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

[Richard Henderson <richard.henderson@linaro.org>]

On 7/29/19 6:43 AM, Peter Maydell wrote:
> On Fri, 26 Jul 2019 at 18:50, Richard Henderson
> <richard.henderson@linaro.org> wrote:
>>
>> With this, we can have the function return a value from the DisasContext.
>>
>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>  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 ?

Ah good point.  Should be 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 ?

Right on both counts.  Because of how the loop in MultiField is set up, x will
always be given 0.  I'll see about cleaning this up.

In the meantime, fyi, this is used for setting the S bit for thumb1 insns,
where S=0 iff the insn is within an IT block.


r~