Re: Proposed major change to combine

[kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)]

    > I'd like to propose getting rid of the mechanisms where it tries to
    > substitute things inside RTL and instead always make new RTL.  This costs
    > most memory, but is less of an issue with GC.

    This will break patterns that require rtl to match, like match_dups and
    in-out asm memory operands.

Well, the only such cases I can think of where there would be an issue
is if the operand were a MEM whose address was being simplified.

So I think we do indeed have to substitute into the address of a MEM,
instead of making another one, but we don't have to in the major cases
of simplifying expressions, which is where the big win occurs.