Re: ICE in change_address at emit_rtl.c

[Zack Weinberg <zack at codesourcery dot com>]

On Sat, Nov 24, 2001 at 11:02:56PM +0000, Neil Booth wrote:
> 
> I've often wondered whether we might do better type-wise to actually
> use separate types, and refer to them as separate types, rather than
> having every function take an effectively typeless "tree" as input.

Once again you seem to be thinking along parallel lines to me - I was
coming at it from the tack of trying to cut down memory consumption,
but it's the same problem.

I see definite benefits from using independent structures for all
constants, identifiers, and possibly types.  I don't think it makes
sense to go as far as separating all the current members of tree_union.
It is genuinely useful to be able to treat the operands of any expression
with the same logic, which argues for e.g. leaving declarations as part of
the tree union (but then, it does the same for constants...)

> Am I right in thinking that the only common thing that every "tree"
> really needs to have is the chain element and tree code?

Less! An identifier has no use for the chain, or for most of the flags.

There are some counter-pressures against shrinking trees too far.  One is
the real danger of running out of tree_code values.  That's an 8-bit field
and C++ has more than 200 tree codes.  Another is the demand for flag bits.
And there are very few trees that don't need at least a cons-cell worth of
pointers; that's eight bytes (on a 32-bit machine) right there.

(Oh, can we please #if 0 out the aux field in tree_common?  The only
code that's using it is on the ast-optimizer-branch which will not be
merged in 3.1 - all it's doing is wasting memory.)

zw