Competing for speed

Article: Bulldozer Benchmarks, What Do They Mean?
By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), April 22, 2011 11:09 am
Room: Moderated Discussions
anon (anon@anon.com) on 4/21/11 wrote:
---------------------------
>Hmm having reread your answer, do you ever transform the input program into an AST
>even when it's used? Or is it the case that you have a unique AST-like format that
>is not canonical but nonetheless serve's its purpose for robust C diagnostics?

I have a really hard time figuring out what you are after..

Of course I transform the program into a syntax tree. That's
what just about any parser does. But the first parsing phase
does _nothing_ else, it literally just builds the syntax
tree that is very close to just the bare necessities.

And what would be "AST-like" about it? It _is_ an AST.

And "canonical" is whatever you define it to be for some
AST that is purely internal. Once sparse decides that a
function or data structure is actually used (ie it either
has global visibility, or it's indirectly used by something
with global visibility), there's a lot of transformations
to simplify the AST, because you need that to get at a lot
of trivial decisions.

Many of those transformations are about normalizing the
tree in many ways, and you can certainly talk about a
"canonical" form for the tree. I just don't think that is
a very interesting issue. Things like re-ordering
associative and commutative expressions into some canonical
form may be important so that you can do all the regular
constant folding (ie turning "1+a+2" into "1+2+a" into
"3+a") on a tree level, but it's not that interesting.

And no, it's not done for all parse trees. That whole
thing is also done only lazily. So parts of the syntax
tree may well be in some raw non-canonical form.

And one reason I don't think the parse tree is even all
that interesting is that it's just a first stage.

Because most of the actual analysis is not done on the
AST at all - the more interesting analysis is done on a
linearized SSA-form IR, where you can do sane CSE and
flow analysis etc. You'd be crazy to do that on the AST,
even if it would be feasible in theory. A syntax tree is
convenient for some things, but very inconvenient for
others.

So if the question was about whether sparse spends a lot
of effort to keep things in a very well-defined canonical
AST form and does a lot of complicated analysis on that
form, then the answer is no. It does _some_ analysis on
the tree form, but the AST isn't some deeply important
thing in the end. It's just a phase.

And the above may not AT ALL be the answer you were looking
for.

For all I know, you may have some very specific notion of
"canonical form" in mind that you were taught as being
the definition of a canonical AST. If so, I would
suggest you just take a look at the sources:

git://git.kernel.org/pub/scm/devel/sparse/chrisl/sparse.git

and then ask whatever specific question you want.

Linus
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                                                                            File size optimizationsRicardo B2011/04/25 08:22 AM
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                                Imperative compilers (and the lack of it)none2011/04/10 06:00 AM
                                  Imperative compilers (and the lack of it)dmsc2011/04/10 06:39 AM
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