On Wed, Nov 18, 2020 at 01:11:27PM -0600, Segher Boessenkool wrote:

Calling this via a different declared function type is undefined
behaviour, but that is independent of how the function is *defined*.
Your program can make ducks appear from your nose even if that function
is never called, if you do that.  Just don't do UB, not even once!

Ah, see, here I think we disagree. UB is a flaw of the spec, but the
real world often has very sane behaviour there (sometimes also very
much not).

In this particular instance the behaviour is UB because the C spec
doesn't want to pin down the calling convention, which is something I
can understand. But once you combine the C spec with the ABI(s) at hand,
there really isn't two ways about it. This has to work, under the
premise that the ABI defines a caller cleanup calling convention.

So in the view that the compiler is a glorified assembler, I'll take UB
every day if it means I can get the thing to do what I want it to.

Obviously in the interest of co-operation and longer term viability, it
would be nice if we can agree on the behaviour and get a language
extention covering it.

Note that we have a fairly extensive tradition of defining away UB with
language extentions, -fno-strict-overflow, -fno-strict-aliasing,
-fno-delete-null-pointer-checks etc..