Re: [Qemu-devel] [PATCH v1 07/14] fpu: introduce hostfloat

[Laurent Vivier]

Le 21/03/2018 à 21:11, Emilio G. Cota a écrit :
> The appended paves the way for leveraging the host FPU for a subset
> of guest FP operations. For most guest workloads (e.g. FP flags
> aren't ever cleared, inexact occurs often and rounding is set to the
> default [to nearest]) this will yield sizable performance speedups.
> 
> The approach followed here avoids checking the FP exception flags register.
> See the comment at the top of hostfloat.c for details.
> 
> This assumes that QEMU is running on an IEEE754-compliant FPU and
> that the rounding is set to the default (to nearest). The
> implementation-dependent specifics of the FPU should not matter; things
> like tininess detection and snan representation are still dealt with in
> soft-fp. However, this approach will break on most hosts if we compile
> QEMU with flags such as -ffast-math. We control the flags so this should
> be easy to enforce though.
> 
> The licensing in softfloat.h is complicated at best, so to keep things
> simple I'm adding this as a separate, GPL'ed file.
> 
> This patch just adds some boilerplate code; subsequent patches add
> operations, one per commit to ease bisection.
> 
> Signed-off-by: Emilio G. Cota <address@hidden>
> ---
>  Makefile.target           |  2 +-
>  include/fpu/hostfloat.h   | 14 +++++++
>  include/fpu/softfloat.h   |  1 +
>  fpu/hostfloat.c           | 96 
> +++++++++++++++++++++++++++++++++++++++++++++++
>  target/m68k/Makefile.objs |  2 +-
>  tests/fp-test/Makefile    |  2 +-
>  6 files changed, 114 insertions(+), 3 deletions(-)
>  create mode 100644 include/fpu/hostfloat.h
>  create mode 100644 fpu/hostfloat.c
> 
...
> diff --git a/target/m68k/Makefile.objs b/target/m68k/Makefile.objs
> index ac61948..2868b11 100644
> --- a/target/m68k/Makefile.objs
> +++ b/target/m68k/Makefile.objs
> @@ -1,5 +1,5 @@
>  obj-y += m68k-semi.o
>  obj-y += translate.o op_helper.o helper.o cpu.o
> -obj-y += fpu_helper.o softfloat.o
> +obj-y += fpu_helper.o softfloat.o hostfloat.o

I don't think you need to add hostfloat.o here,
the softfloat.o in this list contains function specific to m68k
emulation, it's not the one from fpu/

Thanks,
Laurent