Re: [PATCH] libgccjit Fix a RTL bug for libgccjit

[David Malcolm <dmalcolm@redhat.com>]

On Mon, 2023-12-11 at 09:06 -0700, Jeff Law wrote:
> 
> 
> On 11/20/23 16:54, David Malcolm wrote:
> > On Mon, 2023-11-20 at 16:38 -0700, Jeff Law wrote:
> > > 
> > > 
> > > On 11/20/23 15:46, David Malcolm wrote:
> > > > On Fri, 2023-11-17 at 14:09 -0700, Jeff Law wrote:
> > > > > 
> > > > > 
> > > > > On 11/17/23 14:08, Antoni Boucher wrote:
> > > > > > In contrast with the other frontends, libgccjit can be
> > > > > > executed
> > > > > > multiple times in a row in the same process.
> > > > > Yup.  I'm aware of that.  Even so calling init_emit_once more
> > > > > than
> > > > > one
> > > > > time still seems wrong.
> > > > 
> > > > There are two approaches we follow when dealing with state
> > > > stored
> > > > in
> > > > global variables:
> > > > (a) clean it all up via the various functions called from
> > > > toplev::finalize
> > > > (b) make it effectively constant once initialized, with
> > > > idempotent
> > > > initialization
> > > > 
> > > > The multiple in-process executions of libgccjit could pass in
> > > > different
> > > > code-generation options.  Does the RTL-initialization logic
> > > > depend
> > > > anywhere on flags passed in, because if so, we're probably
> > > > going to
> > > > need to re-run the initialization.
> > > The INIT_EXPANDERS code would be the most concerning as it's
> > > implementation is totally hidden and provided by the target. I
> > > wouldn't
> > > be at all surprised if one or more do something evil in there. 
> > > That
> > > probably needs to be evaluated on a target by target basis.
> > > 
> > > The rest really do look like single init, even in a JIT
> > > environment
> > > kinds of things -- ie all the shared constants in RTL.
> > 
> > I think Antoni's patch can we described as implementing "single
> > init",
> > in that it ensures that at least part of init_emit_once is single
> > init.
> > 
> > Is the posted patch OK by you, or do we need to rework things, and
> > if
> > the latter, what would be the goal?
> What I'm struggling with is perhaps a problem of naming. 
> Conceptually 
> "init_emit_once" in my mind should be called once and only once.   
> If I 
> read Antoni's change correctly, we call it more than once.

I'm afraid we're already doing that, Antoni's proposed patch doesn't
change that.

In toplev::finalize we try to clean up as much global state as possible
to allow toplev::main to be runnable again.  From that point of view
"once" could mean "once within an invocation of toplev::main" (if that
makes it feel any less gross).

>   That just
> feels conceptually wrong -- add to it the opaqueness of
> INIT_EXPANDERS 
> and it feels even more wrong -- we don't know what's going on behind
> the 
> scenes in there.

Given these various concerns, I think we should go with approach (a)
from above: add an emit_rtl_cc::finalizer function, and have it reset
all of the globals in emit-rtl.cc.  That seems like the most clear way
to handle this awkward situation.

Dave