[RFC PATCH 0/2] Enable hardfloat for PPC

From: BALATON Zoltan <balaton@eik.bme.hu>
To: qemu-devel@nongnu.org
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>,
	John Arbuckle <programmingkidx@gmail.com>,
	qemu-ppc@nongnu.org, Paul Clarke <pc@us.ibm.com>,
	Howard Spoelstra <hsp.cat7@gmail.com>,
	David Gibson <david@gibson.dropbear.id.au>
Subject: [RFC PATCH 0/2] Enable hardfloat for PPC
Date: Mon, 17 Feb 2020 02:54:21 +0100	[thread overview]
Message-ID: <cover.1581904461.git.balaton@eik.bme.hu> (raw)

Hello,

This is an RFC series to start exploring the possibility of enabling
hardfloat for PPC target that haven't progressed in the last two years.
Hopefully we can work out something now. Previously I've explored this
here:

https://lists.nongnu.org/archive/html/qemu-ppc/2018-07/msg00261.html

where some ad-hoc benchmarks using lame mp3 encoder is also explained
that has two versions: one using VMX and another only using FP. Both
are mostly floating point bounded. I've run this test on mac99 under
MorphOS before and after my patches, also verifying that md5sum of
resulting mp3 matches (this is no proof for correctness but maybe
shows it did not break too much at least those ops used by this
program).

I've got these measurements on an Intel i7-9700K CPU @ 3.60GHz (did
not bother to take multiple samples so these are just approximate):

1) before patch series using softfloat:
lame: 4:01, lame_vmx: 3:14

2) only enabling hardfloat in fpu/softfloat.c without other changes:
lame: 4:06, lame_vmx: 2:06
(this shows why hardfloat was disabled but VMX can benefit from this)

3) with this series, hardfloat=true:
lame: 3:15, lame_vmx: 1:59
(so the patch does something even if there should be more places to
preserve inexact flag to fully use hardfloat)

4) with this series but forcing softfloat with hardfloat=false:
lame: 4:11, lame_vmx: 2:08
(unfortunately it's slower than before, likely due to adding if () to
helper_reset_fpstatus() that should be avoided to at least get back
previous hardfloat enabled case that's still slower than softfloat so
this series only makes sense if the default can be hardfloat=true at
the moment but even that would need more testing)

I hope others can contribute to this by doing more testing to find out
what else this would break or give some ideas how this could be
improved.

Regards,
BALATON Zoltan

BALATON Zoltan (2):
  target/ppc/cpu: Add hardfloat property
  target/ppc: Enable hardfloat for PPC

 fpu/softfloat.c                 | 14 +++++++-------
 target/ppc/cpu.h                |  2 ++
 target/ppc/fpu_helper.c         |  7 ++++++-
 target/ppc/translate_init.inc.c |  2 ++
 4 files changed, 17 insertions(+), 8 deletions(-)

-- 
2.21.1