Re: [Qemu-arm] [PATCH 01/67] decodetree: Allow !function with no input bits

[Peter Maydell]

On Fri, 26 Jul 2019 at 18:50, Richard Henderson
<address@hidden> wrote:
>
> With this, we can have the function return a value from the DisasContext.
>
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  scripts/decodetree.py             | 5 ++++-
>  tests/decode/succ_function.decode | 2 ++
>  2 files changed, 6 insertions(+), 1 deletion(-)
>  create mode 100644 tests/decode/succ_function.decode
>
> diff --git a/scripts/decodetree.py b/scripts/decodetree.py
> index d7a59d63ac..4259d87a95 100755
> --- a/scripts/decodetree.py
> +++ b/scripts/decodetree.py
> @@ -195,7 +195,10 @@ class MultiField:
>      """Class representing a compound instruction field"""
>      def __init__(self, subs, mask):
>          self.subs = subs
> -        self.sign = subs[0].sign
> +        if len(subs):
> +            self.sign = subs[0].sign
> +        else:
> +            self.sign = 0
>          self.mask = mask
>
>      def __str__(self):
> diff --git a/tests/decode/succ_function.decode 
> b/tests/decode/succ_function.decode
> new file mode 100644
> index 0000000000..632a9de252
> --- /dev/null
> +++ b/tests/decode/succ_function.decode
> @@ -0,0 +1,2 @@
> +%foo  !function=foo
> +foo   00000000000000000000000000000000 %foo
> --
> 2.17.1

Could you also update the documentation in docs/devel/decodetree.rst ?

This code change looks like it also now allows definitions
of fields that specify nothing at all (ie there's no check
that a field definition with no "unnamed_field" parts has
a !function specifier) -- what do they do, or should they
be made syntax errors ?

Is one of these functions which just returns a constant
from no input bits still a "static int func(DisasContext *s, int x)"
taking a pointless input argument, or is it now a
"static int func(DisasContext *s)" ? I guess from the fact
this code doesn't change the way a call is output that it
is the former, but would the latter be cleaner ?
(This would probably be implemented something like allowing
FunctionField to be passed a base == None instead of
allowing MultiFields with len(subs) == 0.)

thanks
-- PMM