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Re: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC

From: BALATON Zoltan
Subject: Re: [RFC PATCH v2] target/ppc: Enable hardfloat for PPC
Date: Tue, 18 Feb 2020 18:38:20 +0100 (CET)
User-agent: Alpine 2.22 (BSF 395 2020-01-19)

On Tue, 18 Feb 2020, BALATON Zoltan wrote:
While other targets take advantage of using host FPU to do floating
point computations, this was disabled for PPC target because always
clearing exception flags before every FP op made it slightly slower
than emulating everyting with softfloat. To emulate some FPSCR bits,
clearing of fp_status may be necessary (unless these could be handled
e.g. using FP exceptions on host but there's no API for that in QEMU
yet) but preserving at least the inexact flag makes hardfloat usable
and faster than softfloat. Since most clients don't actually care
about this flag, we can gain some speed trading some emulation

This patch implements a simple way to keep the inexact flag set for
hardfloat while still allowing to revert to softfloat for workloads
that need more accurate albeit slower emulation. (Set hardfloat
property of CPU, i.e. -cpu name,hardfloat=false for that.) There may
still be room for further improvement but this seems to increase
floating point performance. Unfortunately the softfloat case is slower
than before this patch so this patch only makes sense if the default
is also set to enable hardfloat.

Because of the above this patch at the moment is mainly for testing
different workloads to evaluate how viable would this be in practice.
Thus, RFC and not ready for merge yet.

Signed-off-by: BALATON Zoltan <address@hidden>

Forgot to say, in case that's not clear that this should currently apply to David's ppc-for-5.0 branch because there are some clean up patches from me queued there so until those reach master this patch may not apply cleanly. Just in case someone wanted to try it and get errors.


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