[RFC] Make cstring constructor from `char*` explicit and `string_view` conversion implicit
asl opened this issue · comments
Thinking more about it... What if we do the opposite?
- Conversion to
string_view
should be implicit - Construction from
const char*
should be explicit
Here is the rationale: currently it is not possible to have a string_view
function argument if the function is assumed to accept both cstring
and const char*
. E.g. the following:
void foo(cstring);
void foo(string_view);
foo("bar")
Is ambiguous as cstring
could be constructed from char*
implicitly, string_view
could be constructed from char*
implicitly, but we cannot drop the cstring
overload as cstring
cannot be implicitly converted to string_view
.
Certainly, there are lots of examples in codebase that do e.g.
cstring getName() const { return "Foo"; }
that will be broken. But IMO these should be all fixed to cstring::literal
call and explicit conversion to cstring
should emphasize that this is not a cheap operation (involving strlen
+ map lookup).
Originally posted by @asl in #4676 (comment)
Tagging @vlstill @fruffy @ChrisDodd for opinions
@vlstill also introduced a
_cs
suffix at some point which should help here? #4342
Yes. _cs
would reduce the amount of typing indeed. As far as I can see, the backends are major abusers of cstrings. ebpf is the nicest example :) This would affect downstream users certainly, though. However, #4481 (comment) clearly shows that there is lots of things here and there. cstring cache contains the whole copy of the input, for example. As all tokens are stored as cstring's.
I like the idea of explicit constructor for cstring
. I think this would be a major undertaking to do, there is a lot of places in the compiler that count on implicit casts. This would be much simpler if there was some tooling support for making changes like this automatic (it almost seems like something that clang-tidy or something similar should be able to do, but I don't know if there is any solution). That would also make it much simpler for the downstream tools.
I've not tried this one I think, but I've tried to make the construction of IR::ID
explicit and that was too much for me. However, without IR::ID
explicit constructor, this one does not make much sense. These implicit casts are defined in cstring
and IR::ID
:
const char *
->cstring
std::string
->cstring
std::stringstream
->cstring
const char *
->IR::ID
cstring
->IR::ID
std::string
->IR::ID
So making cstring
explicit is not enough, as there would still be chain of implicit casts const char *
-> IR::ID
-> cstring
:-(.
I've also checked how the standard library does similar conversions and it seems to be similar case as you propose (with the additional problem that the implicit cast in stdlib can introduce dangling reference but cstring
-> string_view
can't). The std library has:
- explicit cast
std::string_view
->std::string
(constructor) - implicit cast
std::string
->std::string_view
(operator)
@vlstill Yeah. That's lots of work. I already started changes here and there. Some observations:
- There are lots of
cstring
's created from string literals. Here_cs
is a rescue. Bonus points: this makes the stuff faster as we do not need to copy the string, we just take the address of the literal and put into the map. - Many places receives
cstring
as an argument. They should really takestring_view
. I'm doing these changes on the way. cstring
itself deserves some love. This includes e.g.cstring::startsWith
/cstring::endsWith
. Currently they take cstring as an argument, essentially caching them. There is certainly no need for thiscstring
concatenations. Lots of cases. Should definitely require refactoring. I'm putting FIXMEs for now
Maybe something else will occur on my way :)
So, I just moved a downstream backend over the changes in #4694
It was not that painful. Important story that it uncovered lots of rusty cruft accumulated over the time:
- cstrings created for temporaries
- cstring literals where just normal string literals would suffice
- unnecessary to / from cstring conversion
- lots of (not so) creative ways to create strings here and there
I would say it was definitely worth the cleanup :)
Some story from dowstream code. It had code like this:
void foo(cstring a, cstring b, ...);
void bar() {
..
std::string s = a->toString().c_str();
if (rare_condition)
s += prefix;
foo(s, s);
}
bar()
is quite hot function for some reason. As std::string
=> cstring
conversion is still implicit we ended with:
- std::string allocation (from cstring)
- two std::string => cstring conversions (that involved map lookup) for no particular case
So, just changing the code to:
void foo(cstring a, cstring b, ...);
void bar() {
..
cstring s = a->toString();
if (rare_condition)
s += prefix;
foo(s, s);
}
yielded a noticeable speedup as no memory allocations and map lookups were performed on most probable hot path. Even on slow path we only pay single price for cstring construction.
This overhead went unnoticed due to implicit conversions...