Hi,
On Fri, 12 Apr 2019, Jeff Law wrote:
> > I don't think this follows. Imagine a pure foo tailcalling a pure bar.
> > To make the tailcall, foo may need to change some of its argument slots
> > to pass new arguments to bar.
> I'd claim that a pure/const call can't tail call another function as
> that would potentially modify the argument slots.
I still don't think that what you want follows. Imagine this:
int foo (int i) { ++i; return i; }
To claim that this function is anything else than const+pure seems weird
(in fact this function doesn't access anything that must lie in memory at
all). Now take your off-the-mill ABI that passes arguments on stack, and
an only slightly bad code generator, i.e. -O0 on i386. You will get an
modification of the argument slot:
foo:
pushl %ebp
movl %esp, %ebp
addl $1, 8(%ebp)
movl 8(%ebp), %eax
popl %ebp
ret
So, if anything then the ownership of argument slots is a property of the
psABI. And while we may have been through this discussion a couple times
over the years, I'm pretty sure that at least I consistently argued to
declare all psABIs that leave argument slot ownerships with the callers
(after the call actually happens) to be seriously broken^Wmisguided (and
yes, also because it can prevent tail calls that otherwise would be
perfectly valid).
Ciao,
Michael.
