[cfe-dev] Meaning of LLVM optimization levels

[Dallman, John]

> 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

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