[cfe-dev] address_space pragma

[Leonard Chan via cfe-dev]

Right now, the goal is to just change the diagnostic that would
normally be printed for mixing address spaces to instead print the
macro in place of the whole attribute that contains this address
space. That is:

```
# define __user __attribute__((noderef, address_space(1)))

char __user *x = ...
char *y = x;  // err: initializing 'char *' with an expression of type
'__attribute__((address_space(1))) char *' changes address space of
pointer
```

The error would instead print `initializing 'char *' with an
expression of type '__user char *' changes address space of pointer`.

It seems that the address space is attached to the type via a
qualifier instead of the type, that is, I don't think something like
`__attribute__((address_space(10))) int *` generates an
AttributedType. This makes me think that I need to attach something to
the Qualifier, but I don't think I want to increase how much it holds
since it gets copied everywhere and only holds a mask of 32 bits.

The only alternative options I can think of is changing the printing
policy to accept information relating to the macro, but am unsure if
this is the right solution.
On Mon, Aug 6, 2018 at 2:26 AM Bevin Hansson <bevin.hansson at ericsson.com> wrote:
>
> Hi,
>
> I'm a bit late to the party, but all this address space stuff sounds
> interesting to me as we are heavy users of address space functionality
> downstream.
>
> If I understand correctly, you want to include the spelling of the AS in
> the type. Would this be restricted purely to spellings as macros, or
> would it also work for keywords? There are a bunch of hard coded strings
> in the type printer to print out the specific names of ASes for OpenCL,
> and we have also added our ASes to that hack. If it were possible to
> encode this straight in the type itself based on keyword/identifier
> instead of special-casing in the printer, that would probably be cleaner.
>
> Feel free to add me as reviewer if you make a patch.
>
> / Bevin
>
>
> On 2018-08-03 01:49, Leonard Chan via cfe-dev wrote:
> > As I have it now, I store a map of source locations to macro
> > definition names as they are being parsed and that's where I attempt
> > to find the attribute. Since the printing of the address_space is
> > controlled by the qualifier, I added an extra field to the qualifier
> > instead for storing the macro name. I can get this name during the
> > address space attribute construction where the source location of
> > address_space is available to me.
> >
> > Would you mind if I added you as a reviewer to this patch after I clean it up?
> > On Thu, Aug 2, 2018 at 3:38 PM John McCall <rjmccall at apple.com> wrote:
> >>> On Aug 2, 2018, at 5:41 PM, Leonard Chan <leonardchan at google.com> wrote:
> >>>
> >>> On Thu, Aug 2, 2018 at 2:33 PM John McCall <rjmccall at apple.com> wrote:
> >>>>> On Aug 2, 2018, at 5:28 PM, Leonard Chan <leonardchan at google.com> wrote:
> >>>>>
> >>>>> Ok. So we will no longer use the pragma. We will instead opt for your
> >>>>> suggestion of printing the macro. So the new goals are now:
> >>>>>
> >>>>> - Having address_space() accept strings in addition to integers to
> >>>>> represent unique address spaces
> >>>> Is this still important if you get the diagnostic improvement?  If you were declaring address
> >>>> spaces somehow, this seems fine, but I don't really want to make address_space("whatever")
> >>>> implicitly declare an address space with specific semantics when it's not hard to imagine
> >>>> wanting that for other purposes in the future.
> >>> Not entirely. This was something that's still remnant of when we were
> >>> using pragma. It's something that would be nice for us now, but could
> >>> also be discussed later down the line.
> >>>
> >>>>> - Changing the type printer to print the macro instead of the whole
> >>>>> attribute if the attribute was passed a string instead of an int.
> >>>> I don't know that the latter needs to be restricted; it seems like a nice usability improvement
> >>>> for all address-space users.
> >>> Ok, so in general if address_space is used in a macro, then print the macro.
> >> Yeah.  You'll need to record this spelling information in the AttributedType, since
> >> the Type won't otherwise have a source location attached.
> >>
> >> I'm not sure we actually make an AttributedType for address_space, but doing
> >> so is reasonable.
> >>
> >> We should do a quick parse of the macro's token sequence to verify that it's just
> >> an attribute.  Unfortunately, I'm not really sure how to do that.
> >>
> >> John.
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>