Re: [PATCH 1/3] do full type check in COPY_ARRAY and MOVE_ARRAY

From: René Scharfe <l.s.r@web.de>
Date: Jan 1, 2023, 10:45 UTC
Message-ID: <bf4e6798-c687-270d-1344-37bacb403e13@web.de>
In-Reply-To: <220515b3-3924-c8d2-ff20-7017caa7dead@web.de>

Am 01.01.23 um 08:41 schrieb René Scharfe:

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> Am 01.01.23 um 04:59 schrieb Junio C Hamano:
>> Junio C Hamano <gitster@pobox.com> writes:
>>
>>> ...  I think what you ideally want to enforce is that
>>> typeof(dst) is exactly typeof(src), or typeof(src) sans constness
>>> (i.e. you can populate non-const array from a const template)?
>
> Yes.
>
> Moving the type check shared between COPY_ARRAY and MOVE_ARRAY to a new
> macro is a good idea.
>
> Using compiler extensions when available, as we already do for other
> purposes, is a good idea as well.  I managed to ignore the existing use
> somehow.

On second thought, what do we gain by using __builtin_types_compatible_p here? Does it catch cases that the assignment check plus the element size check wouldn't?

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>
>>
>> IOW, I am wondering if something like this is an improvement.
>>
>>  git-compat-util.h | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git c/git-compat-util.h w/git-compat-util.h
>> index a76d0526f7..be435615f0 100644
>> --- c/git-compat-util.h
>> +++ w/git-compat-util.h
>> @@ -97,8 +97,13 @@ struct strbuf;
>>  # define BARF_UNLESS_AN_ARRAY(arr)						\
>>  	BUILD_ASSERT_OR_ZERO(!__builtin_types_compatible_p(__typeof__(arr), \
>>  							   __typeof__(&(arr)[0])))
>> +# define ARRAYS_COPYABLE_OR_ZERO(src,dst) \
>> +	(BUILD_ASSERT_OR_ZERO(!__builtin_types_compatible_p(__typeof__(src), \
>> +							    __typeof__(dst))) + \

We actually need to compare the types of the elements here, because otherwise we'd disallow copies between arrays (int arr[7]) and pointers (int *p), which would be unnecessarily strict.

>> +			      (0 ? *(dst) = *(src) : 0))

You dropped the ", 0" after the assignment, but we need it to make the type of this expression int instead of whatever type *dst and *src have.

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>>  #else
>>  # define BARF_UNLESS_AN_ARRAY(arr) 0
>> +# define ARRAYS_COPYABLE_OR_ZERO(src,dst) (0 ? *(dst) = *(src) : 0))
>>  #endif
>>  /*
>>   * ARRAY_SIZE - get the number of elements in a visible array