llvm dev - Jun 2013 - [LLVMdev] [cfe-dev] Meaning of LLVM optimization levels

I'm not a LLVM or Clang developer, but I do spend a lot of time teasing
software into
working with the highest possible optimisation levels where it still works
correctly.

These guidelines are pretty good, but there are a few details worth considering.

It needs to be possible to debug code at any optimisation level. It's
acceptable for
that to be harder at high optimisation levels, but it should be possible. I find
myself
doing this when I hit optimizer bugs, and want to make coherent bug reports. The
reports are much better if I can work out what's wrong in the generated
code. I
haven't had to report many problems with Clang ... but I haven't turned
up the
optimisation all the way either.

Related to optimisation levels, it's quite helpful to have a way of
controlling
optimisation on a function-by-function level. This is very useful when
you're trying
to work out where in a file with many functions an optimiser problem is
happening;
it isn't foolproof, but it helps a lot.

--
John Dallman

From: cfe-dev-bounces at cs.uiuc.edu [mailto:cfe-dev-bounces at cs.uiuc.edu] On
Behalf Of Renato Golin
Sent: 06 June 2013 21:41
To: LLVM Dev; Clang Dev
Subject: [cfe-dev] Meaning of LLVM optimization levels

Folks,

I'm trying to rationalize about optimization levels and maybe we should come
up with a document like this:

http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Though, I remember a discussion a few months ago, and some people recommended we
had names, rather than numbers, to dissociate the idea that 3 is better than 2.
Regardless, would be good to have some guidelines on what goes where, so we
don't end up in yet another long discussion about where to put the
optimization <insert-name-here>.

As far as I can get from our side is:

-O3 : throw everything and hope it sticks
-O2 : optimized build, but should not explode in code size nor consume all
resources while compiling
-O1 : optimized debug binaries, don't change the execution order but remove
dead code and stuff
-O0 : don't touch it
-Os : optimize, but don't run passes that could blow up code. Try to be a
bit more drastic when removing code. When in doubt, prefer small, not fast code.
-Oz : only perform optimizations that reduce code size. Don't even try to
run things that could potentially increase code size.

I've been thinking about this, and I think, regarding those criteria, it
would make sense to use a try/compare/rollback approach to some passes, at least
the most dramatic ones.

For instance, the vectorizer keeps the old loops hanging, and under Os/Oz, it
should be possible to rollback the pass if the end result is bigger. Of course,
IR size has little to do with final code size, but that's why we have (and
rely so much on) heuristics.

AFAIK, for that to work on any pass as they are, we'd have to implement a
transactional model on IRBuilder, which is not trivial, but could be done. Does
anyone have a strong opinion about this?

cheers,
--renato

-----------------
Siemens Industry Software Limited is a limited company registered in England and
Wales.
Registered number: 3476850.
Registered office: Faraday House, Sir William Siemens Square, Frimley, Surrey,
GU16 8QD.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20130607/008c1457/attachment.html>

> after O1, sequential execution is a big impediment for optimizations, and
> keeping the debug information valid after so many transformations might
> pose a big penalty on the passes (time & memory). That was the whole
> idea of metadata being a second-class citizen.
I'm afraid I don't know much about how debug information is expressed,
so this idea may
be nonsense.

Is it possible for the debug information to mark all the instructions that arise
from a
language statement as coming from that statement, even though the instructions
may
be widely scattered? That in itself would be quite helpful. Instructions whose
effects
are used in the logic from more than one statement would have to be included
with
all those statement.

I felt the lack of something like this severely when digging out dozens of
compiler bugs
on Microsoft's Itanium compiler, over a decade ago. That processor
"naturally" mixed
instructions from many source statements, which prefigured this kind of problem.

I'm reasonably happy for debugging at high optimisation levels to be
primarily done
with a disassembly listing rather than source code, provided I can get some idea
of
which instructions come from which source statements, and which variables are
being
accessed. The absence of debug information at that level tends to require going
through an entire function figuring out what every instruction does, and how it
relates to the source, which is rather time-consuming.

thanks,

--
John Dallman

From: Renato Golin [mailto:renato.golin at linaro.org]
Sent: 07 June 2013 17:39
To: Dallman, John
Cc: LLVM Dev; Clang Dev
Subject: Re: [cfe-dev] Meaning of LLVM optimization levels

On 7 June 2013 13:53, Dallman, John <john.dallman at
siemens.com<mailto:john.dallman at siemens.com>> wrote:
It needs to be possible to debug code at any optimisation level.

Yes, I agree. But after O1, sequential execution is a big impediment for
optimizations, and keeping the debug information valid after so many
transformations might pose a big penalty on the passes (time & memory). That
was the whole idea of metadata being a second-class citizen.


Related to optimisation levels, it's quite helpful to have a way of
controlling
optimisation on a function-by-function level. This is very useful when
you're trying
to work out where in a file with many functions an optimiser problem is
happening;
it isn't foolproof, but it helps a lot.

There are already people working on that, and discussions on the list about this
very topic. I agree that it would be extremely helpful for debugging large
programs.

cheers,
--renato

-----------------
Siemens Industry Software Limited is a limited company registered in England and
Wales.
Registered number: 3476850.
Registered office: Faraday House, Sir William Siemens Square, Frimley, Surrey,
GU16 8QD.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20130607/cc28921c/attachment.html>

Of course it's *possible*, in a fundamental sense. It's even pretty easy
to get right in a compiler back end (in a conceptual sense). You have to touch a
LOT of code, but all the changes are trivial.  We did this at Tartan Labs back
in the 90s. Done with only a bit of care, it makes debugging possible at any
optimization level.  The idea is to make the debug information reflect what the
optimizer and code generator actually did, rather than restricting them to the
linear mapping supported by most debuggers.  If anyone cares, I can even give
details now that the NDAs have finally expired.

Sadly, you can't express the resulting source line information in the debug
directives used by any commonly available debugger (that I am aware of).  So --
at the very most optimistic -- this approach won't get you anything any time
soon.

Dean Sutherland
dsutherland at cert.org<mailto:dsutherland at cert.org>

On Jun 7, 2013, at 12:38 PM, Renato Golin <renato.golin at
linaro.org<mailto:renato.golin at linaro.org>> wrote:

On 7 June 2013 13:53, Dallman, John <john.dallman at
siemens.com<mailto:john.dallman at siemens.com>> wrote:
It needs to be possible to debug code at any optimisation level.

Yes, I agree. But after O1, sequential execution is a big impediment for
optimizations, and keeping the debug information valid after so many
transformations might pose a big penalty on the passes (time & memory). That
was the whole idea of metadata being a second-class citizen.


Related to optimisation levels, it's quite helpful to have a way of
controlling
optimisation on a function-by-function level. This is very useful when
you're trying
to work out where in a file with many functions an optimiser problem is
happening;
it isn't foolproof, but it helps a lot.

There are already people working on that, and discussions on the list about this
very topic. I agree that it would be extremely helpful for debugging large
programs.

cheers,
--renato
_______________________________________________
cfe-dev mailing list
cfe-dev at cs.uiuc.edu<mailto:cfe-dev at cs.uiuc.edu>
http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20130607/29cf2a1d/attachment.html>