Hi,
On Tue, 16 Apr 2019, Jeff Law wrote:
> > 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:
> Ugh. Good point. But then we're back to the problem with handling of
> REG_EQUIV notes for stores into the argument space. If the callee owns,
> has been declared pure/const, but can clobber things like you've shown,
> then a REG_EQUIV note on those slots must be avoided.
Yes, if we want to be extra precise a REG_EQUAL note would be okay before
the call insn (but not very useful post regalloc), but a REG_EQUIV note is
plain wrong for argument slots if there are uses of the destination
register after the call.
> Certainly wouldn't disagree that the ABIs ought to own defining this
> stuff. It's just an area where they've been weak in the past.
Oh yes, indeed. Even relatively new psABIs don't specify this explicitely
:-/
Ciao,
Michael.
