[native] Remove std::format from the CoreCLR host by simonrozsival · Pull Request #12534 · dotnet/android

simonrozsival · 2026-08-27T07:08:51Z

Part of #12533 (the CoreCLR follow-up to #12139). Stacked on top of #12524.

Why

std::format is by far the biggest single contributor of libc++ symbols in the native host. Every translation unit that formats any value pulls in std::to_chars for float, doubleandlong double, plus std::locale, std::numpunct and std::use_facet — about 15 symbols per object file, whether or not the code ever formats a floating point number.

What

Converts every std::format-based logging call site reachable from the CoreCLR lane to the printf-style log_debugf / log_infof / log_warnf / log_errorf functions that already exist in common/include/shared/log_functions.hh.

This is not just a mechanical swap — unlike the std::format macros, these are annotated with __attribute__((format(printf, ...))), so the compiler now type-checks every format specifier against its argument. -Wformat / -Werror=format-security are already enabled, and the build is clean.

Helpers::abort_application (CAT, std::format (…)) call sites move to the previously unused Helpers::abort_applicationf overload. That overload was only defined in the CoreCLR lane, so an identical definition is added to mono/shared/helpers.cc.

Several of the converted files live in common/ rather than clr/. Those are header-inlined into CoreCLR objects (dso-loader.hh, mainthread-dso-loader.hh, monodroid-dl.hh, …), so leaving them alone would have left the std::format payload in host.cc.o regardless. log_*f is defined in both the CoreCLR and MonoVM lanes, so converting them is safe for Mono too — verified by building it.

Bonus: fixes a latent NativeAOT logging bug

clr/host/bridge-processing.cc is compiled into both the CoreCLR and the NativeAOT hosts. NativeAOT's log_types.hh is a 5-line stub, so those calls fell through to the printf-style macros in java-interop-logger.h — which have no format(printf) attribute, so nothing warned. In NativeAOT builds they printed a literal {} instead of the value. Both sites are now correct in both lanes.

Results

Measured on the arm64 Release CoreCLR archive (libnet-android.release-static-release.a), counting undefined std::__ndk1::* / operator new / operator delete / __cxa_* symbols. Baseline is this PR's base, #12524:

	before	after
undefined libc++ refs	142	76 (−46%)
objects with the `std::format` fingerprint	4	0

Azure DevOps build #1572420 measured the resulting arm64 Release CoreCLR APK savings:

artifact	before	after	saving
`libmonodroid.so` (uncompressed APK entry)	1,094,848 B	564,736 B	530,112 B (48.4%)
Simple APK (R8 on/off)	7,034,299 B	6,854,075 B	180,224 B (2.56%)
XForms APK	18,468,429 B	18,284,109 B	184,320 B (1.00%)
XForms APK (R8)	16,317,971 B	16,133,651 B	184,320 B (1.13%)

The unexpectedly large libmonodroid.so decrease was independently checked against the CI logs. Build #1572420's apkdiff output read the actual signed APK and reported the 564,736-byte entry; the 1,094,848-byte baseline was generated by earlier CI and passed the immediately preceding base validation in build #1572418. The PR-only native patch is unchanged after rebasing. The size reduction is plausible because libc++_static.a is linked as a normal archive under --gc-sections: removing the last std::format references prevents the linker from pulling in its locale and floating-point formatting closure. That code compressed well, which is why the signed APK decreases by 180–184 KB rather than the full 530 KB uncompressed ELF reduction.

Per-object, before → after:

object	before	after
`host.cc.o`	41	27
`assembly-store.cc.o`	35	20
`typemap.cc.o`	22	5
`bridge-processing.cc.o`	22	2

The std::format fingerprint (co-occurrence of to_chars<float/double/long double> with locale / numpunct / use_facet) is now absent from every object file in the archive.

Deleting the machinery

With the last call site gone, the std::format macros and templates in clr/include/shared/log_types.hh have no users left, so this PR deletes them too. That file is now identical to the existing NativeAOT stub. This generates no code change on its own — the templates were never instantiated, which is why the counts above already show the std::format fingerprint gone — but it means std::format can no longer be reintroduced into the CoreCLR host by accident.

The std::string_view overload of log_write goes with it. It was duplicated in the CoreCLR and MonoVM copies of log_types.hh and existed only so call sites passing a string literal wouldn't have to write .data (). It had four users, all in timing-internal.cc: three pass a literal and now bind to the plain const char* overload, and the fourth passes a view produced by FastTiming::dump () and now uses log_writef () with %.*s.

