[common] Fix RDTSC nanosecond conversion (#4422)

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.

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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>

2 files changed, 5 insertions, 3 deletions

diff --git a/src/common/cpu_features.cpp b/src/common/cpu_features.cpp
index 2e02c47826..8ccb6847f7 100644
--- a/src/common/cpu_features.cpp
+++ b/src/common/cpu_features.cpp
@@ -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
diff --git a/src/common/cpu_features.h b/src/common/cpu_features.h
index 7dcb550f8f..61aa2767ab 100644
--- a/src/common/cpu_features.h
+++ b/src/common/cpu_features.h
@@ -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;