On Dec 5, 2005, at 5:27 PM, Mark Mitchell wrote:
Steven Bosscher wrote:
IMVHO dumping for "export" and front-end tools and for the optimizers
should not be coupled like this. Iff we decide to dump trees, then I
would hope the dumper would dump GIMPLE only, not the full front end
and middle-end tree representation.
It's not that I would object to waking up one day to find out that the
C++ front-end no longer used Tree, but it just doesn't seem very
compelling to me.
I agree with you. The 'tree' data structure is conceptually what we
want for the front-ends to represent the code. They are quite
similar in spirit to many AST representations.
Sharing a tree dumper between the front ends and the middle-end would
only make it more difficult again to move to sane data structures for
the middle end and to cleaner data structures for the front ends.
The differences between GIMPLE and C++ Trees are small, structurally;
there are just a lot of extra nodes in C++ that never reach
GIMPLE. If
we had a tree dumper for one, we'd get the other one almost for free.
So, I don't think sharing the tree dumper stands in the way of
anything;
you can still switch either part of the compiler to use non-Tree
whenever you like. You'll just need a new dumper, which you would
have
wanted anyhow.
The point that I'm arguing (and I believe Steven agrees with) is that
trees make a poor representation for optimization. Their use in tree-
ssa has lead to a representation that takes hundreds of bytes and
half a dozen separate allocations for each gimple operation. From
the efficiency standpoint alone, it doesn't make sense to use trees
for optimization.
Further, I would point out that it actually HURTS the front-ends to
have the optimizers using trees. We are getting very close to the
time when there are not enough tree codes to go around, and there is
still a great demand for new ones. Many of these tree codes are
front-end specific (e.g. BIND_EXPR and various OpenMP nodes) and
many of them are backend specific (e.g. the various nodes for the
vectorizer). Having the front-end and the back-end using the same
enum *will* have a short term cost if the size of the tree enum field
needs to be increased.
-Chris