That last one also retires a fragile invariant. The overload called .data (), so it required a NUL-terminated string — but dump () builds its views from a buffer plus an explicit length. They are all NUL-terminated today and nothing enforced it. %.*s honours the length instead.

std::format is now completely absent from the clr/, common/ and nativeaot/ trees:

lane	`std::format`	`#include <format>`
`clr/`	0	0
`common/` (shared)	0	0
`nativeaot/`	0	0
`mono/`	45	2

Verification

Built all three runtime lanes locally for arm64-v8a Release — CoreCLR, MonoVM and NativeAOT — with zero errors and zero new warnings. Building MonoVM caught a real link error (the missing abort_applicationf definition) that a CoreCLR-only build would have missed.

Not in this PR

Converting the remaining mono/-only call sites, and deleting the std::format machinery from mono/shared/log_types.hh.
Removing the remaining std::string / std::function / std::mutex usage from host.cc and assembly-store.cc (the 76 remaining refs).
Dropping the CplusPlusArchive entries from NativeRuntimeComponents.cs — the last step, once the archives are genuinely unreferenced.

Copilot

Pull request overview

This PR continues the CoreCLR-host libc++ reduction work by removing std::format usage from CoreCLR-reachable native host code paths and switching logging/abort call sites to the existing printf-annotated log_*f / Helpers::abort_applicationf APIs. This reduces libc++ symbol pull-in while also making format/argument mismatches compiler-checked.

Changes:

Replaced std::format-style logging with log_debugf / log_infof / log_warnf / log_errorf across CoreCLR host/runtime-base code and shared native headers.
Added Helpers::abort_applicationf definition to the Mono shared lane and migrated CoreCLR abort call sites to the abort_applicationf overload.
Adjusted a few debug-only type/name helpers and log strings to avoid std::string_view/std::format dependencies in CoreCLR code paths.

Show a summary per file

File	Description
src/native/mono/shared/helpers.cc	Adds `Helpers::abort_applicationf` implementation for the Mono lane.
src/native/common/include/runtime-base/timing-internal.hh	Switches warning logs to `log_warnf` (printf-style).
src/native/common/include/runtime-base/system-loadlibrary-wrapper.hh	Converts debug logs to `log_debugf`.
src/native/common/include/runtime-base/mainthread-dso-loader.hh	Removes `<format>` usage and converts logs/abort to printf-style formatting.
src/native/common/include/runtime-base/dso-loader.hh	Converts loader tracing/error logs to printf-style formatting.
src/native/clr/runtime-base/android-system.cc	Converts environment/system-property diagnostics to printf-style logs.
src/native/clr/pinvoke-override/precompiled.cc	Removes `<format>` usage and converts abort/log statements to `abort_applicationf`/`log_debugf`.
src/native/clr/include/runtime-base/monodroid-dl.hh	Converts DSO cache and symbol-lookup logs to printf-style formatting.
src/native/clr/include/runtime-base/android-system.hh	Converts debug log to `log_debugf`.
src/native/clr/include/host/typemap.hh	Replaces debug label `std::string_view` constants with `const char*` and updates debug signatures.
src/native/clr/include/host/pinvoke-override-impl.hh	Converts p/invoke override diagnostics to printf-style logging.
src/native/clr/host/typemap.cc	Converts typemap debug/release tracing to printf-style logging and updates helper signatures.
src/native/clr/host/host.cc	Converts CoreCLR host logging/abort paths to printf-style formatting and adds `<cinttypes>` usage.
src/native/clr/host/fastdev-assemblies.cc	Converts FastDev logs and aborts to printf-style formatting.
src/native/clr/host/bridge-processing.cc	Converts GC-related logs to `log_*f` formatting (including NativeAOT-shared TU).
src/native/clr/host/assembly-store.cc	Removes `std::format` usage (including hex formatting) and converts cache/store logging and aborts to printf-style formatting.

Review details

Files reviewed: 16/16 changed files
Comments generated: 2
Review effort level: Lite

simonrozsival · 2026-09-02T08:27:16Z

/review

github-actions · 2026-09-02T08:27:53Z

✅ Android PR Reviewer completed successfully!

Generated by Android PR Reviewer for #12534

github-actions

⚠️Needs Changes — 0 errors, 1 warning.

The printf-style migration removes the CoreCLR std::format dependency, keeps format arguments type-checked, handles bounded string views correctly, and the updated APK descriptors agree with the measured size reduction. One hot type-map path loses the old macros’ lazy argument evaluation and now performs GUID-to-text conversion even when assembly logging is disabled; please restore a category guard or equivalent lazy wrapper.

