pkg/codesearch: reduce memory consumption when building index

With all references in the index, it become quite big.
Merge and dedup the resulting index on the fly.
Also intern all strings b/c there are tons of duplicates.

This also removes unnecessary duplicates (effectively ODR violations in the kernel)
due to use of BUILD_BUG_ON. The macro produces different function calls
in different translations units, so the same function may contain
__compiletime_assert_N1 call in one TU and __compiletime_assert_N2 in another.

Over this reduces resource consumption of index building from:
time:296.11s user:16993.71s sys:6661.03s memory:82707MB
to:
time:194.28s user:16860.01s sys:6647.01s memory: 3243MB

25x reduction in memory consumption.

4 files changed, 73 insertions, 25 deletions

diff --git a/pkg/clangtool/clangtool.go b/pkg/clangtool/clangtool.go
index 8711b5411..dd7c22f0a 100644
--- a/pkg/clangtool/clangtool.go
+++ b/pkg/clangtool/clangtool.go
@@ -32,7 +32,7 @@ type Config struct {
 
 type OutputDataPtr[T any] interface {
 	*T
-	Merge(*T)
+	Merge(*T, *Verifier)
 	SetSourceFile(string, func(filename string) string)
 	Finalize(*Verifier)
 }
@@ -73,21 +73,22 @@ func Run[Output any, OutputPtr OutputDataPtr[Output]](cfg *Config) (OutputPtr, e
 	}
 	close(files)
 
+	v := NewVerifier(cfg.KernelSrc, cfg.KernelObj)
 	out := OutputPtr(new(Output))
 	for range cmds {
 		res := <-results
 		if res.err != nil {
 			return nil, res.err
 		}
-		out.Merge(res.out)
+		out.Merge(res.out, v)
 	}
 	// Finalize the output (sort, dedup, etc), and let the output verify
 	// that all source file names, line numbers, etc are valid/present.
 	// If there are any bogus entries, it's better to detect them early,
 	// than to crash/error much later when the info is used.
 	// Some of the source files (generated) may be in the obj dir.
-	srcDirs := []string{cfg.KernelSrc, cfg.KernelObj}
-	if err := Finalize(out, srcDirs); err != nil {
+	out.Finalize(v)
+	if err := v.Error(); err != nil {
 		return nil, err
 	}
 	if cfg.CacheFile != "" {
@@ -103,24 +104,26 @@ func Run[Output any, OutputPtr OutputDataPtr[Output]](cfg *Config) (OutputPtr, e
 	return out, nil
 }
 
-func Finalize[Output any, OutputPtr OutputDataPtr[Output]](out OutputPtr, srcDirs []string) error {
-	v := &Verifier{
-		srcDirs:   srcDirs,
+type Verifier struct {
+	srcDirs   []string
+	fileCache map[string]int // file->line count (-1 is cached for missing files)
+	err       strings.Builder
+}
+
+func NewVerifier(src ...string) *Verifier {
+	return &Verifier{
+		srcDirs:   src,
 		fileCache: make(map[string]int),
 	}
-	out.Finalize(v)
+}
+
+func (v *Verifier) Error() error {
 	if v.err.Len() == 0 {
 		return nil
 	}
 	return errors.New(v.err.String())
 }
 
-type Verifier struct {
-	srcDirs   []string
-	fileCache map[string]int // file->line count (-1 is cached for missing files)
-	err       strings.Builder
-}
-
 func (v *Verifier) Filename(file string) {
 	if _, ok := v.fileCache[file]; ok {
 		return
diff --git a/pkg/clangtool/tooltest/tooltest.go b/pkg/clangtool/tooltest/tooltest.go
index 11aae2e88..4f0b3bced 100644
--- a/pkg/clangtool/tooltest/tooltest.go
+++ b/pkg/clangtool/tooltest/tooltest.go
@@ -42,14 +42,16 @@ func TestClangTool[Output any, OutputPtr clangtool.OutputDataPtr[Output]](t *tes
 
