[libc++] Clean up and update deployment target features (#96312)

This patch removes many annotations that are not relevant anymore since
we don't support or test back-deploying to macOS < 10.13. It

[libc++] Clean up and update deployment target features (#96312)

This patch removes many annotations that are not relevant anymore since
we don't support or test back-deploying to macOS < 10.13. It also cleans
up raw usage of target triples to identify versions of dylibs shipped on
prior versions of macOS, and uses the target-agnostic Lit features
instead. Finally, it reorders both the Lit backdeployment features and
the corresponding availability macros in the library in a way that makes
more sense, and reformulates the Lit backdeployment features in terms of
when a version of LLVM was introduced instead of encoding the system
versions on which it hasn't been introduced yet. Although one can be
derived from the other, encoding the negative form is extremely
error-prone.

Fixes #80901

show more ...

[libc++] Fix deployment target Lit features (#94791)

We were not making any distinction between e.g. the "Apple-flavored"
libc++ built from trunk and the system-provided standard library on
Apple

[libc++] Fix deployment target Lit features (#94791)

We were not making any distinction between e.g. the "Apple-flavored"
libc++ built from trunk and the system-provided standard library on
Apple platforms. For example, any test that would be XFAILed on a
back-deployment target would unexpectedly pass when run on that
deployment target against the tip of trunk Apple-flavored libc++. In
reality, that test would be expected to pass because we're running
against the latest libc++, even if it is Apple-flavored.

To solve this issue, we introduce a new feature that describes whether
the Standard Library in use is the one provided by the system by
default, and that notion is different from the underlying standard
library flavor. We also refactor the existing Lit features to make a
distinction between availability markup and the library we're running
against at runtime, which otherwise limit the flexibility of what we can
express in the test suite. Finally, we refactor some of the
back-deployment versions that were incorrect (such as thinking that LLVM
10 was introduced in macOS 11, when in reality macOS 11 was synced with
LLVM 11).

Fixes #82107

show more ...

Unconditionally lower std::string's alignment requirement from 16 to 8. (#68925)

Unconditionally change std::string's alignment to 8.

This change saves memory by providing the allocator more free

Unconditionally lower std::string's alignment requirement from 16 to 8. (#68925)

Unconditionally change std::string's alignment to 8.

This change saves memory by providing the allocator more freedom to
allocate the most
efficient size class by dropping the alignment requirements for
std::string's
pointer from 16 to 8. This changes the output of std::string::max_size,
which makes it ABI breaking.

That said, the discussion concluded that we don't care about this ABI
break. and would like this change enabled universally.

The ABI break isn't one of layout or "class size", but rather the value
of "max_size()" changes, which in turn changes whether `std::bad_alloc`
or `std::length_error` is thrown for large allocations.

This change is the child of PR #68807, which enabled the change behind
an ABI flag.

show more ...

Lower std::string's alignment requirement from 16 to 8. (#68807)

This allows smaller allocations to occur, closer to the actual
std::string's required size. This is particularly effective in
decre

Lower std::string's alignment requirement from 16 to 8. (#68807)

This allows smaller allocations to occur, closer to the actual
std::string's required size. This is particularly effective in
decreasing the allocation size upon initial construction (where
__recommend is called to determine the size).

Although the memory savings per-string are never more than 8 bytes per
string initially, this quickly adds up. And has lead to not insigficant
memory savings at Google.

Unfortunately, this change is ABI breaking because it changes the value
returned by max_size. So it has to be guarded.

show more ...