[LLVMdev] [cfe-dev] [PROPOSAL] per-function optimization level control

[Eric Christopher]

On Fri, Jun 14, 2013 at 11:52 AM, Xinliang David Li
<xinliangli at gmail.com> wrote:
> On Fri, Jun 14, 2013 at 11:36 AM, Eric Christopher <echristo at gmail.com> wrote:
>> On Fri, Jun 14, 2013 at 10:36 AM, Xinliang David Li
>> <xinliangli at gmail.com> wrote:
>>> On Fri, Jun 14, 2013 at 4:06 AM,  <Andrea_DiBiagio at sn.scee.net> wrote:
>>>> Hi David,
>>>>
>>>>> Regarding Andrea's proposal -- the new #pragma can be useful (in rare
>>>>> cases when there is a compiler bug), the intended use cases are
>>>>> questionable:
>>>>> 1) it should not be used as a mechanism to triage compiler bugs -- the
>>>>> compiler backend should have mechanism to allow any pass to be
>>>>> disabled for any (range of) function(s) via command line options so
>>>>> that it can be automated -- you should not expect doing this via
>>>>> source modification
>>>>> 2) Improve debuggability of optimized code. GCC has -Og option that
>>>>> can be used to generate well optimized code with good debuggability.
>>>>> 3) there is a much bigger issue if the customer needs to resort to
>>>>> this pragmas frequently to hide optimizer bugs.
>>>>
>>>> Just to clarify, although in our opinion the ability to workaround
>>>> optimization
>>>> bugs is a useful side effect of this proposal, we don't consider it the
>>>> main
>>>> use case.
>>>> That's probably my fault, I should have put more emphasis on the main use
>>>> case:
>>>>
>>>> "The main motivation of our customers is to be able to selectively disable
>>>> optimizations when debugging one function in a compilation unit, in the
>>>> case
>>>> where compiling the whole unit at -O0 would make the program run too
>>>> slowly."
>>>>
>>>> In general, it is something that should help improve the debugging
>>>> experience when customers have to debug specific portions of their code
>>>> without sacrificing any of their overall runtime performances.
>>>> As a side note, using GCC is not a option for us.
>>>>
>>>
>>> To be clear, I am not advocating GCC at all here. Just pointing out
>>> there is room for improvement for DOC so that user does not need to
>>> resort to these workarounds. I know Eric Christopher  is working very
>>> hard to make this happen for LLVM :)
>>>
>>
>> Assuredly. FWIW I like Andrea's new proposal as a good first step. I
>> definitely acknowledge the usefulness of having almost all of the app
>> running at full speed and only having a small function at O0 to get a
>> better debugging experience. The full set of passes enabled this way?
>> Not quite convinced yet :)
>
> The problem with this approach (selective O0) is that you can only
> look at the function itself with full debug. Very often you will need
> to examine the context of the call. Besides what if the function is
> the hottest one in the program?  Having said that, I did this a lot
> myself when debugging a program -- just recompile the module i care
> about with -O0 -g and relink.
>

Totally agree. I've done exactly that myself. :)

and the first part is exactly why I was resistant to the idea in the
first place - besides my ivory tower idea that the compiler should
handle everything perfectly ;)

-eric