[superdimwit]

Clang recently got a new calling convention that makes these tail calls much cheaper (avoids the need for the caller to preserve some registers). I can never remember the name - it’s either preserve_all or preserve_none (whose perspective is the preservation from?).

[haberman]

preserve_none is the new one. It can be applied to the functions performing tail calls to allow them use of the full register space.

I even saw an enhancement recently that will make preserve_none allocate arguments in the registers that are usually callee-saved: https://github.com/llvm/llvm-project/pull/88333

This will make [[musttail]] + preserve_none a winning combination when used together, particularly when making non-tail calls to fallback functions that use a regular calling convention, because all the arguments to [[musttail]] functions can stay pinned in the callee-save registers.

I'm delighted, because this matches what I originally proposed back in 2021, except I called it "reverse_ccc" instead of "preserve_none": https://github.com/haberman/llvm-project/commit/e8d9c75bb35c...

preserve_all also exists, and has existed for a while. You could use it on fallback functions to help the tail calling functions avoid spilling. But this always seemed like an unsatisfactory solution to me, because it's too intrusive (and requires too much diligence) to tag a bunch of regular functions as preserve_all. It's much more practical to tag all the core interpreter functions as preserve_none.

[jeffbee]

I see signs that Google itself is using preserve_none internally, since the public protobuf repo has PROTOBUF_CC (Calling Convention) but it is defined as nothing

  #define PROTOBUF_CC

Is there any chance of this getting out into the wild or is it too dangerous for us mortals?

[haberman]

Since preserve_none is Clang-only and only available on recent compilers, it would introduce an ABI hazard between the core library and generated code. We don't want to cause crashes if you compile protobuf with one compiler and generated code with another.

Also, until quite recently preserve_none was incompatible with the sanitizers, but I believe this may have been fixed by: https://github.com/llvm/llvm-project/pull/81048

[superdimwit]

Thanks!