[llvm-dev] RFC: To add __attribute__((regmask("preserve/clobbered list here"))) in clang

[Joerg Sonnenberger via llvm-dev]

On Fri, Jul 15, 2016 at 11:57:35PM +0530, vivek pandya via llvm-dev wrote:
> So for IPRA we have a situation where a function is calling a function
> which is written in assembly and it is not defined in current module. IPRA's scope is
> limited to a module so for such externally defined function it uses default
> calling convention but here as the function is written in assembly user can
> provide exact register usage detials. So we dicided to mark declration of
> such
> function with __attribute__((regmask("clobbered list here"))) so LLVM can
> construct regmask out of it and use it with IPRA to improve register
> allocation.

This situation is actually far more common and not restricted to
assembly at all. There are a number of functions already that have
special ABIs with much larger set of preserved registers. A typical
is __tls_get_addr in many ABIs. At the moment, we need hacks in the
target specific part of LLVM for handling this. Related (limited)
approaches for this are the preserve_most and preserve_all calling
conventions.

As mentioned in the IRC discussion, there are two important issues to be
considered here from my perspective.

(1) I really dislike an attribute providing a clobber list. Whether a
given register is clobbered or not is an implementation detail of a
specific version and can easily change. It is also something difficult
to reason about. The invariance that should be put into the ABI contract
is the inverse -- what registers a function is going to preserve. That
is even more important when looking at long time ABI stability. New
registers are introduced every so often. That shouldn't change the
meaning of a declaration.

The main reason for using a clobber list seems to be a concern about
verbosity. I think that can be mostly avoided by allowing the use of
register classes in the specifier, e.g. all-fp for i387 register,
all-sse2 for the SSE2 register set, all-avx for the AVX register etc.
At the same time, I consider a certain verbosity to be useful, since
ultimately, implementation and interface definition need to be carefully
compared.

(2) Should the attribute extend or replace the normal preserved
registers? Randomly clobbering registers is going to create all kinds of
fun issues with the backend assumptions. We already have such fun with
inline assembler. Extend-only semantic is much easier to support. It can
also be combined with a special CC with minimal default preservation and
well defined meanings e.g. for arguments passed in registers.

Joerg