 func LoadOutput[Output any, OutputPtr clangtool.OutputDataPtr[Output]](t *testing.T) OutputPtr {
 	out := OutputPtr(new(Output))
+	v := clangtool.NewVerifier("testdata")
 	forEachTestFile(t, func(t *testing.T, file string) {
 		tmp, err := osutil.ReadJSON[OutputPtr](file + ".json")
 		if err != nil {
 			t.Fatal(err)
 		}
-		out.Merge(tmp)
+		out.Merge(tmp, v)
 	})
-	if err := clangtool.Finalize(out, []string{"testdata"}); err != nil {
+	out.Finalize(v)
+	if err := v.Error(); err != nil {
 		t.Fatal(err)
 	}
 	return out
diff --git a/pkg/codesearch/database.go b/pkg/codesearch/database.go
index 93f194793..dbea6632c 100644
--- a/pkg/codesearch/database.go
+++ b/pkg/codesearch/database.go
@@ -4,6 +4,9 @@
 package codesearch
 
 import (
+	"fmt"
+	"maps"
+	"slices"
 	"strings"
 
 	"github.com/google/jsonschema-go/jsonschema"
@@ -13,6 +16,10 @@ import (
 
 type Database struct {
 	Definitions []*Definition `json:"definitions,omitempty"`
+
+	mergeCache   map[string]*Definition
+	reverseCache map[*Definition]string
+	stringCache  map[string]string
 }
 
 type Definition struct {
@@ -52,21 +59,45 @@ var DatabaseFormatHash = func() string {
 	return hash.String(schema, semanticVersion)
 }()
 
-func (db *Database) Merge(other *Database) {
-	db.Definitions = append(db.Definitions, other.Definitions...)
-}
-
-func (db *Database) Finalize(v *clangtool.Verifier) {
-	db.Definitions = clangtool.SortAndDedupSlice(db.Definitions)
-
-	for _, def := range db.Definitions {
+func (db *Database) Merge(other *Database, v *clangtool.Verifier) {
+	if db.mergeCache == nil {
+		db.mergeCache = make(map[string]*Definition)
+		db.reverseCache = make(map[*Definition]string)
+		db.stringCache = make(map[string]string)
+	}
+	for _, def := range other.Definitions {
+		id := fmt.Sprintf("%v-%v-%v", def.Kind, def.Name, def.Body.File)
+		if _, ok := db.mergeCache[id]; ok {
+			continue
+		}
+		db.mergeCache[id] = def
+		db.reverseCache[def] = id
 		v.LineRange(def.Body.File, def.Body.StartLine, def.Body.EndLine)
 		if def.Comment.File != "" {
 			v.LineRange(def.Comment.File, def.Comment.StartLine, def.Comment.EndLine)
 		}
+		db.intern(&def.Kind)
+		db.intern(&def.Name)
+		db.intern(&def.Type)
+		db.intern(&def.Body.File)
+		db.intern(&def.Comment.File)
+		for _, ref := range def.Refs {
+			db.intern(&ref.Kind)
+			db.intern(&ref.Name)
+			db.intern(&ref.EntityKind)
+		}
 	}
 }
 
+func (db *Database) Finalize(v *clangtool.Verifier) {
+	db.Definitions = slices.Collect(maps.Values(db.mergeCache))
+	slices.SortFunc(db.Definitions, func(a, b *Definition) int {
+		return strings.Compare(db.reverseCache[a], db.reverseCache[b])
+	})
+	db.mergeCache = nil
+	db.reverseCache = nil
+}
+
 // SetSoureFile attaches the source file to the entities that need it.
 // The clang tool could do it, but it looks easier to do it here.
 func (db *Database) SetSourceFile(file string, updatePath func(string) string) {
@@ -78,3 +109,15 @@ func (db *Database) SetSourceFile(file string, updatePath func(string) string) {
 		}
 	}
 }
+
+func (db *Database) intern(str *string) {
+	if *str == "" {
+		return
+	}
+	v, ok := db.stringCache[*str]
+	if !ok {
+		v = strings.Clone(*str)
+		db.stringCache[v] = v
+	}
+	*str = v
+}
diff --git a/pkg/declextract/entity.go b/pkg/declextract/entity.go
index 3b5e13a6d..82bf00446 100644
--- a/pkg/declextract/entity.go
+++ b/pkg/declextract/entity.go
@@ -228,7 +228,7 @@ type EntityGlobalAddr struct {
 	Name string
 }
 
-func (out *Output) Merge(other *Output) {
+func (out *Output) Merge(other *Output, v *clangtool.Verifier) {
 	out.Functions = append(out.Functions, other.Functions...)
 	out.Consts = append(out.Consts, other.Consts...)
 	out.Enums = append(out.Enums, other.Enums...)