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Re: [RFC] sched: Do not move expensive insns speculatively (PR68664)
On Fri, Jan 27, 2017 at 1:38 PM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> 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 ;-)
Well, the testcase as written doesn't but if you speculate the sqrt it might?
> 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.
I understood that but then moving sth inside a loop is almost never a win.
Can't "not modeled" insns not be marked somehow in the pipeline description?
Richard.
>
>
> Segher