jpountz commented on code in PR #14906:
URL: https://github.com/apache/lucene/pull/14906#discussion_r2189422844
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lucene/core/src/java/org/apache/lucene/search/ScorerUtil.java:
##########
@@ -157,11 +157,10 @@ static void filterCompetitiveHits(
int newSize = 0;
for (int i = 0; i < buffer.size; ++i) {
- if (buffer.scores[i] >= minRequiredScore) {
- buffer.docs[newSize] = buffer.docs[i];
- buffer.scores[newSize] = buffer.scores[i];
- newSize++;
- }
+ int inc = buffer.scores[i] >= minRequiredScore ? 1 : 0;
+ buffer.docs[newSize] = buffer.docs[i];
+ buffer.scores[newSize] = buffer.scores[i];
+ newSize += inc;
Review Comment:
I checked the assembly, it doesn't use cmov, it looks like it does what the
Java code suggests by adding the result of the comparison to `newSize`.
However, your suggestion made me want to look into making C2 generate cmov
instructions, and the below approach worked:
```java
@Benchmark
public int branchlessCandidateCmov() {
int newSize = 0;
for (int i = 0; i < size; ++i) {
int doc = docs[i];
double score = scores[i];
docs[newSize] = doc;
scores[newSize] = score;
if (score >= minScoreInclusive) {
newSize += 1;
}
}
return newSize;
}
```
Plus it's faster:
```
Benchmark (minScoreInclusive) (size)
Mode Cnt Score Error Units
CompetitiveBenchmark.branchlessCandidate 0 128
thrpt 5 16462.769 ± 1385.801 ops/ms
CompetitiveBenchmark.branchlessCandidate 0.2 128
thrpt 5 8681.387 ± 510.207 ops/ms
CompetitiveBenchmark.branchlessCandidate 0.4 128
thrpt 5 8469.440 ± 279.038 ops/ms
CompetitiveBenchmark.branchlessCandidate 0.5 128
thrpt 5 8403.283 ± 371.047 ops/ms
CompetitiveBenchmark.branchlessCandidate 0.8 128
thrpt 5 8497.105 ± 250.696 ops/ms
CompetitiveBenchmark.branchlessCandidateCmov 0 128
thrpt 5 16974.162 ± 386.891 ops/ms
CompetitiveBenchmark.branchlessCandidateCmov 0.2 128
thrpt 5 10308.811 ± 115.632 ops/ms
CompetitiveBenchmark.branchlessCandidateCmov 0.4 128
thrpt 5 10583.388 ± 434.330 ops/ms
CompetitiveBenchmark.branchlessCandidateCmov 0.5 128
thrpt 5 10368.750 ± 539.356 ops/ms
CompetitiveBenchmark.branchlessCandidateCmov 0.8 128
thrpt 5 10306.593 ± 499.033 ops/ms
```
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