btf: introduce caching to the string table to speed up ext info loading

paulcacheux · lmb · commit 22ffff556aa4 · 2025-06-24T14:21:42.000+01:00
when parsing line infos, we end up querying multiple times the same
offest (especially the file name), and we currently create a new string
each time. This commit deduplicates this allocation by caching the
string.

Signed-off-by: Paul Cacheux &lt;paul.cacheux@datadoghq.com&gt;
diff --git a/btf/ext_info.go b/btf/ext_info.go
@@ -559,12 +559,12 @@ func LoadLineInfos(reader io.Reader, bo binary.ByteOrder, recordNum uint32, spec
 }
 
 func newLineInfo(li bpfLineInfo, strings *stringTable) (LineOffset, error) {
-	line, err := strings.Lookup(li.LineOff)
+	line, err := strings.LookupCached(li.LineOff)
 	if err != nil {
 		return LineOffset{}, fmt.Errorf("lookup of line: %w", err)
 	}
 
-	fileName, err := strings.Lookup(li.FileNameOff)
+	fileName, err := strings.LookupCached(li.FileNameOff)
 	if err != nil {
 		return LineOffset{}, fmt.Errorf("lookup of filename: %w", err)
 	}
diff --git a/btf/strings.go b/btf/strings.go
@@ -12,6 +12,8 @@ import (
 type stringTable struct {
 	base  *stringTable
 	bytes []byte
+
+	cache map[uint32]string
 }
 
 // sizedReader is implemented by bytes.Reader, io.SectionReader, strings.Reader, etc.
@@ -88,6 +90,30 @@ func (st *stringTable) lookupSlow(offset uint32) ([]byte, error) {
 	return st.bytes[offset : offset+uint32(i)], nil
 }
 
+// LookupCache returns the string at the given offset, caching the result
+// for future lookups.
+func (cst *stringTable) LookupCached(offset uint32) (string, error) {
+	// Fast path: zero offset is the empty string, looked up frequently.
+	if offset == 0 {
+		return "", nil
+	}
+
+	if str, ok := cst.cache[offset]; ok {
+		return str, nil
+	}
+
+	str, err := cst.Lookup(offset)
+	if err != nil {
+		return "", err
+	}
+
+	if cst.cache == nil {
+		cst.cache = make(map[uint32]string)
+	}
+	cst.cache[offset] = str
+	return str, nil
+}
+
 // stringTableBuilder builds BTF string tables.
 type stringTableBuilder struct {
 	length  uint32

Original file line number	Diff line number	Diff line change
`@@ -559,12 +559,12 @@ func LoadLineInfos(reader io.Reader, bo binary.ByteOrder, recordNum uint32, spec`
`559`	`559`	`}`
`560`	`560`
`561`	`561`	`func newLineInfo(li bpfLineInfo, strings *stringTable) (LineOffset, error) {`
`562`		`- line, err := strings.Lookup(li.LineOff)`
	`562`	`+ line, err := strings.LookupCached(li.LineOff)`
`563`	`563`	`if err != nil {`
`564`	`564`	`return LineOffset{}, fmt.Errorf("lookup of line: %w", err)`
`565`	`565`	`}`
`566`	`566`
`567`		`- fileName, err := strings.Lookup(li.FileNameOff)`
	`567`	`+ fileName, err := strings.LookupCached(li.FileNameOff)`
`568`	`568`	`if err != nil {`
`569`	`569`	`return LineOffset{}, fmt.Errorf("lookup of filename: %w", err)`
`570`	`570`	`}`