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){
if(m.count(42)>0)__builtin_unreachable();
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
