frontend: TrackedInst stores FileIndex instead of path digest

The purpose of using path digest was to reference a file in a
serializable manner. Now that there is a stable index associated with
files, it is a superior way to accomplish that goal, since removes one
layer of indirection, and makes TrackedInst 8 bytes instead of 20.

The saved Zig Compiler State file for "hello world" goes from 1.3M to
1.2M with this change.

1 files changed, 21 insertions, 4 deletions

diff --git a/src/InternPool.zig b/src/InternPool.zig
index e79de26516..18cd21d08d 100644
--- a/src/InternPool.zig
+++ b/src/InternPool.zig
@@ -92,12 +92,27 @@ dep_entries: std.ArrayListUnmanaged(DepEntry) = .{},
 /// garbage collection pass.
 free_dep_entries: std.ArrayListUnmanaged(DepEntry.Index) = .{},
 
+/// Elements are ordered identically to the `import_table` field of `Zcu`.
+///
+/// Unlike `import_table`, this data is serialized as part of incremental
+/// compilation state.
+///
+/// Key is the hash of the path to this file, used to store
+/// `InternPool.TrackedInst`.
+///
+/// Value is the `Decl` of the struct that represents this `File`.
+files: std.AutoArrayHashMapUnmanaged(Cache.BinDigest, OptionalDeclIndex) = .{},
+
+pub const FileIndex = enum(u32) {
+    _,
+};
+
 pub const TrackedInst = extern struct {
-    path_digest: Cache.BinDigest,
+    file: FileIndex,
     inst: Zir.Inst.Index,
     comptime {
         // The fields should be tightly packed. See also serialiation logic in `Compilation.saveState`.
-        assert(@sizeOf(@This()) == Cache.bin_digest_len + @sizeOf(Zir.Inst.Index));
+        assert(@sizeOf(@This()) == @sizeOf(FileIndex) + @sizeOf(Zir.Inst.Index));
     }
     pub const Index = enum(u32) {
         _,
@@ -126,11 +141,11 @@ pub const TrackedInst = extern struct {
 pub fn trackZir(
     ip: *InternPool,
     gpa: Allocator,
-    path_digest: Cache.BinDigest,
+    file: FileIndex,
     inst: Zir.Inst.Index,
 ) Allocator.Error!TrackedInst.Index {
     const key: TrackedInst = .{
-        .path_digest = path_digest,
+        .file = file,
         .inst = inst,
     };
     const gop = try ip.tracked_insts.getOrPut(gpa, key);
@@ -4597,6 +4612,8 @@ pub fn deinit(ip: *InternPool, gpa: Allocator) void {
     ip.dep_entries.deinit(gpa);
     ip.free_dep_entries.deinit(gpa);
 
+    ip.files.deinit(gpa);
+
     ip.* = undefined;
 }