On 04/12/2020 00:00, Andrea Corallo wrote:
David Brown <david.brown@xxxxxxxxxxxx> writes:
Hi David,
As "f()" runs, the value of "y->fun_ptr" changes with each step as you
get alternating "foo1", "foo2" outputs. Thus it is clear (I hope) that
the compiler cannot assume that "y->fun_ptr" is not clobbered by the
function call.
Indeed that's clear. The question is more if there's some way to express
this to the compiler qualifying the variables in discussion without
changing the structure of the code. This because as I wrote may be not
trivial to modify the code generator for optimal results.
I understand this is probably not possible and there's no magic
attribute or qualifier to express this.
In order to add a qualifier or attribute, you'd still need to change the
code in some way. Is it possible to make /some/ changes to the code
generator, but not others? (I appreciate that code generators can be
difficult to change.)
On trick that sometimes works when you want to give the compiler more
information is to use "if (!x) __builtin_unreachable();" as a way of
saying "x is always true". But I can't see any way that would help
here, I'm afraid.
