[native] Drop std::binary_semaphore and the last string in the DSO loader by simonrozsival · Pull Request #12560 · dotnet/android

simonrozsival · 2026-08-28T06:33:47Z

PR relationship: No code prerequisite; this PR is based directly on main. The absolute CoreCLR totals below were measured in the former serialized stack.

Part of the drop-libc++ effort.

This clears the remaining libc++ references from android-system.cc.o, taking that object from 5 to 0 and the CoreCLR total from 31 to 26.

None of the five references had anything to do with path handling — they came from two small header-only helpers:

`std::binary_semaphore` -> `sem_t`

try_acquire_for pulled in std::chrono::steady_clock::now and libc++'s __libcpp_timed_backoff_policy, and release pulled in __cxx_atomic_notify_all. (This was the unexplained Release-mode steady_clock reference noted earlier in the stack.)

The replacement is plain POSIX sem_t from <semaphore.h> — a real semaphore that lives in libc, not libc++, and needs no wrapper type.

An earlier revision of this PR hand-rolled a BinarySemaphore on top of pthread_mutex_t/pthread_cond_t so the timeout could use CLOCK_MONOTONIC. That was reimplementing a primitive libc already ships, so it is gone: sem_timedwait() supports CLOCK_REALTIME only until API 28 (sem_timedwait_monotonic_np() is __INTRODUCED_IN(28), sem_clockwait() is API 30, and we support API 24), which means a wall clock adjustment inside the window can cut the 3s wait short or stretch it. For a sanity timeout on loading a shared library that is a fine trade for deleting 99 lines of hand-written synchronization code.

sem_timedwait() takes an absolute deadline, so retrying after EINTR cannot extend the total wait, and no nanosecond normalization is needed because the timeout is a whole number of seconds.

The gratuitous `virtual` destructor

MainThreadDsoLoader is never derived from and only ever lives on the stack (dso-loader.hh:169), but its destructor was virtual. That made the compiler emit the deleting destructor D0Ev, which references operator delete.

The NUL-termination `std::string`

SystemLoadLibraryWrapper::load created a std::string purely to get a NUL-terminated copy of a std::string_view to hand to NewStringUTF. It now uses a stack buffer with a heap fallback, with the actual loading split into a const char* overload so there is a single place to free the copy.

Results

	before	after
`android-system.cc.o` refs	5	0
CoreCLR total refs	31	26
`libnet-android.release.so`	536,368	525,904 (−10,464)

NativeAOT stays at 0 refs; MonoVM and NativeAOT both still build clean (these are common/ headers, so MonoVM benefits too).

Copilot

Copilot review overview

Review tier: Lite
Findings: 2

New issues introduced by this change (2)

Severity	Finding
	`src/native/common/include/runtime-base/system-loadlibrary-wrapper.hh` — ❌ error: `java_lib_name` is a local JNI reference created via `NewStringUTF()` and is never…
	`src/native/common/include/shared/binary-semaphore.hh` — 💡 suggestion: `clock_gettime()` return value is currently ignored. If it ever fails, `deadline`…

What changed in this PR

This PR advances the “drop-libc++” effort by removing remaining libc++-pulling constructs from the DSO loading path: replacing std::binary_semaphore with a pthread-based BinarySemaphore, removing an unnecessary virtual destructor, and avoiding std::string allocation just to NUL-terminate a std::string_view for JNI.

Changes:

Introduces BinarySemaphore (pthread mutex/condvar) as a replacement for std::binary_semaphore in the main-thread DSO loader path.
Refactors SystemLoadLibraryWrapper::load to use a stack buffer with heap fallback instead of std::string for JNI NUL-terminated input.
Removes the virtual destructor from MainThreadDsoLoader to avoid emitting a deleting destructor (and operator delete).

File	Description
src/native/common/include/shared/binary-semaphore.hh	Adds pthread-based binary semaphore with monotonic-clock timeout logic.
src/native/common/include/runtime-base/system-loadlibrary-wrapper.hh	Replaces `std::string` copy with stack/heap buffer and splits out `const char*` overload for JNI loading.
src/native/common/include/runtime-base/mainthread-dso-loader.hh	Switches to `BinarySemaphore` and removes unnecessary virtual destructor.

