[LLVMdev] [RFC] The coding standard for "struct" should be relaxed or removed

[Duncan P. N. Exon Smith]

Committed in r202728.

On 2014 Mar 2, at 13:43, Chris Lattner <clattner at apple.com> wrote:

> Works for me,
> 
> -Chris
> 
>> On Mar 1, 2014, at 9:53 PM, "Duncan P. N. Exon Smith" <dexonsmith at apple.com> wrote:
>> 
>> 
>>> On 2014 Mar 1, at 22:33, Chris Lattner <clattner at apple.com> wrote:
>>> 
>>>> On Mar 1, 2014, at 7:15 PM, Chandler Carruth <chandlerc at google.com> wrote:
>>>> On Sat, Mar 1, 2014 at 5:59 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>>>> The current guidelines [1] on the use of the struct keyword are too
>>>> restrictive and apparently ignored.  They limit the use of struct to
>>>> PODs, citing broken compilers.
>>>> 
>>>> The guidelines are out-of-date and should be relaxed.  Here’s why:
>>>> 
>>>> 1. Our updated list of supported compilers should all deal correctly
>>>>   with non-POD structs.
>>>> 
>>>> 2. A quick grep of the source suggests that no one paid attention
>>>>   anyway.
>>>> 
>>>> I’ve attached a patch that removes the guideline entirely (matching
>>>> the apparent current practice), but does anyone feel a new explicit
>>>> guideline is in order?
>>>> 
>>>> We should at least remove it, the rule is nonsense as you point out.
>>> 
>>> This rule was specifically about brokenness with some version of MSVC.  That version (I have no idea which, or if it still does that) mangled classes and structs differently.  If this isn't the case for the currently supported version of MSVC, we should definitely remove this guideline.
>>> 
>>> -Chris
>> 
>> According to <http://www.agner.org/optimize/calling_conventions.pdf>
>> (see table 9), MSVC still mangles classes and structs differently.
>> 
>> Nevertheless, the rule is not being followed.  E.g., df_ext_iterator,
>> APInt::ms, ilist_traits<SparseBitVectorElement<ElementSize>>, and
>> CaptureTracker are four quite different examples that run afoul (none
>> of these is POD).  It’s easy to get this wrong; there are many more
>> examples.
>> 
>> I’m not even sure the rule is helpful.  Even POD types can be
>> alternately declared “class” or “struct”, changing their mangling in
>> MSVC.
>> 
>> I’ve attached a patch that clarifies the mangling problem and
>> describes the actual practice in the codebase.  How does this look?
>> 
>> <clarify-struct-guidelines.patch>