[Qemu-devel] [PATCH v2 0/3] ARM/softfloat: support flushing denormals on input

[Peter Maydell]

On ARM, the FPSCR FZ bit (which controls whether denormals should be
flushed to zero) is supposed to cause this flushing to occur both
when the output of a calculation is a denormal (already implemented in
softfloat) and also when the input to a calculation is a denormal
(not implemented, as noted by a FIXME comment).

This patchset adds the support to softfloat for flushing denormals on
input. This is controlled using a new status flag to enable it (so that
CPUs which only flush on output continue to work). There is a new
exception status bit to indicate when input flushing has happened
(because on ARM it is reported via a different FPSCR bit to that used
when an output denormal is flushed to zero).

I have deliberately only implemented this for input float32 and
float64 values because that is what ARM requires (on ARM float16
inputs must not be flushed to zero and floatx80 and float128 are
not used) so other changes would be totally untested code.

Existing CPUs should be unaffected as there is no behaviour change
unless the mode is enabled.

(I suspect that MIPS should be able to use this to implement the
FCSR FO bit if desired.)

Tested using random instruction generation for vadd/vsub/vmul/vdiv
with the FPSCR FZ bit set.

The only change from v1 is that I have split the old patch 2/2
into two parts, since it accidentally included a one-liner bugfix
where we were setting the softfloat cumulative exception flags
wrongly if the FPSCR was written to explicitly.

Peter Maydell (3):
  softfloat: Implement flushing input denormals to zero
  ARM: Set softfloat cumulative exc flags from correct FPSCR bits
  ARM: wire up the softfloat flush_input_to_zero flag

 fpu/softfloat.c     |  104 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 fpu/softfloat.h     |   22 ++++++++++-
 target-arm/helper.c |   10 ++++-
 3 files changed, 131 insertions(+), 5 deletions(-)