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

Richard Smith richard at metafoo.co.uk
Wed Apr 24 07:50:59 PDT 2013

On Wed, Apr 24, 2013 at 6:00 AM, <Andrea_DiBiagio at sn.scee.net> wrote:

> Hello,
> We've had a high priority feature request from a number of our customers
> to
> provide per-function optimization in our Clang/LLVM compiler.
> I would be interested in working with the community to implement this.
> The idea is to allow the optimization level to be overridden
> for specific functions.
> The rest of this proposal is organized as follows:
>  - Section 1. describes this new feature and explains why and when
>    per-function optimization options are useful;
>  - Sections 2. and 3. describe how the optimizer could be adapted/changed
>    to allow the definition of per-function optimizations;
>  - Section 4. tries to outline a possible workflow for implementing this
>    new feature.
> I am looking for any feedback or suggestions etc.
> Thanks!
> Andrea Di Biagio
> SN Systems Ltd.
> http://www.snsys.com
> 1. Description
> ==============
> The idea is to add pragmas to control the optimization level on functions.
> A similar approach has been implemented by GCC as well.
> Since GCC 4.4, new function specific option pragmas have been added to
> allow
> users to set the optimization level on a per function basis.
> http://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html
> describes the pragmas as
>   #pragma GCC optimize ("string")
>   #pragma GCC push_options
>   #pragma GCC pop_options
>   #pragma GCC reset_options
> Instead of imitating GCC's syntax, I think it would be better to use a
> syntax
> consistent with existing pragma clang diagnostics:
>   #pragma clang optimize push
>   #pragma clang optimize "string"
>   #pragma clang optimize pop

Since the intent is to provide overrides on a per-function basis, have you
considered using a function attribute instead of a pragma?
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