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

[Eric Christopher]

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 :)

-eric