Re: Denormal input handling

[Peter Maydell]

On Wed, 26 May 2021 at 20:07, Michael Morrell <mmorrell@tachyum.com> wrote:
> I see support in QEMU for architectures which have a denormal
> input flag bit and those that have a "flush inputs to zero" control
> bit, but the implementation is not specializable and seems wrong
> for x86 at least.

> For example, in sf_canonicalize, if the input is denormal and
> "flush_inputs_to_zero" is true, the "input denormal" flag is set
> and then the value is set to a zero value, and if the input is
> denormal and "flush_inputs_to_zero" is false, then the input
> is simply normalized.

This is the intended behaviour -- if a target arch needs
"denormalized inputs should be flushed to zero", it sets
the float_status flush_inputs_to_zero flag. If it also
wants to be able to detect when this has happened, it can
then look at the input_denormal flag. This matches the behaviour
that Arm needs, and it is for Arm that the flush_inputs_to_zero
and input_denormal flags were introduced.

> I think the behavior should be for denormal inputs that if
> "flush_inputs_to_zero" is true, then the value is set to zero;
> and if "flush_inputs_to_zero" is false, set the "input denormal"
> flag and normalize the input.

> This matches what x86 does (I'm not sure about other architectures).

What in particular does x86 want that it isn't getting at
the moment? If it needs some additional variation of behaviour
we can look at adding a new status flag to control that.

thanks
-- PMM