[native] Drop <chrono> from the timing code

[simonrozsival]

Part of the drop-libc++ work for CoreCLR.

FastTiming::get_time() has always read the clock with clock_gettime() directly — the comment above it even says we do that to avoid calling into libc++:

// We cheat a bit here, by avoiding a call to libc++ code that performs the same action.
// We can do it because we know our target platform.

std::chrono::steady_clock was then used only as the type tag of the chrono::time_point we wrapped the result in. So we were paying for <chrono> without using the clock it provides.

This PR stores timestamps as a plain uint64_t nanosecond count and removes <chrono> from the four files that included it. In mainthread-dso-loader.hh the include was entirely unused.

Sharing the interval formatting

Four places repeated the same seconds / milliseconds / nanoseconds-within-the-millisecond split, so they now share one helper:

struct time_interval
{
	unsigned long long seconds;
	unsigned long long milliseconds;
	unsigned long long nanoseconds;

	explicit constexpr time_interval (time_point interval) noexcept
		: seconds { interval / NANOSECONDS_PER_SECOND },
		  milliseconds { interval / NANOSECONDS_PER_MILLISECOND },
		  nanoseconds { interval % NANOSECONDS_PER_MILLISECOND }
	{}
};

The split reproduces chrono::duration_cast exactly, including the fact that the middle field is the total milliseconds rather than a remainder. The timing output is byte-for-byte unchanged, which matters because the format after the first colon is parsed by our performance measuring utilities.

I verified this rather than assuming it: a standalone harness compared the old chrono computation against time_interval over the nine interesting edge cases (0, 999999, 1000000, 1000001, 999999999, 1000000000, …, INT64_MAX) plus 2,000,000 random values — 2,000,009 checked, 0 mismatches.

CLOCK_MONOTONIC_RAW → CLOCK_MONOTONIC

We now read CLOCK_MONOTONIC, the clock steady_clock is specified to use, instead of CLOCK_MONOTONIC_RAW. The two differ only in that CLOCK_MONOTONIC is slewed by NTP, which is irrelevant at the granularity we measure.

Results

This does not remove any undefined libc++ symbols — the count stays at 59. <chrono> is header-only, so it never contributed any. What it does do is shrink the binary slightly and remove one more libc++ header from the build, which is a prerequisite for eventually building without libc++ headers at all:

	before	after	delta
libnet-android.release.so	539,912	539,832	−80 B
timing-internal.cc.o	39,880	39,872	−8 B

(Measured against a real rebuild of the parent commit, not remembered numbers.)

Testing

• CoreCLR and MonoVM both build clean (only the pre-existing format_managed_type_name warning).
• time_interval verified against chrono over 2,000,009 inputs as described above.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​native/​common/​include/​runtime-base/​timing-internal.hh — Suggestion (documentation): The comment above time_interval says “whole milliseconds”, which can…

What changed in this PR

This PR advances the CoreCLR “drop-libc++ headers” effort by removing <chrono> from native timing code paths and representing timing points/intervals as a plain uint64_t nanosecond count, while preserving the existing timing output format relied on by performance tooling.

Changes:

• Replace std::chrono-tagged time points with uint64_t nanosecond timestamps (time_point) and compute durations via integer arithmetic.
• Introduce a shared time_interval helper to centralize the seconds / total-milliseconds / nanoseconds-within-millisecond split used in multiple log formats.
• Switch the timing clock source from CLOCK_MONOTONIC_RAW to CLOCK_MONOTONIC and remove an unused <chrono> include.

File	Description
src/​native/​mono/​monodroid/​monodroid-glue.cc	Use time_interval for JIT timing log formatting instead of std::chrono duration casts.
src/​native/​common/​runtime-base/​timing-internal.cc	Remove <chrono> usage and format accumulated timing results via time_interval.
src/​native/​common/​include/​runtime-base/​timing.hh	Use time_interval for managed timing sequence log formatting; initialize start/end as 0.
src/​native/​common/​include/​runtime-base/​timing-internal.hh	Redefine time_point as uint64_t, add time_interval, remove <chrono>, and update get_time() implementation/clock.
src/​native/​common/​include/​runtime-base/​mainthread-dso-loader.hh	Drop unused <chrono> include.

[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.