stage2/wasm: write exports on flush, cleanup

Exports now have a dirty flag and are rewritten on flush if this flag
has been set.

A couple other minor changes have been made based on Andrew's review.

src-self-hosted/Module.zig

@@ -974,7 +974,7 @@ pub fn update(self: *Module) !void {
     }
 
     // This is needed before reading the error flags.
-    try self.bin_file.flush();
+    try self.bin_file.flush(self);
 
     self.link_error_flags = self.bin_file.errorFlags();
 

src-self-hosted/codegen/wasm.zig

@@ -52,7 +52,7 @@ pub fn genCode(buf: *ArrayList(u8), decl: *Decl) !void {
     const writer = buf.writer();
 
     // Reserve space to write the size after generating the code
-    try writer.writeAll(&([1]u8{undefined} ** 5));
+    try buf.resize(5);
 
     // Write the size of the locals vec
     // TODO: implement locals

src-self-hosted/link.zig

@@ -168,16 +168,15 @@ pub const File = struct {
         }
     }
 
-    /// Commit pending changes and write headers.
-    pub fn flush(base: *File) !void {
+    pub fn flush(base: *File, module: *Module) !void {
         const tracy = trace(@src());
         defer tracy.end();
 
         try switch (base.tag) {
-            .elf => @fieldParentPtr(Elf, "base", base).flush(),
-            .macho => @fieldParentPtr(MachO, "base", base).flush(),
-            .c => @fieldParentPtr(C, "base", base).flush(),
-            .wasm => @fieldParentPtr(Wasm, "base", base).flush(),
+            .elf => @fieldParentPtr(Elf, "base", base).flush(module),
+            .macho => @fieldParentPtr(MachO, "base", base).flush(module),
+            .c => @fieldParentPtr(C, "base", base).flush(module),
+            .wasm => @fieldParentPtr(Wasm, "base", base).flush(module),
         };
     }
 
@@ -285,7 +284,7 @@ pub const File = struct {
             };
         }
 
-        pub fn flush(self: *File.C) !void {
+        pub fn flush(self: *File.C, module: *Module) !void {
             const writer = self.base.file.?.writer();
             try writer.writeAll(@embedFile("cbe.h"));
             var includes = false;
@@ -1038,7 +1037,8 @@ pub const File = struct {
         pub const abbrev_pad1 = 5;
         pub const abbrev_parameter = 6;
 
-        pub fn flush(self: *Elf) !void {
+        /// Commit pending changes and write headers.
+        pub fn flush(self: *Elf, module: *Module) !void {
             const target_endian = self.base.options.target.cpu.arch.endian();
             const foreign_endian = target_endian != std.Target.current.cpu.arch.endian();
             const ptr_width_bytes: u8 = self.ptrWidthBytes();

src-self-hosted/link/MachO.zig

@@ -73,7 +73,7 @@ fn createFile(allocator: *Allocator, file: fs.File, options: link.Options) !Mach
     }
 }
 
-pub fn flush(self: *MachO) !void {}
+pub fn flush(self: *MachO, module: *Module) !void {}
 
 pub fn deinit(self: *MachO) void {}
 

src-self-hosted/link/Wasm.zig

@@ -59,34 +59,37 @@ pub fn openPath(allocator: *Allocator, dir: fs.Dir, sub_path: []const u8, option
 
     try file.writeAll(&(spec.magic ++ spec.version));
 
-    wasm.base = .{
-        .tag = .wasm,
-        .options = options,
-        .file = file,
-        .allocator = allocator,
-    };
-
     // TODO: this should vary depending on the section and be less arbitrary
     const size = 1024;
     const offset = @sizeOf(@TypeOf(spec.magic ++ spec.version));
 
-    wasm.types = try Types.init(file, offset, size);
-    wasm.funcs = try Funcs.init(file, offset + size, size, offset + 3 * size, size);
-    wasm.exports = try Exports.init(file, offset + 2 * size, size);
-    try file.setEndPos(offset + 4 * size);
+    wasm.* = .{
+        .base = .{
+            .tag = .wasm,
+            .options = options,
+            .file = file,
+            .allocator = allocator,
+        },
 
-    wasm.sections = [_]*Section{
-        &wasm.types.typesec.section,
-        &wasm.funcs.funcsec,
-        &wasm.exports.exportsec,
-        &wasm.funcs.codesec.section,
+        .types = try Types.init(file, offset, size),
+        .funcs = try Funcs.init(file, offset + size, size, offset + 3 * size, size),
+        .exports = try Exports.init(file, offset + 2 * size, size),
+
+        // These must be ordered as they will appear in the output file
+        .sections = [_]*Section{
+            &wasm.types.typesec.section,
+            &wasm.funcs.funcsec,
+            &wasm.exports.exportsec,
+            &wasm.funcs.codesec.section,
+        },
     };
 
+    try file.setEndPos(offset + 4 * size);
+
     return &wasm.base;
 }
 
 pub fn deinit(self: *Wasm) void {
-    if (self.base.file) |f| f.close();
     self.types.deinit();
     self.funcs.deinit();
 }
@@ -112,7 +115,8 @@ pub fn updateDecl(self: *Wasm, module: *Module, decl: *Module.Decl) !void {
 
     decl.fn_link.wasm = .{ .typeidx = typeidx, .funcidx = funcidx };
 
-    try self.exports.writeAll(module);
+    // TODO: we should be more smart and set this only when needed
+    self.exports.dirty = true;
 }
 
 pub fn updateDeclExports(
@@ -121,11 +125,7 @@ pub fn updateDeclExports(
     decl: *const Module.Decl,
     exports: []const *Module.Export,
 ) !void {
-    // TODO: updateDeclExports() may currently be called before updateDecl,
-    // presumably due to a bug. For now just rely on the following call
-    // being made in updateDecl().
-
-    //try self.exports.writeAll(module);
+    self.exports.dirty = true;
 }
 
 pub fn freeDecl(self: *Wasm, decl: *Module.Decl) void {
@@ -138,7 +138,9 @@ pub fn freeDecl(self: *Wasm, decl: *Module.Decl) void {
     }
 }
 
-pub fn flush(self: *Wasm) !void {}
+pub fn flush(self: *Wasm, module: *Module) !void {
+    if (self.exports.dirty) try self.exports.writeAll(module);
+}
 
 /// This struct describes the location of a named section + custom section
 /// padding in the output file. This is all the data we need to allow for
@@ -373,11 +375,14 @@ const Exports = struct {
     /// Size in bytes of the contents of the section. Does not include
     /// the "header" containing the section id and this value.
     contents_size: u32,
+    /// If this is true, then exports will be rewritten on flush()
+    dirty: bool,
 
     fn init(file: fs.File, offset: u64, initial_size: u64) !Exports {
         return Exports{
             .exportsec = (try VecSection.init(spec.exports_id, file, offset, initial_size)).section,
             .contents_size = 5,
+            .dirty = false,
         };
     }
 
@@ -410,6 +415,8 @@ const Exports = struct {
 
         for (module.decl_exports.entries.items) |entry|
             for (entry.value) |e| try writeExport(writer, e);
+
+        self.dirty = false;
     }
 
     /// Return the total number of bytes an export will take.