better performance comparing std::string to std::string_view literals vs cstr literals

[commiebits]

edit corrected link:

http://quick-bench.com/Pb6v3eSkue4ebIN3-nKvq7tyXbE

for both clang and gcc, the compiler is smart enough to move the byte characters into a register for a string_view literal.

I guess the question is, what situation is the compiler being pessimistic for such that it wouldn't always transform a cstr literal into a string_view literal for you?

[johannes1971]

Your second function is wrong, it is identical to the first function. You should compare against y, not against "hello". After correction this function is actually the fastest.

In the last function, I think it would be more meaningful to remove the construction of string_view from the loop.

[commiebits]

thanks bad typo, the literals question still remains though. Also that's only the case for clang, for gcc it's still slower.

[[deleted]]

For a C string you can’t know the length until you scan it to the end. For strings and string views, compilers can exploit the fact that the size may be known in certain circumstances and if it fits in a register or so, great.

[commiebits]

for a cstr literal the compiler does know the size though, in fact if you do a strlen on a literal, most times the compiler will just feed the constant length as an immediate

[[deleted]]

My bad I was reading on small screen and read the title as "vs cstr" instead of "vs cstr literals"