livie
/
phoenix-firestorm
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

DRTVWR-575: Fix bug in macOS micro_sleep(). 

The compiler was deducing an unsigned type for the difference (U64 desired
microseconds - half KERNEL_SLEEP_INTERVAL_US). When the desired sleep was less
than that constant, the difference went hugely positive, resulting in a very
long snooze.

Amusingly, forcing that U64 result into an S32 num_sleep_intervals worked only
*because* of integer truncation: the high-order bits were discarded, resulting
in a negative result as intended.

Ensuring that both integer operands are signed at the outset, though, produces
a more formally correct result.
master
Nat Goodspeed 2023-01-04 12:04:56 -05:00
parent bb718155bd
commit aa112ef17f
1 changed files with 7 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
indra/llcommon/lltimer.cpp
View File

@ -121,9 +121,14 @@ U32 micro_sleep(U64 us, U32 max_yields)
U64 start = get_clock_count();
// This is kernel dependent. Currently, our kernel generates software clock
// interrupts at 250 Hz (every 4,000 microseconds).
const U64 KERNEL_SLEEP_INTERVAL_US = 4000;
const S64 KERNEL_SLEEP_INTERVAL_US = 4000;
auto num_sleep_intervals = (us - (KERNEL_SLEEP_INTERVAL_US >> 1)) / KERNEL_SLEEP_INTERVAL_US;
// Use signed arithmetic to discover whether a sleep is even necessary. If
// either 'us' or KERNEL_SLEEP_INTERVAL_US is unsigned, the compiler
// promotes the difference to unsigned. If 'us' is less than half
// KERNEL_SLEEP_INTERVAL_US, the unsigned difference will be hugely
// positive, resulting in a crazy long wait.
auto num_sleep_intervals = (S64(us) - (KERNEL_SLEEP_INTERVAL_US >> 1)) / KERNEL_SLEEP_INTERVAL_US;
if (num_sleep_intervals > 0)
{
U64 sleep_time = (num_sleep_intervals * KERNEL_SLEEP_INTERVAL_US) - (KERNEL_SLEEP_INTERVAL_US >> 1);