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

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

On Fri, Jan 27, 2017 at 01:15:27PM +0100, Richard Biener wrote:
> As both SQRT and DIV may trap I wonder how we can end up speculating
> them at all?

The testcase uses -ffast-math.

> Ok, maybe with -fno-trapping-math we don't consider that case but even
> then generating
> a NaN is usually dreadfully slow so avoiding speculation of such insns
> looks good in
> any case (w/o considering its cost).

And -ffast-math includes -ffinite-math-only.  No, the testcase never
takes the square root of number smaller than zero, it isn't *that* slow ;-)

Things slow down so much because there is a loop immediately followed
by a square root insn, and sched-rgn decides it is a good idea to move
it to inside the loop.  Which is a bad idea no matter what the frequency
of the loop is because 1) we do not get such profiles very correct, and
2) sqrt is really expensive.


Segher