[Patch AArch64] Use software sqrt expansion always for -mlow-precision-recip-sqrt

[James Greenhalgh]

On Mon, Jan 11, 2016 at 04:57:56PM -0600, Evandro Menezes wrote:
> On 01/11/2016 05:53 AM, James Greenhalgh wrote:
> >I'd like to switch the logic around in aarch64.c such that
> >-mlow-precision-recip-sqrt causes us to always emit the low-precision
> >software expansion for reciprocal square root. I have two reasons to do
> >this; first is consistency across -mcpu targets, second is enabling more
> >-mcpu targets to use the flag for peak tuning.
> >
> >I don't much like that the precision we use for -mlow-precision-recip-sqrt
> >differs between cores (and possibly compiler revisions). Yes, we're
> >under -ffast-math but I take this flag to mean the user explicitly wants the
> >low-precision expansion, and we should not diverge from that based on an
> >internal decision as to what is optimal for performance in the
> >high-precision case. I'd prefer to keep things as predictable as possible,
> >and here that means always emitting the low-precision expansion when asked.
> >
> >Judging by the comments in the thread proposing the reciprocal square
> >root optimisation, this will benefit all cores currently supported by GCC.
> >To be clear, we would still not expand in the high-precision case for any
> >cores which do not explicitly ask for it. Currently that is Cortex-A57
> >and xgene, though I will be proposing a patch to remove Cortex-A57 from
> >that list shortly.
> >
> >Which gives my second motivation for this patch. -mlow-precision-recip-sqrt
> >is intended as a tuning flag for situations where performance is more
> >important than precision, but the current logic requires setting an
> >internal flag which also changes the performance characteristics where
> >high-precision is needed. This conflates two decisions the target might
> >want to make, and reduces the applicability of an option targets might
> >want to enable for performance. In particular, I'd still like to see
> >-mlow-precision-recip-sqrt continue to emit the cheaper, low-precision
> >sequence for floats under Cortex-A57.
> >
> >Based on that reasoning, this patch makes the appropriate change to the
> >logic. I've checked with the current -mcpu values to ensure that behaviour
> >without -mlow-precision-recip-sqrt does not change, and that behaviour
> >with -mlow-precision-recip-sqrt is to emit the low precision sequences.
> >
> >I've also put this through bootstrap and test on aarch64-none-linux-gnu
> >with no issues.
> >
> >OK?
> 
> Yes, it LGTM.

Thanks.

> I appreciate the idea of uniformity whne an option is specified,
> which led me to think if it wouldn't be a good ide to add an option
> that would have the effect of focring the emission of the reciprocal
> square root, effectively forcing the flag
> AARCH64_EXTRA_TUNE_RECIP_SQRT on, regardless of the tuning flags for
> the given core.  I think that this flag would be particularly useful
> when specifying flags for specific functions, irrespective of the
> core.
> 
> Thoughts?

Currently you can do this using the (mostly unsupported) -moverride
mechanism as -moverride=tune=recip_sqrt from the command line.
I'm not sure how reliable using this from
__attribute__((target("override=tune=recip_sqrt"))) would be, I wrote a small
testcase that didn't work as intended, but whether that is a bug or a
design decision I'm not yet sure. I think the logic for parsing the
target attribute is set up to reapply the command-line override string
over whichever tuning options you apply through the attribute, rather than
to allow you to apply a per-function override.

As to whether we'd want to expose this as a fully supported,
user-visible setting, I'd rather not. Our claim is that for the
higher-precision sequences the results are close enough that we can
consider this like reassociation width or other core-specific tuning
parameters that we don't expose. What I'm hoping to avoid is a
proliferation of supported options which are not in anybody's regular
testing matrix. This one would not be so bad as it is automatically
enabled by some cores. For now I'd rather not add the option.

Thanks,
James