GCC 11 Now Defaults To C++17 Dialect By Default

[phoronix Administrator Find all posts View Profile Close phoronix]

Phoronix: GCC 11 Now Defaults To C++17 Dialect By Default

Following the proposal at the end of last year over GCC 11 aiming to default to C++17 for its C++ front-end, that change is now in place for GNU Compiler Collection 11...

http://www.phoronix.com/scan.php?pag...Cpp-17-Default

[JacekJagosz Junior Member Find all posts View Profile Close JacekJagosz]

How much does enabling newer standards slow down compiling of older code? Has anybody tested that?

[JackLilhammers Senior Member Find all posts View Profile Close JackLilhammers]

Yes, Michael usually does that too.
But as new standards get usually a little safer, you might spend more time compiling but less time debugging things the older ones allowed

[pal666 Senior Member Find all posts View Profile Close pal666]

How much does enabling newer standards slow down compiling of older code? Has anybody tested that?

why should it slow down compiling and why you aren't interested at all how it affects something more important, like code generation ? old code will use newer standard library, which will use newer language features which usually speed up compiling

[carewolf Senior Member Find all posts View Profile Close carewolf]

How much does enabling newer standards slow down compiling of older code? Has anybody tested that?

Why on earth would it take longer? It is not extra optimization passes, just some more stuff allowed. Stuff already parsed with lower standard limits so that the compiler can output helpful error messages.

[entropy Senior Member Find all posts View Profile Close entropy]

Being a bit nit-picking her

c++17 is a base standard.
gnu17 would be a dialect.

[Mangix Senior Member Find all posts View Profile Close Mangix]

Why on earth would it take longer? It is not extra optimization passes, just some more stuff allowed. Stuff already parsed with lower standard limits so that the compiler can output helpful error messages.

With higher standards, more and more stuff becomes constexpr, making compilation slower.

Not the only reason but that's one of them.

[carewolf Senior Member Find all posts View Profile Close carewolf]

With higher standards, more and more stuff becomes constexpr, making compilation slower.

Not the only reason but that's one of them.

That doesn't make anything slower. Constexpr is only a helper for the programmer to ensure they program in a way that can be const propagated, a compiler does const propagation in any case.

The only difference C++17 makes is allow you to use those expressions in new places, but if you have code that compiled with C++14 you are not doing that. So you are just getting the const propagation that has always taken place.

[Mangix Senior Member Find all posts View Profile Close Mangix]

I was under the impression that constexpr enables compile time evaluation.

[archibald Senior Member Find all posts View Profile Close archibald]

That doesn't make anything slower. Constexpr is only a helper for the programmer to ensure they program in a way that can be const propagated, a compiler does const propagation in any case.

The only difference C++17 makes is allow you to use those expressions in new places, but if you have code that compiled with C++14 you are not doing that. So you are just getting the const propagation that has always taken place.

I'm sorry to say but you're mistaken here - constexpr is a keyword that allows compile-time evaluation: https://en.cppreference.com/w/cpp/language/constexpr. It can indeed slow down compilation time, but it results in runtime speedups and compile-time checking of results, so the tradeoff is generally considered worth it.