…ader This clears the remaining `libc++` references from `android-system.cc.o`, bringing that object down from 5 to 0 and the CoreCLR total from 31 to 26. None of the five references had anything to do with path handling: * `std::binary_semaphore::try_acquire_for` pulled in `std::chrono::steady_clock::now` and libc++'s timed backoff policy, and `release` pulled in `__cxx_atomic_notify_all`. Replace it with a small `BinarySemaphore` built directly on `pthread_mutex_t`/`pthread_cond_t`, which is the primitive already used elsewhere in the tree. As a bonus it waits on `CLOCK_MONOTONIC`, so the timeout is no longer affected by wall clock adjustments. * `MainThreadDsoLoader`'s destructor was `virtual` even though the class is never derived from and only ever lives on the stack. That made the compiler emit the deleting destructor, which references `operator delete`. * `SystemLoadLibraryWrapper::load` created a `std::string` purely to get a NUL-terminated copy of a `std::string_view`. Use a stack buffer with a heap fallback instead, and split the actual loading into an overload taking a `const char*` so there is a single place to free the copy. `libnet-android.release.so` shrinks by 10,464 bytes (536,368 -> 525,904). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

- `SystemLoadLibraryWrapper::load ()` never released the local reference returned by `NewStringUTF ()`. This runs while loading the application's shared libraries, before control returns to Java, so nothing reclaims the references in between and the local reference table can fill up. Delete the reference once `CallStaticVoidMethod ()` returns. `DeleteLocalRef ()` is safe to call with a pending exception, so it can happen before the exception check. - `BinarySemaphore::try_acquire_for ()` ignored the return value of `clock_gettime ()`. On failure `deadline` stayed zero, which makes `pthread_cond_timedwait ()` return `ETIMEDOUT` right away and turns the wait into a silent spurious timeout. Abort instead. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

The previous commit replaced `std::binary_semaphore` with a `BinarySemaphore` class built on `pthread_mutex_t`/`pthread_cond_t`. That was reimplementing a primitive libc already provides: `sem_t` from `<semaphore.h>` is a POSIX semaphore, lives in libc rather than libc++, and needs no wrapper at all. Delete the 99-line header and use `sem_init()`/`sem_post()`/`sem_timedwait()` directly. The only reason to prefer a condition variable here was that `sem_timedwait()` supports `CLOCK_REALTIME` only until API 28 (`sem_timedwait_monotonic_np()` is `__INTRODUCED_IN(28)` and `sem_clockwait()` is API 30, while we support API 24), so a wall clock adjustment inside the window can cut the 3s wait short or stretch it. For a sanity timeout on loading a shared library that is an acceptable trade for deleting a hand-written synchronization primitive. `sem_timedwait()` also takes an *absolute* deadline, so unlike the relative timeout it replaces, retrying after `EINTR` cannot extend the total wait, and the deadline needs no nanosecond normalization because the timeout is a whole number of seconds. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

The deadline arithmetic and the `EINTR` retry loop obscured what `load()` is actually doing. Move them into a small `try_acquire_for()` helper so the wait reads as a single line again, as it did when this was a `std::binary_semaphore`. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

simonrozsival · 2026-09-03T10:19:46Z

/review

github-actions · 2026-09-03T10:20:34Z

✅ Android PR Reviewer completed successfully!

Generated by Android PR Reviewer for #12560

github-actions

✅ LGTM

Findings: 0 errors · 0 warnings · 1 suggestion

The POSIX semaphore replacement preserves the one-shot synchronization semantics while removing the libc++ dependency, and the JNI local reference is now released on both success and exception paths. I left one non-blocking inline suggestion to retain explicit clock_gettime() failure handling. All 44 reported CI checks completed successfully.

Generated by Android PR Reviewer for #12560 · gpt56 · 73.9 AIC · ⌖ 20.2 AIC · ⊞ 25.7K
_{Comment /review to run again}

…rapper (#12568) Part of the [drop-libc++](#12533) effort. Split out of #12560, where it had been sitting alongside the DSO loader changes despite being unrelated to them. `jstring_array_wrapper` had two problems. ### A JNI local reference leak `jstring_wrapper::release ()` bailed out early when it had no UTF chars to release: ```cpp if (cstr == nullptr) { return; } ``` But `jstring_array_wrapper::operator[]` fetches the array element's reference eagerly, while `cstr` is only populated on the first `get_cstr ()` call. So any element that was indexed but never read kept its local reference until the frame was popped. The reference and the UTF chars have independent lifetimes, so they are now released independently. ### `new[]` / `delete[]` The wrapper allocated its elements with `new jstring_wrapper[len]`, which is where `_Znam` and `_ZdaPv` in `host.cc.o` came from. It now uses `malloc()` with explicit placement construction and destruction. `jstring_wrapper` is not an implicit-lifetime type — it has a user-provided destructor — so `malloc()` alone cannot begin its lifetime; placement new is required. Its default constructor is private, with `jstring_array_wrapper` as a friend, which is what makes that legal here. ## Results | | before | after | |---|---|---| | `host.cc.o` refs | 11 | **9** | | CoreCLR total refs | 23 | **21** | All three runtime lanes build clean.

