Is there somewhere in the compiler that we can easily see these defaults?

My understanding is that if the ABI is not specified, the default is always "C". Is there something that says "C" is equivalent to these various ABIs for different targets? Or am I misunderstanding what it means by default?

Is aapcs the default for softfloat arm32?

My understanding is that if the ABI is not specified, the default is always "C".

Correct. And "C" can be identical to other ABIs depending on the target. This PR documents my understanding of these equivalences. Cc @workingjubilee @Amanieu for people that know more about the nitty-gritty details.

Basically, extern "C" is always the default for C code on the current target, but some of these ABI strings (such as sysv64) let one use that ABI on other targets as well. On non-Windows x86_64, "C" and "sysv64" are equivalent, but on win64 they are not.

Is aapcs the default for softfloat arm32?

I would think so but I don't know.

Is aapcs the default for softfloat arm32?

Yes, "C" == "aapcs" on soft-float arm32.

So if I am understanding correctly, the only way to determine what the behavior of the "C" calling convention is to dig through LLVM source, and each target seems to implement it differently, is that correct? For example aarch32/aarch64 use a method called CCAssignFnForCall. And x86 doesn't have any single place, but has methods like isCallingConvWin64.

I'm not entirely clear for stdcall, it says "default for the Win32 API". Why does it explicitly say "API"? Is this saying, this is the default that Microsoft uses for the Win32 API functions but is not the default of "C". That is, on i686-pc-windows-msvc, "C" is different from "stdcall"? Is that correct?

"thiscall" is also confusing, as it uses the term "default" for "C++ member functions". So it's not the default for the "C" calling convention?

(I'm finding the wording of "default" here confusing, so I'm trying to get clarity here.)

I'm not entirely clear for stdcall, it says "default for the Win32 API". Why does it explicitly say "API"? Is this saying, this is the default that Microsoft uses for the Win32 API functions but is not the default of "C". That is, on i686-pc-windows-msvc, "C" is different from "stdcall"? Is that correct?

Yes, that is correct. The default for win32 in general ("C" on that target) is "cdecl".

(I'm finding the wording of "default" here confusing, so I'm trying to get clarity here.)

Yeah that's fair. I went for a most minimal edit here to correct some wrong information. This section could probably use a larger rewrite, but I'm not sure if/when I will have the time for that.