[libc++] Rename __libcpp_assertion_handler to __libcpp_verbose_abort

With the goal of reusing that handler to do other things besides
handling assertions (such as terminating when an exception is th

[libc++] Rename __libcpp_assertion_handler to __libcpp_verbose_abort

With the goal of reusing that handler to do other things besides
handling assertions (such as terminating when an exception is thrown
under -fno-exceptions), the name `__libcpp_assertion_handler` doesn't
really make sense anymore.

Furthermore, I didn't want to use the name `__libcpp_abort_handler`,
since that would give the impression that the handler is called
whenever `std::abort()` is called, which is not the case at all.

Differential Revision: https://reviews.llvm.org/D130562

(cherry picked from commit 507125af3d0b953cb56bce2e5b8000249fe1ef53)

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[libc++] Generalize the customizeable assertion handler

Instead of taking a fixed set of arguments, use variadics so that
we can pass arbitrary arguments to the handler. This is the first
step towar

[libc++] Generalize the customizeable assertion handler

Instead of taking a fixed set of arguments, use variadics so that
we can pass arbitrary arguments to the handler. This is the first
step towards using the handler to handle other non-assertion-related
failures, like std::unreachable and an exception being thrown in
-fno-exceptions mode, which would improve user experience by including
additional information in crashes (right now, we call abort() without
additional information).

Differential Revision: https://reviews.llvm.org/D130507

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[libc++] Make the Debug mode a configuration-time only option

The debug mode has been broken pretty much ever since it was shipped
because it was possible to enable the debug mode in user code witho

[libc++] Make the Debug mode a configuration-time only option

The debug mode has been broken pretty much ever since it was shipped
because it was possible to enable the debug mode in user code without
actually enabling it in the dylib, leading to ODR violations that
caused various kinds of failures.

This commit makes the debug mode a knob that is configured when
building the library and which can't be changed afterwards. This is
less flexible for users, however it will actually work as intended
and it will allow us, in the future, to add various kinds of checks
that do not assume the same ABI as the normal library. Furthermore,
this will make the debug mode more robust, which means that vendors
might be more tempted to support it properly, which hasn't been the
case with the current debug mode.

This patch shouldn't break any user code, except folks who are building
against a library that doesn't have the debug mode enabled and who try
to enable the debug mode in their code. Such users will get a compile-time
error explaining that this configuration isn't supported anymore.

In the future, we should further increase the granularity of the debug
mode checks so that we can cherry-pick which checks to enable, like we
do for unspecified behavior randomization.

Differential Revision: https://reviews.llvm.org/D122941

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[libcxx] Add flag to disable __builtin_assume in _LIBCPP_ASSERT

Introduce _LIBCPP_ASSERTIONS_DISABLE_ASSUME which makes _LIBCPP_ASSERT
not call __builtin_assume when _LIBCPP_ENABLE_ASSERTIONS == 0.

[libcxx] Add flag to disable __builtin_assume in _LIBCPP_ASSERT

Introduce _LIBCPP_ASSERTIONS_DISABLE_ASSUME which makes _LIBCPP_ASSERT
not call __builtin_assume when _LIBCPP_ENABLE_ASSERTIONS == 0.

Calling __builtin_assume was introduced in D122397.

__builtin_assume is generally supposed to improve optimizations, but can
cause regressions when LLVM has trouble handling the calls to
`llvm.assume()` (see comments in D122397).

Reviewed By: philnik

Differential Revision: https://reviews.llvm.org/D123175

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[libc++] Use __builtin_expect and __builtin_assume in _LIBCPP_ASSERT

Since we expect the condition to be true most of the time, we might
as well tell the compiler. And when assertions are disabled,

[libc++] Use __builtin_expect and __builtin_assume in _LIBCPP_ASSERT

Since we expect the condition to be true most of the time, we might
as well tell the compiler. And when assertions are disabled, we
might as well tell the compiler that it's allowed to assume that
the condition holds.

Differential Revision: https://reviews.llvm.org/D122397

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[libc++] Add a lightweight overridable assertion handler

This patch adds a lightweight assertion handler mechanism that can be
overriden at link-time in a fashion similar to `operator new`.

This is

[libc++] Add a lightweight overridable assertion handler

This patch adds a lightweight assertion handler mechanism that can be
overriden at link-time in a fashion similar to `operator new`.

This is a third take on https://llvm.org/D121123 (which allowed customizing
the assertion handler at compile-time), and https://llvm.org/D119969
(which allowed customizing the assertion handler at runtime only).

This approach is, I think, the best of all three explored approaches.
Indeed, replacing the assertion handler in user code is ergonomic,
yet we retain the ability to provide a custom assertion handler when
deploying to older platforms that don't have a default handler in
the dylib.

As-is, this patch provides a pretty good amount of backwards compatibility
with the previous debug mode:

- Code that used to set _LIBCPP_DEBUG=0 in order to get basic assertions
in their code will still get basic assertions out of the box, but
those assertions will be using the new assertion handler support.
- Code that was previously compiled with references to __libcpp_debug_function
and friends will work out-of-the-box, no changes required. This is
because we provide the same symbols in the dylib as we used to.
- Code that used to set a custom __libcpp_debug_function will stop
compiling, because we don't provide that declaration anymore. Users
will have to migrate to the new way of setting a custom assertion
handler, which is extremely easy. I suspect that pool of users is
very limited, so breaking them at compile-time is probably acceptable.

The main downside of this approach is that code being compiled with
assertions enabled but deploying to an older platform where the assertion
handler didn't exist yet will fail to compile. However users can easily
fix the problem by providing a custom assertion handler and defining
the _LIBCPP_AVAILABILITY_CUSTOM_ASSERTION_HANDLER_PROVIDED macro to
let the library know about the custom handler. In a way, this is
actually a feature because it avoids a load-time error that one would
otherwise get when trying to run the code on the older target.

Differential Revision: https://reviews.llvm.org/D121478

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[libc++] Remove workarounds for re-defining _LIBCPP_ASSERT in the test suite

As a fly-by fix, enable the complexity-changing assertions in __debug_less
only when the full debug mode is enabled, sinc

[libc++] Remove workarounds for re-defining _LIBCPP_ASSERT in the test suite

As a fly-by fix, enable the complexity-changing assertions in __debug_less
only when the full debug mode is enabled, since debugging level 0 is usually
understood to only contain basic assertions that do not change the complexity
of algorithms.

Differential Revision: https://reviews.llvm.org/D121129

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[libc++] Move everything related solely to _LIBCPP_ASSERT to its own file

This is the first step towards disentangling the debug mode and assertions
in libc++. This patch doesn't make any functional

[libc++] Move everything related solely to _LIBCPP_ASSERT to its own file

This is the first step towards disentangling the debug mode and assertions
in libc++. This patch doesn't make any functional change: it simply moves
_LIBCPP_ASSERT-related stuff to its own file so as to make it clear that
libc++ assertions and the debug mode are different things. Future patches
will make it possible to enable assertions without enabling the debug
mode.

Differential Revision: https://reviews.llvm.org/D119769

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