CopilotAI lite review requested due to automatic review settings August 28, 2026 06:33

simonrozsival requested a review from jonathanpeppers as a code owner August 28, 2026 06:33

Copilotstarted reviewing on behalf of simonrozsival August 28, 2026 06:34 View session

CopilotAI reviewed Aug 28, 2026
View reviewed changes

Comment threadsrc/native/common/include/runtime-base/system-loadlibrary-wrapper.hh
Comment threadsrc/native/common/include/shared/binary-semaphore.hh Outdated

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from 8707bd1 to 43bdbbbCompare August 28, 2026 07:14

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from ed042fa to 4908d80Compare August 28, 2026 07:14

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from 43bdbbb to cc2c3a8Compare August 28, 2026 07:54

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from 4908d80 to 9eaa808Compare August 28, 2026 07:54

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from cc2c3a8 to 6b74b72Compare August 28, 2026 08:47

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from 9eaa808 to 7fa16a9Compare August 28, 2026 08:47

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from 6b74b72 to f2a7c53Compare August 28, 2026 08:56

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from 7fa16a9 to d28ecc6Compare August 28, 2026 08:56

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from f2a7c53 to 7ee6aa9Compare August 28, 2026 09:52

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from d28ecc6 to b6471e9Compare August 28, 2026 09:52

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from 7ee6aa9 to 8c831f8Compare August 28, 2026 10:29

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch 4 times, most recently from 04f2a89 to d4d8482Compare August 28, 2026 11:50

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from f9a1d0e to d4409ceCompare August 28, 2026 12:06

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from d4d8482 to ba36ae9Compare August 28, 2026 12:06

simonrozsival added the drop-libcpp Work to remove the libc++ dependency from Android NativeAOT label Aug 28, 2026

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from f61c6e5 to e808135Compare August 28, 2026 12:35

simonrozsival mentioned this pull request Aug 28, 2026
[native] Fix a JNI reference leak and drop new[] from jstring_array_wrapper #12568
Merged

simonrozsivaland others added 4 commits September 3, 2026 08:15

simonrozsival force-pushed the dev/simonrozsival/clr-android-system-paths branch from 56d80d9 to a752d5aCompare September 3, 2026 06:15

simonrozsival force-pushed the dev/simonrozsival/clr-drop-std-semaphore branch from e808135 to 54a7fa6Compare September 3, 2026 06:15

simonrozsival changed the base branch from dev/simonrozsival/clr-android-system-paths to mainSeptember 3, 2026 06:15

simonrozsival mentioned this pull request Sep 3, 2026
[native] Remove the last libc++ dependencies from the CoreCLR host #12571
Draft

simonrozsival added the ready-to-review This PR is ready to review/merge, I think any CI failures are just flaky (ignorable). label Sep 3, 2026

github-actionsBot reviewed Sep 3, 2026
View reviewed changes

Comment threadsrc/native/common/include/runtime-base/mainthread-dso-loader.hh

rolfbjarne approved these changes Sep 3, 2026
View reviewed changes

simonrozsival merged commit 411cc92 into mainSep 3, 2026
44 checks passed

simonrozsival deleted the dev/simonrozsival/clr-drop-std-semaphore branch September 3, 2026 16:37

Navigation Menu

Uh oh!

[native] Drop std::binary_semaphore and the last string in the DSO loader - #12560

[native] Drop std::binary_semaphore and the last string in the DSO loader#12560
simonrozsival merged 4 commits into
mainfrom
dev/simonrozsival/clr-drop-std-semaphore

simonrozsival commented Aug 28, 2026•
edited
Loading

Uh oh!

CopilotAI left a comment

Uh oh!

Uh oh!

Uh oh!

simonrozsival commented Sep 3, 2026

Uh oh!

github-actionsBot commented Sep 3, 2026•
edited
Loading

Uh oh!

github-actionsBot left a comment

Uh oh!

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Uh oh!

Conversation

simonrozsival commented Aug 28, 2026• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

std::binary_semaphore -> sem_t

The gratuitous virtual destructor

The NUL-termination std::string

Results

Uh oh!

CopilotAI left a comment

Choose a reason for hiding this comment

Copilot review overview

Uh oh!

Uh oh!

Uh oh!

simonrozsival commented Sep 3, 2026

Uh oh!

github-actionsBot commented Sep 3, 2026• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

github-actionsBot left a comment

Choose a reason for hiding this comment

✅ LGTM

Uh oh!

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

simonrozsival commented Aug 28, 2026•
edited
Loading

`std::binary_semaphore` -> `sem_t`

The gratuitous `virtual` destructor

The NUL-termination `std::string`

github-actionsBot commented Sep 3, 2026•
edited
Loading