mirrors/prometheus
1
0
Fork 0
mirror of https://github.com/prometheus/prometheus.git synced 2026-04-01 03:41:05 +02:00
Code Issues Projects Releases Wiki Activity

perf(PromQL): allow inlining kahansum.Inc (#18319)

Browse Source 
Signed-off-by: Domantas Jadenkus <djadenkus@gmail.com>
Signed-off-by: Domantas <djadenkus@gmail.com>
Co-authored-by: Linas Medžiūnas <linasm@users.noreply.github.com>
This commit is contained in:
Domantas 2026-03-19 16:31:20 +02:00 committed by GitHub
parent a9d90952ba
commit 2129702dff
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
1 changed files with 17 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
util/kahansum/kahansum.go
View File

@ -16,10 +16,21 @@ package kahansum
import "math"
// Inc performs addition of two floating-point numbers using the Kahan summation algorithm.
// We get incorrect results if this function is inlined; see https://github.com/prometheus/prometheus/issues/16714.
//
//go:noinline
func Inc(inc, sum, c float64) (newSum, newC float64) {
// We've seen Kahan summation return less accurate results when Inc function is
// allowed to be inlined (see https://github.com/prometheus/prometheus/pull/16895).
// Go permits fusing float operations (e.g. using fused multiply-add, which allows
// calculating a*b+c without rounding the result of a*b to precision available in float64),
// and Kahan sum is sensitive to float rounding behavior. Instead of forbidding inlining
// (which only disallows fusing operations outside of Inc with operations happening inside)
// and eating the performance cost of non-inlined function calls, we forbid just the fusing
// across Inc call boundary. We can do that by explicitly requesting Inc arguments and results
// to be rounded to float64 precision, as documented in go spec (https://go.dev/ref/spec#Floating_point_operators).
// The following casts are not no-ops!
inc = float64(inc)
sum = float64(sum)
c = float64(c)
t := sum + inc
switch {
case math.IsInf(t, 0):
@ -31,6 +42,9 @@ func Inc(inc, sum, c float64) (newSum, newC float64) {
default:
c += (inc - t) + sum
}
t = float64(t)
c = float64(c)
return t, c
}