[llvm-dev] Proposal for string keys for address_space

Leonard Chan via llvm-dev
Thu, 10 Jan 2019 21:53:50 -0000

Hello,

We would like to propose a way for improving the diagnostics for
address_space by being able to pass strings as an argument to it
instead of just an integer. This was initially proposed before
(http://lists.llvm.org/pipermail/cfe-dev/2018-August/058702.html) but
did not focus on it at the time.

Reasoning:
Clang's __attribute__((address_space(...))) feature uses an arbitrary
integer as the discriminator of address spaces. This is effectively a
global name space of integers in an API. The meaningless integers are
used in diagnostics, which is not very informative. Moreover, the
maintenance of the integer assignments is error-prone. It would be
better if this could be a string rather than an integer. It's easy
enough to pick string prefixes that distinguish the use in one part of
an API from others in separately-maintained APIs, just as with symbol
name prefixes in C APIs.

Goals:
- Allow for address_space to accept strings so various APIs do not
need to share the same global name space of integers
(address_space("API1") != address_space("API2"))
- Print the string argument instead of an integer if address_space is
provided with a string for more understandable error messages.

(Possible) Implementation:
Off the top of my head, it seems that a large chunk of this would be
changing how address spaces are managed. Currently, it seems that all
address_spaces are limited to just integers with a few spaces reserved
for OpenCL and CUDA under the LangAS enum. I'm thinking that LangAS
itself could be a base class with 2 different subclasses that take
either integers or strings and when storing them in a qualifier, part
of the address space mask could be dedicated to indicating if this
LangAS is a string or integer. The only thing I can't think of is how
printing the AS would work in a diagnostic since, based off the
existing TypePrinter/Qualifiers::print() method, I do not think I
would be able to store a reference back to some mapping of addr spaces
to strings since it seems that the Qualifiers class is meant to be 32
bits long and passed by value.

I was going to come up with a proof of concept in the meantime, but
wanted to ask sooner to see if anyone had ideas on this or ideas on
how this could be implemented. Any feedback is welcome and I don't
mind answering questions.

Thanks,
Leonard