Moves some isbig checks into string.h, enabling smarter optimizations to
be made on small strings. Also we no longer zero out our string prior to
calling the various constructors, buying back the performance we lost on
big strings when we made the small-string optimization. We further add a
little optimization to the big_string copy constructor: if the string is
using half or more of its capacity, then we don’t recompute capacity and
just take the old string’s. As well, the copy constructor always makes a
small string when it will fit, even if copied from a big string that got
truncated.
This also reworks the test to follow the idiom adopted elsewhere re stl,
and adds a helper function to tell if a string is small based on data().
The mangled name of a C++ function will typically not vary by const-ness
of a by-value parameter; in other words, there is no meaning to a const-
qualified by-value parameter in a function prototype. However, the const
keyword _does_ matter at function _definition_ time, like it does with a
variable declared in the body. So for prototypes, we strip out const for
by-value parameters; but for definitions, we leave them alone.
At function definition (as opposed to prototype), we add const to values
in parameters by default, unless we’re going to mutate them.
This commit also changes a couple of const string_view& to be simply by-
value string_view. A string_view is only two words; it rarely ever makes
sense to pass one by reference if it’s not going to be mutated.
We now fully initialize a ctl::string’s memory, so that it is always set
to a well-defined value, thus making it always safe to memcpy out of it.
This incidentally makes our string::swap function legal, which it wasn’t
before. This also saves us a store in string::reserve.
Now that we have made both big_string and small_string POD, I believe it
is safe to elide the launder calls, and have done so, thus cleaning up a
lot of the blob-related code.
I also got rid of set_big_capacity and replaced it with a set_big_string
that leaves us in a well-defined state afterwards. This function also is
able to be somewhat simpler; rather than delicate bit-twiddling, it just
reaches straight into blob and rewrites it wholesale.
Overall, this shaves about 1–2ns off of most benchmarks, and adds 1ns to
only one of them - creating a string from a char *.
`big_string` is not pod which means it needs to be properly constructed
and destroyed. Instead make it POD and destroy it manually in `string`
destructor.
There were a few errors in how capacity and memory was being handled for
small strings. The capacity errors meant that small strings would become
big strings too soon, and the memory error introduced undefined behavior
that was caught by CheckMemoryLeaks in our test file but only sometimes.
The crucial change is in reserve: we only copy n bytes into p2, and then
we manually set the null terminator instead of expecting it to have been
there already. (E.g. it might not be there for an empty small string.)
We also fix one other doozy in append when we were exactly at the small-
to-big string boundary: we set the last byte (i.e., the remainder field)
to 0, then decremented it, giving us size_t max. Whoops. We boneheadedly
fix this by setting the 0 byte after we've fixed up the remainder, so it
is at worst a no-op.
Otherwise, capacity now works the same for small strings as it does with
big strings: it's the amount of space available including the null byte.
We test all of this with a new test that only gets included if our class
under test is not std::string (presumably meaning it's ctl::string.) The
test manually verifies that the small string optimization behaves how we
expect.
Since this test checks against std::string, we go ahead and include that
other header from the STL.
Also modifies the new test we introduced to also run on std::string, but
it just does the append without expecting anything about how its data is
stored. We also check that the string has the right value afterwards.
A small-string optimization is a way of reusing inline storage space for
sufficiently small strings, rather than allocating them on the heap. The
current approach takes after an old Facebook string class: it reuses the
highest-order byte for flags and small-string size, in such a way that a
maximally-sized small string will have its last byte zeroed, making it a
null terminator for the C string.
The only flag we have is in the highest-order bit, that says whether the
string is big (set) or small (cleared.) Most of the logic switches based
on the value of this bit; e.g. data() returns big()->p if it's set, else
small()->buf if it's cleared. For a small string, the capacity is always
fixed at sizeof(string) - 1 bytes; we store the length in the last byte,
but we store it as the number of remaining bytes of capacity, so that at
max size, the last byte will read zero and serve as our null terminator.
Morally speaking, our class's storage is a union over two POD C structs.
For now I gravitated towards a slightly more obtuse approach: the string
class itself contains a blob of the right size, and we alias that blob's
pointer for the two structs, taking some care not to run afoul of object
lifetime rules in C++. If anyone wants to improve on this, contributions
are welcome.
This commit also introduces the `ctl::__` namespace. It can't be legally
spelled by library users, and serves as our version of boost's "detail".
We introduced a string::swap function, and we now use that in operator=.
operator= now takes its argument by value, so we never need to check for
the case where the pointers are equal and can just swap the entire store
of the argument with our own, leaving the C++ destructor to free our old
storage afterwards.
There are probably still a few places where our capacity is slightly off
and we grow too fast, although there don't appear to be any where we are
too slow. I will leave these to be fixed in future changes.
