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.
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?
--
Evandro Menezes
