Re: Transform assertion into optimization hints

[Moritz Klammler <moritz at klammler dot eu>]

One potential problem I can see is that the expansion of the new macro
is not a void expression as (if I am not mistaken) mandated by the
standard (§ 7.2.1.1 N1570) meaning that (admittedly questionable) code
like the following won't compile.

int n = (assert(1), 42);

Would the new macro still be able to give the desired hint to the
compiler if the logic were moved into an inline function and have the
macro expand to a simple call to that function?

Or, since this is only relevant for GCC anyway, maybe a statement
expression could be used?

https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

But I'm not sure what potential problems this might cause with regards
to C++ object lifetime if the _Condition is of a user-defined type.


On 9/17/18 10:38 PM, François Dumont wrote:
> We talk about it a while back.
> 
> I've run testsuite several times since I have this patch on my local
> copy. Note that when I implemented it the wrong way tests started to
> fail so it is clearly having an effect on the generated code.
> 
> * include/bits/c++config [__OPTIMIZE__](__glibcxx_assert): Define as
> optimization hint
> 
> using __builtin_unreachable.
> 
> Ok to commit ?
> 
> François
>

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