https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109112
--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
In an email thread Jakub wrote:
----
Only the simplest assumptions in [[assume(cond)]] where there clearly aren't
any
side-effects no risks of them are lowered to if (!cond) __builtin_unreachable
();
in the IL, anything else goes into the condition being outlined in a
separate artificial function and an internal function recording the
assumption in the IL.
The optimizers then can optimize both the artificial function and the
caller.
The missed optimization thing is that currently only the value range
propagation is able to take advantage of the assumptions, and VRP
is only able to deal with scalars.
We have interprocedural optimizations like IPA scalar replacement of
aggregates etc., where we can optimize passing aggregates at function
boundaries to passing just some scalars from them if the rest isn't needed
etc., but because the assumptions aren't normal calls they'd need tweaks to
be able to optimize the assumptions too so that VRP could take advantage of
those.
----
Why don't the existing optimizations work on the artificial function the same
as any other function? i.e. like
struct S { bool x; };
void do_something();
inline void assumption_1 (const S& s) noexcept {
if (s.x) __builtin_unreachable ();
}
void fn(S s) {
assumption_1 (s);
if (s.x) do_something();
}
which is also optimized as expected.
