This is the mail archive of the
gcc-help@gcc.gnu.org
mailing list for the GCC project.
Re: Optimize based on possible enum class values?
- From: Mathieu Malaterre <mathieu dot malaterre at gmail dot com>
- To: Mikhail Maltsev <maltsevm at gmail dot com>
- Cc: âJosh Chia (èää) <joshchia at gmail dot com>, gcc-help <gcc-help at gcc dot gnu dot org>
- Date: Thu, 19 May 2016 12:13:51 +0200
- Subject: Re: Optimize based on possible enum class values?
- Authentication-results: sourceware.org; auth=none
- References: <CALxtSbTS96dhLiLa7ujcbpdjPSsU20SWFNjk4h6PYccq-bXHWg at mail dot gmail dot com> <573D8F70 dot 3040007 at gmail dot com>
On Thu, May 19, 2016 at 12:03 PM, Mikhail Maltsev <maltsevm@gmail.com> wrote:
> On 05/19/2016 04:37 AM, âJosh Chia (èää) wrote:
>> Is there a way to tell gcc to assume that the underlying integer value
>> is for a valid enum value and optimize accordingly? Even more
>> generally, is there a way to give optimization hints to the compiler
>> that a variable can only have certain values at a certain point in the
>> code?
>>
> You can use this idiom to tell GCC that 'condition' is true:
>
> if (!condition)
> __builtin_unreachable();
>
> GCC is able to infer value ranges from conditions like e.g. (x >= a && x <= b).
Or,
#define __assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0)
for portability.
--
Mathieu