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8380651: [ubsan] gc/logging/TestGCId.java triggers runtime error: division by zero in shenandoahAdaptiveHeuristics #30692

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8380651: [ubsan] gc/logging/TestGCId.java triggers runtime error: division by zero in shenandoahAdaptiveHeuristics #30692

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d989235
do not divide by zero
kdnilsen Apr 6, 2026
639fef8
refactor the fix for better generality
kdnilsen Apr 10, 2026
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27 changes: 19 additions & 8 deletions src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -247,15 +247,26 @@ void ShenandoahAdaptiveHeuristics::add_gc_time(double timestamp, double gc_time)
_gc_time_b = gc_time;
_gc_time_sd = 0.0;
} else if (_gc_time_num_samples == 2) {
// Two points define a line
double delta_y = gc_time - _gc_time_samples[_gc_time_first_sample_index];
double delta_x = timestamp - _gc_time_timestamps[_gc_time_first_sample_index];
_gc_time_m = delta_y / delta_x;

// y = mx + b
// so b = y0 - mx0
_gc_time_b = gc_time - _gc_time_m * timestamp;
_gc_time_sd = 0.0;
if (delta_x != 0.0) {
// Two points define a line
double delta_y = gc_time - _gc_time_samples[_gc_time_first_sample_index];
_gc_time_m = delta_y / delta_x;
// y = mx + b
// so b = y0 - mx0
_gc_time_b = gc_time - _gc_time_m * timestamp;
_gc_time_sd = 0.0;
} else {
// Avoid divide by zero. Predict with a horizontal line at the average value.
_gc_time_m = 0;
uint first_index = _gc_time_first_sample_index;
uint second_index = index;
assert(second_index == (first_index + 1) % GC_TIME_SAMPLE_SIZE, "don't break this invariant");
_gc_time_b = (_gc_time_samples[first_index] + _gc_time_samples[second_index]) / 2;
double deviation = _gc_time_samples[first_index] - _gc_time_b;
double sum_of_square_deviations = 2 * deviation * deviation;
_gc_time_sd = sqrt(sum_of_square_deviations / 2);
}
} else {
_gc_time_m = ((_gc_time_num_samples * _gc_time_sum_of_xy - _gc_time_sum_of_timestamps * _gc_time_sum_of_samples) /
(_gc_time_num_samples * _gc_time_sum_of_xx - _gc_time_sum_of_timestamps * _gc_time_sum_of_timestamps));
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