Re: [gomp] OpenMP IL design notes

[Ian Lance Taylor <ian at airs dot com>]

Diego Novillo <dnovillo@redhat.com> writes:

> > I personally find it kind of baffling to have the same tree code act
> > differently in GENERIC and GIMPLE, a la SWITCH_EXPR.  It seems to add
> > confusion for minimal benefit.  If you are suggesting that the single
> > tree code GOMP_PARALLEL have different operands in GENERIC and GIMPLE,
> > can I suggest that you instead use two different tree coes?
> > 
> That is a fundamental feature of both GENERIC and GIMPLE. GIMPLE
> is a non-strict subset of GENERIC.  Every program in GIMPLE form
> is also in GENERIC form.  The reverse, however, is not true.

Well, sure.

But it seems to me that there is a difference.  PLUS_EXPR always takes
two operands and adds them together.  Given a PLUS_EXPR, you always
where to find the operands, and more or less what they mean.  I don't
find that confusing, although I agree that the operands themselves may
be different in GENERIC and GIMPLE.

SWITCH_EXPR is different.  Sometimes you use SWITCH_BODY, and
sometimes you use SWITCH_LABELS.  How do you know which one to use?
It depends on whether you have GENERIC or GIMPLE.

In particular, this matters for functions like block_may_fallthru,
which are called with both GENERIC and GIMPLE.

> If we did that we would need different codes for every tree code
> (MODIFY_EXPR, PLUS_EXPR, etc).  Similarly, we would need
> different codes when we move from High-GIMPLE into Low-GIMPLE.
> I'm not sure that's worth the effort.  From the point of view of
> analysis and optimizations the differences between the different
> ILs are mostly in the grammar, not their syntax.

If I understand what you are saying, I am complaining about the
specific cases where the difference is in the syntax.  If a tree takes
a different set of operands in GENERIC and GIMPLE, or if the operands
have significantly different meanings, then I think we should use a
different tree code.  If the operands are more or less the same, then
I think using the same tree code is fine.

This is obviously all just my opinion, as somebody who came late to
this stuff and is trying to understand it.

Ian