CI is still in progress with no failures reported so far.

Generated by Android PR Reviewer for #12534 · gpt56 · 280.8 AIC · ⌖ 19.3 AIC · ⊞ 25.7K
_{Comment /review to run again}

Part of the "drop libc++" effort (#12533). `std::format` is by far the biggest single contributor of libc++ symbols in the native host: each translation unit that formats a value pulls in `std::to_chars` for `float`/`double`/`long double`, `std::locale`, `std::numpunct` and `std::use_facet`. This converts every `std::format`-based logging call site in the CoreCLR lane to the printf-style `log_debugf`/`log_infof`/`log_warnf`/`log_errorf` functions that already exist in `common/include/shared/log_functions.hh`. Unlike the `std::format` macros, those are annotated with `__attribute__((format(printf, ...)))`, so the compiler now type-checks every format specifier against its argument. `Helpers::abort_application (CAT, std::format (...))` call sites move to the previously unused `Helpers::abort_applicationf` overload. That overload was only defined in the CoreCLR lane, so an identical definition is added to `mono/shared/helpers.cc`. Also fixes a latent bug: `clr/host/bridge-processing.cc` is compiled into both the CoreCLR and the NativeAOT hosts, but NativeAOT's `log_types.hh` is a stub, so those calls fell through to the printf-style macros in `java-interop-logger.h` and logged a literal `{}` instead of the value. Measured on the arm64 Release CoreCLR archive (`libnet-android.release-static-release.a`), undefined libc++ symbols drop from 144 to 78 (-46%), and the `std::format` fingerprint is gone from every object file. Verified by building the CoreCLR, MonoVM and NativeAOT lanes. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

- Retry the main-thread DSO loader pipe write on EINTR and treat any incomplete write as a failure, rather than only checking for -1. - Drop the duplicated word in the "Trying to load loading shared JNI library" log message. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

With every CoreCLR-lane call site converted to the printf-style `log_*f` functions, the `std::format`-based macros and templates in `clr/include/shared/log_types.hh` have no users left. They generated no code (the templates were never instantiated), but they kept `<format>` and the whole `std::format` API reachable from every CoreCLR translation unit. The file is now identical to the NativeAOT stub. The `std::string_view` overload of `log_write` goes away as well. It was duplicated in the CoreCLR and MonoVM copies of `log_types.hh` and existed only so that call sites using a string literal wouldn't have to write `.data ()`. It had just four users, all in `timing-internal.cc`: three pass a literal and now rely on the plain `const char*` overload, and the fourth passes a `std::string_view` produced by `FastTiming::dump ()` and now uses `log_writef ()` with `%.*s`. That last one also removes a fragile invariant: the overload called `.data ()` and so required a NUL-terminated string, but `dump ()` builds its views from a buffer and an explicit length. They happen to be NUL-terminated today, and nothing enforced it. `%.*s` honours the length. `std::format` is now entirely absent from the `clr/`, `common/` and `nativeaot/` trees; only MonoVM still uses it. Verified by building the CoreCLR, MonoVM and NativeAOT lanes. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

The `log_time` helper in `FastTiming::dump ()` formatted into a fixed 256-byte buffer and silently clamped the length when the message didn't fit, so an oversized line would have been quietly cut short. `snprintf` already reports how much room the message needs, so use it: format into a stack buffer, and on overflow allocate a heap buffer of exactly the required size and format again. This matches the existing `format_message`/`build_message` pair used for the individual timing events, including freeing the buffer only when it differs from the stack one. The stack buffer is now `Constants::MAX_LOGCAT_MESSAGE_LENGTH`, the same size `dump ()` already uses for each event message, so the heap path is not expected to be taken in practice. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Guard the converted printf-style debug calls before constructing MonoGuidString so disabled assembly logging retains the previous lazy argument evaluation. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

simonrozsival · 2026-09-02T16:25:18Z

/azp run

azure-pipelines · 2026-09-02T16:25:42Z

Azure Pipelines: Successfully started running 1 pipeline(s).

simonrozsival · 2026-09-02T19:35:12Z

/azp run

azure-pipelines · 2026-09-02T19:35:35Z

Azure Pipelines: Successfully started running 1 pipeline(s).

