Explicitly value-initializes the deleter, even though I have not found a
way to get the deleter to act like it’s been default-initialized in unit
tests so far.
Uses auto in reset. The static cast is apparently not needed (unless I’m
missing some case I didn’t think of.)
Implements the general move constructor - turns out that the reason this
didn’t work before was that default_delete<U> was not move constructible
from default_delete<T>.
Drop inline specifiers from functions defined entirely inside the struct
definition since they are implicitly inline.
* Cleans up reset to match spec
Remove the variants from the T[] specialization. Also follow the spec on
the order of operations in reset, which may matter if we are deleting an
object that has a reference to the unique_ptr that is being reset. (?)
* Tests Base/Derived reset.
* Adds some constexpr declarations.
* Adds default_delete specialization for T[].
* Makes parameters const.
The STL says that these should be replaceable by user code.
new.cc now defines only a few direct functions (including a free wrapper
that perplexingly is needed since g++ didn’t want to alias "free".) Now,
all of the operators are weak references to those functions.
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().
* Add ctl utility.h
Implements forward, move, swap, and declval. This commit also adds a def
for nullptr_t to cxx.inc. We need it now because the CTL headers stopped
including anything from libc++, so we no longer get their basic types.
* Use ctl::swap in string
The STL spec says that swap is located in the string_view header anyawy.
Performance-wise this is a noop, but it’s slightly cleaner.
The way unique_ptr is supposed to work is as a purely compile-time check
that your raw pointers are getting deleted when they go out of scope. It
should ideally emit the same exact machine code as if you were using raw
pointers with manual deletes.
Part of what this means is that under normal circumstances, a unique_ptr
shouldn’t take up more space than a raw pointer - in other words, sizeof
unique_ptr<T> should == sizeof(T*).
The present PR doesn’t bother with the specialization for array types. I
also left a couple other parts of the STL API unimplemented. I’d love to
see someone else implement these, or I’ll get to them at some point.
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.
🚨 clang-format changes output per version!
This is with version 19.0.0. The modifications seem to be fixing the old
version’s errors - mainly involving omitted whitespace around binary ops
and inserted whitespace between goto labels and colons (if followed by a
curly brace.)
Also fixes a few mistakes made by e.g. someone (ahem) forgetting to pass
his ctl/string.h modifications through it.
We should add this to .git-blame-ignore-revs once we have its final hash
on master.
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 *.
This replaces the STL <new> header. Mainly, it defines a global operator
new and operator delete, as well as the placement versions of these. The
placement versions are required to not get compile errors when trying to
write a placement new statement.
Each of these operators is defined with many, many different variants. A
glance at new.cc is recommended followed by a chaser of the Alexandrescu
talk "std::allocator is to Allocation as std::vector is to Vexation". We
must provide a global-namespace source-level definition of each operator
and it is illegal for any of them to be marked inline, so here we are.
The upshot is that we no longer need to include <new>, and our optional/
vector headers are self-contained.
`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.
Manually manage the lifetime of `value_` by using an anonymous
`union`. This fixes a bunch of double-frees and double-constructs.
Additionally move the `present_` flag last. When `T` has padding
`present_` will be placed there saving `alignof(T)` bytes from
`sizeof(optional<T>)`.
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.
