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

[Ralf Karrenberg]

Hi,

On 13.06.2013 20:15, Xinliang David Li wrote:
> GCC's optimize attribute should work fine (at least with trunk):
>
> __attribute__((optimize("O3","no-tree-pre"))) int foo( ...)
> {
>      ...
> }
>
> will turn on -O3 for 'foo', but disable PRE pass for it.

Indeed, the optimize attribute should do the job if you require 
optimization control on function level only.

If you need finer-grained control mechanisms, you need to resort to a 
pragma or attributes (or whatever kind of annotation) approach.
For instance, noise allows you to annotate loops or compound statements.

> 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.

I agree, these are indeed questionable.
However, there is a very important use case that I would call 
"performance tuning of critical code segments".

Cheers,
Ralf

>
> David
>
>
> On Wed, Jun 12, 2013 at 8:11 AM, Dallman, John <john.dallman at siemens.com> wrote:
>>> Although both cases would be nice and our users have expressed some
>>> interest in both, the critical one is the second case of making sure that
>>> some functions are never optimized is the most critical one.  The major
>>> use-case for this is for ease of debugging optimized builds.
>>
>> I have a similar usage case: I work on code that tends to show up optimiser
>> bugs, possibly because it is very thoroughly tested. Optimization control
>> pragmas are invaluable for locating optimizer bugs in a particular function;
>> the lack of them is one of the reasons why my GCC and Clang builds don't have
>> optimization turned up so high as on some other compilers.
>>
>> --
>> John Dallman
>> -----------------
>> Siemens Industry Software Limited is a limited company registered in England and Wales.
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