On Dec 5, 2005, at 11:48 AM, Steven Bosscher wrote:
On Saturday 03 December 2005 20:43, Mark Mitchell wrote:
There is one advantage I see in the LTO design over LLVM's
design. In
particular, the LTO proposal envisions a file format that is
roughly at
the level of GIMPLE. Such a file format could easily be extended
to be
at the source-level version of Tree used in the front-ends, so that
object files could contain two extra sections: one for LTO and one
for
source-level information. The latter section could be used for
things
like C++ "export" -- but, more importantly, for other tools that need
source-level information, like IDEs, indexers, checkers, etc.
I actually see this as a disadvantage.
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.
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.
I totally agree with Steven on this one. It is *good* for the
representation hosting optimization to be different from the
representation you use to represent a program at source level. The
two have very different goals and uses, and trying to merge them into
one representation will give you a representation that isn't very
good for either use.
In particular, the optimization representation really does want
something in "three-address" form. The current tree-ssa
implementation emulates this (very inefficiently) using trees, but at
a significant performance and memory cost. The representation you
want for source-level information almost certainly *must* be a tree.
I think it is very dangerous to try to artificially tie link-time
(and other) optimization together with source-level clients. The
costs are great and difficult to recover from (e.g. as difficult as
it is to move the current tree-ssa work to a lighter-weight
representation) once the path has been started.
That said, having a good representation for source-level exporting is
clearly useful. To be perfectly clear, I am not against a source-
level form, I am just saying that it should be *different* than the
one used for optimization.
-Chris
