[llvm-dev] Redefining optnone to help LTO

[Robinson, Paul via llvm-dev]

In D28404, Mehdi wanted to use the 'optnone' attribute as a way to record
"I was compiled with -O0" in the IR, because it seems like a good idea to
remember that fact in an LTO compilation and there is no way to remember
that fact currently.  A couple of people felt it might be better to have
this idea discussed on the dev list, where it might get better exposure,
so I'm volunteering to get that discussion started.

While 'optnone' does cause lots of optimizations to bypass a function,
exactly matching -O0 was not the motivation and never a hard requirement.
The implementation makes a distinct effort to get close to the behavior
of -O0, but it's not an exact match and for the intended purpose (allowing
a given function to be un-optimized to help debugging) it worked fine.

Using 'optnone' to convey -O0 to LTO is something of a redefinition, or
at least a re-purposing, of the attribute.  To get there from here, I
think we would need a couple of things to happen, separately from the
minor grunt work of adding 'optnone' to function IR at -O0.

1) Update the LangRef definition of 'optnone' to reflect this intent.
The current definition doesn't provide a motivation, and the description
is (deliberately) a bit vague.  If we want 'optnone' to intentionally
match -O0, that should be tightened up.

2) Make a concerted effort to teach 'optnone' to targets.  Currently
I know the X86 target is aware of it, but I'm not so sure about others.

3) Take another look at what 'optnone' currently does *not* turn off,
and see if there is something we can do about that.  In some cases this
will not be practical, and we may just have to live with that.

(Okay, we need 3 things to happen.)

I won't say this is blocking Mehdi's work, but it would remove a
point of contention and allow the review to proceed more smoothly.
--paulr