perf(PromQL): make floatHistogram.KahanAdd inlineable on Go 1.26 (#18568)

* perf(PromQL): make kahansum.Inc inlineable on Go 1.26

Signed-off-by: linasm <linas.medziunas@gmail.com>
Signed-off-by: Linas Medžiūnas <linasm@users.noreply.github.com>
Co-authored-by: George Krajcsovits <krajorama@users.noreply.github.com>

util/kahansum/kahansum.go

@@ -15,6 +15,12 @@ package kahansum
 
 import "math"
 
+// isInf reports whether f is positive or negative infinity.
+// It avoids math.IsInf to prevent inflating Inc's inlining cost.
+func isInf(f float64) bool {
+	return f > math.MaxFloat64 || f < -math.MaxFloat64
+}
+
 // Inc performs addition of two floating-point numbers using the Kahan summation algorithm.
 func Inc(inc, sum, c float64) (newSum, newC float64) {
 	// We've seen Kahan summation return less accurate results when Inc function is
@@ -33,7 +39,7 @@ func Inc(inc, sum, c float64) (newSum, newC float64) {
 
 	t := sum + inc
 	switch {
-	case math.IsInf(t, 0):
+	case isInf(t):
 		c = 0
 
 		// Using Neumaier improvement, swap if next term larger than sum.

util/kahansum/kahansum_test.go

@@ -0,0 +1,36 @@
+// Copyright The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package kahansum
+
+import (
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsInf(t *testing.T) {
+	for _, f := range []float64{
+		math.Inf(1),
+		math.Inf(-1),
+		math.MaxFloat64,
+		-math.MaxFloat64,
+		1.0,
+		-1.0,
+		0,
+		math.NaN(),
+	} {
+		require.Equal(t, math.IsInf(f, 0), isInf(f), "%e", f)
+	}
+}