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libstdc++: Improve comments on __wait_args::_M_setup_proxy_wait

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libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (__wait_args): Improve comments.
	* src/c++20/atomic.cc (__wait_args::_M_setup_proxy_wait):
	Improve comment.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Jonathan Wakely
2026-01-10 13:44:49 +00:00
committed by Jonathan Wakely
parent da5a5c5528
commit 59bc37debf
2 changed files with 21 additions and 14 deletions
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11
libstdc++-v3/include/bits/atomic_wait.h
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@@ -162,9 +162,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
struct __wait_args_base
{
__wait_flags _M_flags;
int _M_order = __ATOMIC_ACQUIRE;
__wait_value_type _M_old = 0;
void* _M_wait_state = nullptr;
int _M_order = __ATOMIC_ACQUIRE; // Memory order for loads from _M_obj.
__wait_value_type _M_old = 0; // Previous value of *_M_obj.
void* _M_wait_state = nullptr; // For proxy wait and tracking contention.
const void* _M_obj = nullptr; // The address of the object to wait on.
unsigned char _M_obj_size = 0; // The size of that object.
@@ -286,9 +286,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return __val;
}
// Returns true if a proxy wait will be used for __addr, false otherwise.
// If true, _M_wait_state, _M_obj, _M_obj_size, and _M_old are set.
// If false, data members are unchanged.
// Prepare `*this` for a call to `__wait_impl` or `__wait_until_impl`.
// See comments in src/c++20/atomic.cc for more details.
bool
_M_setup_proxy_wait(const void* __addr);

24
libstdc++-v3/src/c++20/atomic.cc
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@@ -308,14 +308,22 @@ namespace
} // namespace
// Return false (and don't change any data members) if we can do a non-proxy
// wait for the object of size `_M_obj_size` at address `addr`.
// Otherwise, we will use a proxy wait. If addr == _M_obj this is the initial
// setup of the proxy wait, so set _M_wait_state to the proxy state for addr,
// and set _M_obj and _M_obj_size to refer to the _M_wait_state->_M_ver proxy.
// For both the initial setup of a proxy wait and for subsequent calls to this
// function for proxy waits, we load the current value from _M_obj (the proxy)
// and store it in _M_old, then return true.
// Returns true if a proxy wait will be used for addr, false otherwise.
//
// For the first call (from `_M_setup_wait`) `_M_obj` will equal `addr`,
// and this function will decide whether to use a proxy wait.
//
// If it returns false, there are no effects (all data members are unchanged),
// and this function should not be called again on `*this` (because calling it
// again will just return false again, so is a waste of cycles).
//
// The first call that returns true does the initial setup of the proxy wait,
// setting `_M_wait_state` to the waitable state for `addr` and setting
// `_M_obj` and `_M_obj_size` to refer to the `_M_wait_state->_M_ver` proxy.
// For subsequent calls (from `_M_on_wake`) the argument can be null,
// because any value not equal to `_M_obj` has the same effect.
// All calls that return true will load a value from `_M_obj` (i.e. the proxy)
// and store it in `_M_old`.
bool
__wait_args::_M_setup_proxy_wait(const void* addr)
{