[PATCH] Add PragmaAttr and Pragma Spelling to Tablegen

[Reid Kleckner]

On Wed, May 28, 2014 at 6:42 AM, Aaron Ballman <aaron at aaronballman.com>wrote:

> On Tue, May 27, 2014 at 6:28 PM, Reid Kleckner <rnk at google.com> wrote:
> > On Sat, May 24, 2014 at 8:28 AM, Aaron Ballman <aaron at aaronballman.com>
> > wrote:
> >>
> >> > My first idea is to add a tablegen property like "HasPrettyPrinter = 0
> >> > or 1"
> >> > that supresses emission of the default printPretty method.  Then
> again,
> >> > we
> >> > kind of want the default version when the GNU and C++11 spellings of
> the
> >> > attribute are used.
> >>
> >> That seems like the wrong way to go about it though, since this is a
> >> new *spelling*, not a wholly new semantic kind of attribute (which I
> >> think we really want to avoid). We could possibly attach a custom
> >> pretty printer to the spelling itself (a la a code block), but that
> >> seems a bit skeezy because the arguments are rather important and the
> >> spelling shouldn't have to care about the arguments.
> >
> >
> > How about this: when generating the prettyPrint implementation, if there
> is
> > a pragma spelling, call MyAttr::printPrettyPragma(...).  Any attribute
> with
> > a pragma spelling will have to provide this method in
> lib/AST/AttrImpl.cpp.
>
> So my ultimate goal is for pragma attributes to mirror all of the
> other attributes, where we can infer syntax and semantics just from
> what's in Attr.td. But it's becoming more obvious to me that the
> amount of work required to get there is rather large (mostly due to
> the fact that pragma argument syntax is ALL over the place).
>
> As a stopgap solution, it seems reasonable to me to rely on an
> AdditionalMember being defined for pragmas (which is basically what
> you're suggesting).
>
> So if an attribute has a pragma spelling, it is required to have:
>
> let AdditionalMembers = [{
>   void printPrettyPragma(raw_ostream &OS, const PrintingPolicy &Policy)
> const {
>   }
> }];
>

Sure, but... I like writing C++ in .cpp files, not .td files.  +1 for
defining it out of line in AttrImpl.cpp.  :)


> Then in ClangAttrEmitter.cpp, the printPretty generator function can
> simply call through to this for a pragma spelling in
> writePrettyPrintFunction. If the attribute has no AdditionalMembers
> field, the generator can spit out a friendly error, but if the author
> simply forgets the signature for this, a compile-time error will have
> to suffice.
>
> Someday, once we solve the parameter syntax issue, we can rip out the
> printPrettyPragma functions. That can be added as a FIXME to
> writePrettyPrintFunction.
>
> ~Aaron
>
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