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[common] Fix RDTSC nanosecond conversion (#4422)

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Split the RDTSC ticks-per-nanosecond ratio into integer and Q0.64 fractional components. This avoids overflowing GetFixedPoint64Factor for TSC frequencies above 1 GHz while preserving the multiply-only conversion path.

Use the invariant clock only above 1 GHz because the reverse nanoseconds-per-tick factor cannot represent an exact ratio of one.

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
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-------------------

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4422
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
This commit is contained in:
Feng Chen
2026-09-17 19:50:19 +02:00
committed by crueter
parent 1203082a8f
commit 0ce29be608
2 changed files with 5 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/common/cpu_features.cpp
+4 -3
View File
@@ -244,7 +244,8 @@ WallClock::WallClock(bool invariant_, u64 rdtsc_frequency_) noexcept
, ns_rdtsc_factor{invariant_ ? GetFixedPoint64Factor(NsRatio::den, rdtsc_frequency_) : 0}
, us_rdtsc_factor{invariant_ ? GetFixedPoint64Factor(UsRatio::den, rdtsc_frequency_) : 0}
, ms_rdtsc_factor{invariant_ ? GetFixedPoint64Factor(MsRatio::den, rdtsc_frequency_) : 0}
, rdtsc_ns_factor{invariant_ ? GetFixedPoint64Factor(rdtsc_frequency_, NsRatio::den) : 1}
, rdtsc_ns_integer{invariant_ ? rdtsc_frequency_ / NsRatio::den : 1}
, rdtsc_ns_factor{invariant_ ? GetFixedPoint64Factor(rdtsc_frequency_ % NsRatio::den, NsRatio::den) : 0}
, cntpct_rdtsc_factor{invariant_ ? GetFixedPoint64Factor(CNTFRQ, rdtsc_frequency_) : 0}
, gputick_rdtsc_factor{invariant_ ? GetFixedPoint64Factor(GPUTickFreq, rdtsc_frequency_) : 0}
, invariant{invariant_}
@@ -291,7 +292,7 @@ bool WallClock::IsNative() const {
}
u64 WallClock::NsToTicks(std::chrono::nanoseconds ns) const {
return invariant ? MultiplyHigh(ns.count(), rdtsc_ns_factor) : ns.count();
return ns.count() * rdtsc_ns_integer + MultiplyHigh(ns.count(), rdtsc_ns_factor);
}
#elif defined(HAS_NCE)
namespace {
@@ -416,7 +417,7 @@ u64 WallClock::NsToTicks(std::chrono::nanoseconds ns) const {
const WallClock g_wall_clock = [] {
#if defined(ARCHITECTURE_x86_64)
auto const& caps = Common::g_cpu_caps;
return WallClock(caps.invariant_tsc && caps.tsc_frequency >= std::nano::den, caps.tsc_frequency);
return WallClock(caps.invariant_tsc && caps.tsc_frequency > std::nano::den, caps.tsc_frequency);
#elif defined(HAS_NCE)
return WallClock(false, 1);
#else
src/common/cpu_features.h
+1
View File
@@ -96,6 +96,7 @@ public:
u64 ns_rdtsc_factor;
u64 us_rdtsc_factor;
u64 ms_rdtsc_factor;
u64 rdtsc_ns_integer;
u64 rdtsc_ns_factor;
u64 cntpct_rdtsc_factor;
u64 gputick_rdtsc_factor;