sched: Do not move expensive insns speculatively (PR68664)

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Jan 26, 2017 at 05:00:44PM -0800, Andrew Pinski wrote:
> On Thu, Jan 26, 2017 at 4:38 PM, Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > Scheduling should never move very expensive instructions to places they
> > are executed more frequently.  This patch fixes that, reducing the
> > execution time of c-ray by over 40% (I tested on a BE Power7 system).
> >
> > Is there some existing way to test for "very expensive insn" or "very
> > expensive insn we should not speculate"?  Should there be a new hook?
> > Is only disallowing (FP) SQRT and DIV a good solution?
> 
> Seems like it should be checking the insn cost and compare that
> against some parameter.  That is possibly set by the target if needed.

But what is "insn cost"?  Latency is no good at all -- we *want* insns
with higher latency to be earlier.  fsqrt is not pipelined, and that is
what makes it so costly.  (This isn't modeled in the scheduling
description btw: that would make the automata sizes explode, the usual
problem).


Segher