…12541) Part of #12533 (drop `libc++` from the CoreCLR host). Stacked on top of #12534. `std::mutex` is a thin wrapper over `pthread_mutex_t`, but using it pulls `<mutex>` and out-of-line libc++ symbols into the native host. This PR stores `pthread_mutex_t` directly and provides a small CoreCLR RAII guard so locked scopes retain automatic unlock behavior without libc++. ### What * Replace CoreCLR's `std::mutex` instances with `pthread_mutex_t`, initialized using `PTHREAD_MUTEX_INITIALIZER`. * Add `xamarin::android::lock_guard` under the CoreCLR include tree. It takes `pthread_mutex_t&` and calls `pthread_mutex_lock` / `pthread_mutex_unlock`. * Update CoreCLR's `StartupAwareLock` to take `pthread_mutex_t&` directly. * Convert shared `Timing` to direct pthread calls and keep it explicitly non-copyable and non-movable. * Leave `src/native/mono` unchanged; MonoVM retains its existing `mutex` and templated `lock_guard` implementation in `mono/shared/cppcompat.hh`. `PTHREAD_MUTEX_INITIALIZER` keeps static instances constant-initialized, so this adds no thread-safe initialization guards. The CoreCLR guard is header-only and compiles down to direct pthread calls. ### Effect This removes the last `#include <mutex>` in the repository: | | files including `<mutex>` | `std::mutex` / `std::lock_guard` uses | |---|---:|---:| | before | 6 | 13 | | after | **0** | **0** | It also drops eight undefined libc++ references: | symbol | before | after | |---|---:|---:| | `std::__ndk1::mutex::lock()` | 3 | 0 | | `std::__ndk1::mutex::unlock()` | 3 | 0 | | `std::__ndk1::mutex::~mutex()` | 2 | 0 | | | refs | `__cxa_guard_*` | |---|---:|---:| | #12534 (base) | 55 | 10 | | this PR | **47** | 10 | The link-time `libc++` requirement only disappears when every cause reaches zero, so this is one of several prerequisites rather than a self-sufficient win. The remaining causes (`operator new`/`delete[]`, `__cxa_guard_*`, `std::string`, `__libcpp_verbose_abort`) are tracked in #12533. ### Verification * CoreCLR, MonoVM and NativeAOT build clean. * `fastdev-assemblies.cc` is `#if defined(DEBUG)` and was additionally compiled with `-DDEBUG`. * The CoreCLR guard compiled with the runtime's no-C++-exceptions settings references only `pthread_mutex_lock` and `pthread_mutex_unlock`; it introduces no C++ exception-runtime or initialization-guard symbols. * Real `libc++` references: **CoreCLR 55 → 47, NativeAOT 0**. The eight removed references are exactly the mutex members listed above. * `__cxa_guard_*` undefined references remain at **10**, confirming that the static mutexes remain constant-initialized.

CopilotAI lite review requested due to automatic review settings August 27, 2026 07:08

simonrozsival requested a review from jonathanpeppers as a code owner August 27, 2026 07:08

simonrozsival mentioned this pull request Aug 27, 2026
[CoreCLR] Remove libc++ from the CoreCLR host #12533
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Comment threadsrc/native/common/include/runtime-base/mainthread-dso-loader.hh
Comment threadsrc/native/common/include/runtime-base/dso-loader.hh Outdated

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simonrozsival changed the base branch from main to dev/simonrozsival/nativeaot-drop-libcpp-packsAugust 27, 2026 07:16

simonrozsival mentioned this pull request Aug 27, 2026
[native] Delete the std::format logging machinery from the CoreCLR host #12536
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This was referenced Aug 27, 2026
[native] Use a raw pointer for the CoreCLR host's Timing instance #12540
Closed
[native] Replace std::mutex with pthread_mutex_t in the CoreCLR host #12541
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simonrozsival added the drop-libcpp Work to remove the libc++ dependency from Android NativeAOT label Aug 27, 2026

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Comment threadsrc/native/clr/host/typemap.cc Outdated

simonrozsival added the ready-to-review This PR is ready to review/merge, I think any CI failures are just flaky (ignorable). label Sep 2, 2026

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simonrozsivaland others added 6 commits September 2, 2026 15:20

[tests] Update CoreCLR APK size references
e3eb218
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Avoid eager GUID formatting in typemap logs
8627170
Guard the converted printf-style debug calls before constructing MonoGuidString so disabled assembly logging retains the previous lazy argument evaluation. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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simonrozsival merged commit 9f9c972 into mainSep 3, 2026
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simonrozsival deleted the dev/simonrozsival/clr-remove-std-format branch September 3, 2026 05:58

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[native] Remove std::format from the CoreCLR host - #12534

[native] Remove std::format from the CoreCLR host#12534
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