Transform assertion into optimization hints

[François Dumont]

On 09/18/2018 08:11 AM, Marc Glisse wrote:
> On Tue, 18 Sep 2018, François Dumont wrote:
>
>> If your concern is rather that the condition got evaluated which will 
>> eventually slow down execution then I need to check generated code. 
>> Any good link explaining how to have a clear view on the generated 
>> code ?
>
> This.
>
> For a file like
>
> #include <map>
>
> void f();
> void g(std::map<int,int> const&m){
> Â  if(m.count(42)>0)__builtin_unreachable();
> Â  f();
> }
>
> compiled with g++ file.c -O3 -S -fdump-tree-optimized, I get a
> file.c.228t.optimized that contains quite a lot of code, not just a call
> to f.

Too bad, and thanks for the tip on how to generate this.

>
>> Maybe a 'if __builtin_constant_p(_Condition)' could help if gcc 
>> doesn't do it itself already.
>
> How would you use that precisely?

I wouldn't, I wasn't totally waken up when I proposed this, it makes no 
sens.

>
> It may be easiest to use a different macro for trivial tests that can go
> with __builtin_unreachable and for expensive tests that cannot.
>
Even if I think that doing this kind of operation in an assert call is 
too much I agree that it makes this change invalid.

I'll see if I need to introduce a macro the day I want to add a 
__builtin_unreachable.

François