[Clang] Add memory attribute

[philnik777]

The LLVM IR has a memory attribute, which allows specifying memory access for different regions. Specifically, all memory, memory passed via arguments and inaccessible memory. All of these fields can be specified to have (1) no access, (2) read only access, (3) write only access or (4) read/write access. Combining these is also possible. That way it is possible to specify that a function can read/write global memory, but only read argument memory. That allows for really nice optimizations like removing dead calls to functions that only write to some memory (e.g. the frexp family of functions). It would be great if clang could expose this flexibility via an attribute. (Except maybe the inaccessible memory part, since that doesn't really seem like a useful concept outside of builtins)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nikolas Klauser (philnik777)

The LLVM IR has a `memory` attribute, which allows specifying memory access for different regions. Specifically, all memory, memory passed via arguments and inaccessible memory. All of these fields can be specified to have (1) no access, (2) read only access, (3) write only access or (4) read/write access. Combining these is also possible. That way it is possible to specify that a function can read/write global memory, but only read argument memory. That allows for really nice optimizations like removing dead calls to functions that only write to some memory (e.g. the `frexp` family of functions). It would be great if clang could expose this flexibility via an attribute. (Except maybe the inaccessible memory part, since that doesn't really seem like a useful concept outside of builtins)

[shafik]

CC @AaronBallman

[AaronBallman]

There's plenty of precedence for __in, __out, and __inout (via various spellings), and conveying this information via the parameter list would also enable some kinds of static analysis to take place.

(Related: I would still like to see some sort of support for SAL; the patent expires in 2026: https://patents.google.com/patent/US20050076331A1/en)

[xcpky]

I think this is really beneficial for optimization, especially for the read only access. In C/C++ const can be cast away, so there are no ways to express read only access for pointer parameter, which prohibit a lot of optimization opportunities when the function is not defined in the same translation unit.

[[gnu::access]] is a way to express this "memory attribute". Clang doesn't support it yet #117890. I prefer to annotate the attribute via the parameter list as @AaronBallman describes though.