tree 897699bb9320ef32b5c912ca9ad92a2b68676a1b
parent 7339447a7f457f1d8efa6322c971e71afc304d32
author Fangrui Song <maskray@google.com> 1706828742 -0800
committer Copybara-Service <copybara-worker@google.com> 1706828819 -0800

Decrease the precision of absl::Now in x86-64 debug builds

CycleClock::Now utilizes ABSL_INTERNAL_CYCLECLOCK_SHIFT to discourage reliance
on the raw CPU cycle counter values. As a side effect, it discourages
strictly-increasing assumption. Apply the idea to discourage over-reliance on
the precision of absl::Now/absl::GetCurrentTimeNanos.

Programs relying on a very high precision often exhibit portability issues on
machines with a lower cycle counter precision, or worse, race conditions. For
example, Apple M1 emulated x86 RDTSC only guarantees weakly-increasing RDTSC
values.

x86 clock speed is usually measured in GHz and is still precise after we drop 8
least significant bits.

PiperOrigin-RevId: 603493500
Change-Id: Ib1b00075109283f5dbcb39a43fe0ab722351a1e2