Re: SIMPLE: A language-independent tree IR

[Diego Novillo <dnovillo at redhat dot com>]

On Wed, 02 Jan 2002, Tom Tromey wrote:

> In Java a `break' in an inner loop can break out of an outer loop (if
> the user uses the `break <label'> form).  In this case the break must
> also deal with enclosing `try-finally' constructs properly.  I don't
> know whether capturing this information is important at this level, or
> if you'd rather we just model this with goto or whatever.  (I don't
> know what we do right now.)  The same holds for `continue'.
> 
Initially, I would just follow what the current tree->RTL
expander does.  Emitting SIMPLE trees instead of RTL, of course.
I won't know for sure until I have sat down and looked at it,
though.

> I looked through the BNF you provided but I didn't see how `goto' or
> `?:' expressions are handled.  I also didn't see how exceptions are
> handled.
> 
Conditional ?: expressions are expanded to the equivalent
if-then-else tree.  GOTOs are a special case.  One of their goals
is to remove all GOTOs from the code.  I don't think this is
crucial in the short term, so I will just keep the GOTOs.

> If we do tree lowering like this before doing any tree-based
> optimizations, then that will make optimizing Java bytecode generation
> much harder (to the point where it is unlikely that it will be done).
> This probably isn't a great loss.
> 
Ah, but for Java bytecodes we can use JIMPLE.  It's essentially
the same IR as SIMPLE, specifically targeted at bytecode
optimizations:  http://www.sable.mcgill.ca/soot/

However, I don't plan on eliminating the optimizations we do on
language-dependent trees.  This lowering would be done after the
ld-trees have been optimized (e.g. inlining)


Diego.