[native] Replace std::function with function pointers

[simonrozsival]

Part of #12533. Stacked on top of #12547.

std::function is a type-erasing wrapper: it has to be able to store, copy and destroy an arbitrary callable, and it pulls <functional> into every translation unit that sees the declaration. Neither of the two remaining uses in the CoreCLR host needs any of that.

FastTiming::dump

The line writer was taken by value:

void dump (size_t entries, bool indent, std::function<void(std::string_view const&)> line_writer) noexcept;

Of its two callers, dump_to_logcat passes a captureless lambda and dump_to_file captures a single FILE*. A plain function pointer plus an opaque context covers both:

using LineWriter = void (*) (FILE *output, std::string_view const& line);

void dump (size_t entries, bool indent, LineWriter line_writer, FILE *output) noexcept;

The context is typed as FILE* rather than void*, since the only caller that needs one writes to a file — that avoids a cast in the writer. dump_to_file passes its FILE* through instead of capturing it, and dump stays out of line — no template, so no code duplication per callback type.

The two writers are plain functions in an anonymous namespace rather than captureless lambdas. A lambda converted to a function pointer goes through a compiler generated static invoker, so using functions directly removes a level of indirection.

AssemblyStore::configure_from_payload

static void configure_from_payload (const void *payload_start, const std::function<std::string()>& get_full_store_path) noexcept;

The callback existed only to produce a store path for diagnostics, but the single caller was:

AssemblyStore::configure_from_payload (payload, [store_path]() -> std::string { return std::string { store_path }; });

That wraps a const char * in a std::string purely so that all three use sites can call .c_str () on it again. One of those three is the success-path log_debugf at the end of the function, so the callback runs on every startup and this allocated a string every time.

It now takes the const char * directly and uses the existing optional_string () helper, which also makes it null-safe. The stale comment describing the callback was updated — it claimed the path was only used for invalid payloads, which was not true.

Results

	before	after
undefined libc++ refs	59	59
libnet-android.release.so	546,904 B	539,912 B

The undefined reference count is unchanged: both uses were fully inlined by the optimizer at -O2, so std::function's machinery was emitted into the objects rather than left as undefined references. What goes away is that generated machinery — 6,992 bytes come off the shared library overall (mostly .text and .bss), plus one std::string allocation per startup.

Verification

• CoreCLR and MonoVM both build clean (only the pre-existing format_managed_type_name warning).
• Reference counts and sizes measured with llvm-nm --undefined-only / llvm-size over the same build tree, before and after.
• <functional> is no longer included by timing-internal.hh or assembly-store.hh. The only remaining std::function references in src/native are in src/native/mono/, which this stack does not touch.

On the unchanged reference count

Worth recording, since it is easy to misread as "no progress": llvm-nm --undefined-only lists each undefined symbol once per object file, so this metric counts (object, symbol) pairs rather than call sites. host.cc.o still has 24 relocations against operator delete and 8 against ~basic_string from its other std::string uses, so dropping one use cannot remove the symbol from its undefined list. Measured per object across this change:

object	undefined symbol set	relocations	.text
host.cc.o	identical	60 → 56	30,550 → 30,026
assembly-store.cc.o	identical	84 → 83	24,424 → 24,106

The count only moves when the last use of a symbol in a given file goes away, so it behaves as a per-file cliff rather than a gradual measure.

The lambdas remaining inside dump are called directly instead of being converted to function pointers, so the optimizer already inlines them; replacing them with named functions measured 2 bytes larger, so they were left as they are.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 3

New issues introduced by this change (3)

Severity	Finding
	src/​native/​common/​include/​runtime-base/​timing-internal.hh — 💡 Documentation — The new comment reads a bit awkwardly (“thread state through”). Consider…
	src/​native/​common/​runtime-base/​timing-internal.cc — 💡 Maintainability — Consider declaring line_writer as LineWriter instead of auto so it’s…
	src/​native/​clr/​include/​host/​assembly-store.hh — 💡 Documentation — Since the new API accepts a raw pointer and the implementation is explicitly…

What changed in this PR

This PR reduces C++ standard library surface area in the CoreCLR host by replacing the remaining std::function callback parameters with lower-overhead alternatives (function pointer + void* context, and const char*), helping the broader effort to remove libc++ from the CoreCLR host and reducing startup allocations / binary size.

Changes:

• Reworked FastTiming::dump to use a function pointer callback (LineWriter) plus an opaque context pointer instead of std::function.
• Updated AssemblyStore::configure_from_payload to take const char *store_path directly, eliminating a per-startup std::string allocation and simplifying diagnostics.
• Removed now-unneeded <functional> includes from the affected headers.

File	Description
src/​native/​common/​runtime-base/​timing-internal.cc	Switches FastTiming::dump callback invocation to (context, line); updates logcat/file dump call sites accordingly.
src/​native/​common/​include/​runtime-base/​timing-internal.hh	Removes <functional>, introduces LineWriter typedef, updates dump signature.
src/​native/​clr/​include/​host/​assembly-store.hh	Removes <functional>, updates configure_from_payload signature and comment.
src/​native/​clr/​host/​host.cc	Updates the single call site to pass store_path directly.
src/​native/​clr/​host/​assembly-store.cc	Replaces callback usage with optional_string (store_path) for null-safe diagnostics and debug logging.

[simonrozsival]

Consolidated into #12545 to reduce the depth of the #12546 stack.

No code changed: the commits from this PR are now part of #12545 unmodified, and the resulting tree is byte-identical. This PR sat directly on top of #12545 and touched the same files, so reviewing them together is easier than reviewing the same file across two intermediate states.