Re: [PATCH 15/20] arm: [MVE intrinsics] add unary_acc shape

[Richard Sandiford <richard.sandiford@arm.com>]

Christophe Lyon <christophe.lyon@arm.com> writes:
> On 5/11/23 10:30, Richard Sandiford wrote:
>> Christophe Lyon <christophe.lyon@arm.com> writes:
>>> On 5/10/23 16:52, Kyrylo Tkachov wrote:
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: Christophe Lyon <christophe.lyon@arm.com>
>>>>> Sent: Wednesday, May 10, 2023 2:31 PM
>>>>> To: gcc-patches@gcc.gnu.org; Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>;
>>>>> Richard Earnshaw <Richard.Earnshaw@arm.com>; Richard Sandiford
>>>>> <Richard.Sandiford@arm.com>
>>>>> Cc: Christophe Lyon <Christophe.Lyon@arm.com>
>>>>> Subject: [PATCH 15/20] arm: [MVE intrinsics] add unary_acc shape
>>>>>
>>>>> This patch adds the unary_acc shape description.
>>>>>
>>>>> 2022-10-25  Christophe Lyon  <christophe.lyon@arm.com>
>>>>>
>>>>> 	gcc/
>>>>> 	* config/arm/arm-mve-builtins-shapes.cc (unary_acc): New.
>>>>> 	* config/arm/arm-mve-builtins-shapes.h (unary_acc): New.
>>>>> ---
>>>>>    gcc/config/arm/arm-mve-builtins-shapes.cc | 28 +++++++++++++++++++++++
>>>>>    gcc/config/arm/arm-mve-builtins-shapes.h  |  1 +
>>>>>    2 files changed, 29 insertions(+)
>>>>>
>>>>> diff --git a/gcc/config/arm/arm-mve-builtins-shapes.cc b/gcc/config/arm/arm-
>>>>> mve-builtins-shapes.cc
>>>>> index bff1c3e843b..e77a0cc20ac 100644
>>>>> --- a/gcc/config/arm/arm-mve-builtins-shapes.cc
>>>>> +++ b/gcc/config/arm/arm-mve-builtins-shapes.cc
>>>>> @@ -1066,6 +1066,34 @@ struct unary_def : public overloaded_base<0>
>>>>>    };
>>>>>    SHAPE (unary)
>>>>>
>>>>> +/* <S0:twice>_t vfoo[_<t0>](<T0>_t)
>>>>> +
>>>>> +   i.e. a version of "unary" in which the source elements are half the
>>>>> +   size of the destination scalar, but have the same type class.
>>>>> +
>>>>> +   Example: vaddlvq.
>>>>> +   int64_t [__arm_]vaddlvq[_s32](int32x4_t a)
>>>>> +   int64_t [__arm_]vaddlvq_p[_s32](int32x4_t a, mve_pred16_t p) */
>>>>> +struct unary_acc_def : public overloaded_base<0>
>>>>> +{
>>>>> +  void
>>>>> +  build (function_builder &b, const function_group_info &group,
>>>>> +	 bool preserve_user_namespace) const override
>>>>> +  {
>>>>> +    b.add_overloaded_functions (group, MODE_none,
>>>>> preserve_user_namespace);
>>>>> +    build_all (b, "sw0,v0", group, MODE_none, preserve_user_namespace);
>>>>> +  }
>>>>> +
>>>>> +  tree
>>>>> +  resolve (function_resolver &r) const override
>>>>> +  {
>>>>> +    /* FIXME: check that the return value is actually
>>>>> +       twice as wide as arg 0.  */
>>>>
>>>> Any reason why we can't add that check now?
>>>> I'd rather not add new FIXMEs here...
>>>
>>> I understand :-)
>>>
>>> That's because the resolver only knows about the arguments, not the
>>> return value:
>>>     /* The arguments to the overloaded function.  */
>>>     vec<tree, va_gc> &m_arglist;
>>>
>>> I kept this like what already exists for AArch64/SVE, but we'll need to
>>> extend it to handle return values too, so that we can support all
>>> overloaded forms of vuninitialized
>>> (see https://gcc.gnu.org/pipermail/gcc-patches/2023-April/616003.html)
>>>
>>> I meant this extension to be a follow-up work when most intrinsics have
>>> been converted and the few remaining ones (eg. vuninitialized) needs an
>>> improved framework.  And that would enable to fix the FIXME.
>> 
>> We can't resolve based on the return type though.  It has to be
>> arguments only.  E.g.:
>> 
>>     decltype(foo(a, b))
>> 
>> has to be well-defined, even though decltype (by design) provides no
>> context about "what the caller wants".
>> 
>
> So in fact we can probably get rid of (most of) the remaining 
> definitions of vuninitializedq in arm_mve.h, but not by looking at the 
> return type (re-reading this I'm wondering whether I overlooked this 
> when I started the series....)
>
> But for things like vaddlvq, we can't check that the result is actually 
> written in a twice-as-large as the argument location?

No.  All we can/should do is to resolve the typeless builtin to a fully-typed
builtin, based on the argument types.  The return type of that fully-typed
builtin determines the type of the function call expression (the CALL_EXPR).
It's then up to the frontend to do semantic/type checking of the
resolved expression type.

In other words, information only flows in one direction:

  argument types -> function overloading -> function return type

Thanks,
Richard