On Tue, 28 Apr 2020, Alex Bennée wrote:
罗勇刚(Yonggang Luo) <luoyongg...@gmail.com> writes:
I am confusing why only inexact are set then we can use hard-float.
The inexact behaviour of the host hardware may be different from the
guest architecture we are trying to emulate and the host hardware may
not be configurable to emulate the guest mode.
Have a look in softfloat.c and see all the places where
float_flag_inexact is set. Can you convince yourself that the host
hardware will do the same?
Can you convince me that it won't? This all seems to be guessing without
evidence so I think what we need first is some tests to prove it either
way. Such tests could then also be used at runtime to decide if the host
and guest FPU are compatible enough to enable hardfloat. Are such tests
available somewhere or what would need to be done to implement them?
This may not solve the problem with PPC target with non-cumulative status
bits but could improve hardfloat performance at least for some host-guest
combinations. To see if it worth the effort we should run such test on
common combinations (say x86_64. ARM and PPC hosts with at least these
guests).
And PPC always clearing inexact flag before calling to soft-float
funcitons. so we can not
optimize it with hard-float.
I need some resouces about ineact flag and why always clearing inexcat in
PPC FP simualtion.
Because that is the behaviour of the PPC floating point unit. The
inexact flag will represent the last operation done.
More precisely additional to the usual cumulative (or sticky) bits there
are two non-sticky bits for inexact and rounded (latter of which is not
emulated) that currently need clearing FP status before every FP op. I
wonder if we can know when the guest reads these and rerun the last FP op
in softfloat to compute them only if these are read, then it's enough to
remember the last FP op. This could be relatively simple and may be used
even if we don't detect accessing the bits within FPSCR just accessing the
FPSCR as likely most guest code does not check that and any cross-platform
code won't check PPC specific non-sticky bits so I'd exepect most guest
code to be fine with hardfloat. Although what about FP exceptions? We also
need to revert to softfloat it FP exceptions are enabled so maybe using
host FP exception for managing status bits could be the way to go to let
hardware manage this and we don't need to implement everything in
software.
Regards,
BALATON Zoltan
