configurer: add search_prefixes back

It is generally best practice to avoid calling this function, instead
relying on the user to provide these paths via the standard build system
interface. However, when integrating with other build systems, the user
may have already provided the information to the other build system, and
thus it is desirable to use that same information without requiring the
user to provide it again.

BRANCH_TODO

@@ -77,3 +77,26 @@ closes #31397
 
 * `b.build_root` (Directory) -> `b.root` (Path)
 * `ConfigHeader.Options`: `include_guard_override` -> `include_guard`
+
+### Perf Data Point: `zig build -h` (cached)
+
+```
+Benchmark 1 (34 runs): master/zig build -h
+  measurement          mean ± σ            min … max           outliers         delta
+  wall_time           150ms ± 5.52ms     145ms …  165ms          4 (12%)        0%
+  peak_rss           84.8MB ±  275KB    84.2MB … 85.1MB          0 ( 0%)        0%
+  cpu_cycles          593M  ± 4.01M      588M  …  608M           2 ( 6%)        0%
+  instructions        995M  ± 52.5K      995M  …  995M           0 ( 0%)        0%
+  cache_references   25.8M  ±  165K     25.4M  … 26.1M           0 ( 0%)        0%
+  cache_misses        651K  ± 20.1K      619K  …  697K           0 ( 0%)        0%
+  branch_misses       918K  ± 7.44K      906K  …  935K           0 ( 0%)        0%
+Benchmark 2 (348 runs): branch/zig build -h
+  measurement          mean ± σ            min … max           outliers         delta
+  wall_time          14.3ms ±  744us    13.2ms … 23.3ms          8 ( 2%)        ⚡- 90.4% ±  0.4%
+  peak_rss           78.5MB ±  562KB    77.1MB … 81.4MB          7 ( 2%)        ⚡-  7.4% ±  0.2%
+  cpu_cycles         24.1M  ±  821K     22.8M  … 27.1M           3 ( 1%)        ⚡- 95.9% ±  0.1%
+  instructions       43.7M  ± 23.8K     43.7M  … 43.8M          56 (16%)        ⚡- 95.6% ±  0.0%
+  cache_references   1.46M  ± 14.6K     1.40M  … 1.50M          19 ( 5%)        ⚡- 94.3% ±  0.1%
+  cache_misses        142K  ± 4.87K      127K  …  157K           2 ( 1%)        ⚡- 78.1% ±  0.4%
+  branch_misses       126K  ± 1.37K      120K  …  129K          12 ( 3%)        ⚡- 86.3% ±  0.1%
+```

lib/compiler/Maker.zig

@@ -446,6 +446,9 @@ pub fn main(init: process.Init.Minimal) !void {
                 else => {},
             }
         }
+        for (c.search_prefixes) |search_prefix| {
+            try graph.search_prefixes.append(arena, search_prefix.slice(c));
+        }
         break :sc .{
             .configuration = configuration,
             .top_level_steps = top_level_steps,

lib/compiler/Maker/ScannedConfig.zig

@@ -20,6 +20,12 @@ pub fn print(sc: *const ScannedConfig, w: *Writer) Writer.Error!void {
     var serializer: Serializer = .{ .writer = w };
     var s = try serializer.beginStruct(.{});
 
+    {
+        var tf = try s.beginTupleField("search_prefixes", .{});
+        for (c.search_prefixes) |string| try tf.field(string.slice(c), .{});
+        try tf.end();
+    }
+
     try s.field("default_step", @intFromEnum(c.default_step), .{});
     {
         var sf = try s.beginStructField("top_level_steps", .{});

lib/std/Build.zig

@@ -295,12 +295,12 @@ fn createChild(
     pkg_deps: AvailableDeps,
     user_input_options: UserInputOptionsMap,
 ) error{OutOfMemory}!*Build {
-    const allocator = parent.allocator;
-    const child = try allocator.create(Build);
+    const arena = parent.graph.arena;
+    const child = try arena.create(Build);
     child.* = .{
         .graph = parent.graph,
         .root = root,
-        .allocator = allocator,
+        .allocator = arena,
         .install_tls = .{
             .step = .init(.{
                 .tag = .top_level,
@@ -334,8 +334,8 @@ fn createChild(
         .pkg_hash = pkg_hash,
         .available_deps = pkg_deps,
     };
-    try child.top_level_steps.put(allocator, child.install_tls.step.name, &child.install_tls);
-    try child.top_level_steps.put(allocator, child.uninstall_tls.step.name, &child.uninstall_tls);
+    try child.top_level_steps.put(arena, child.install_tls.step.name, &child.install_tls);
+    try child.top_level_steps.put(arena, child.uninstall_tls.step.name, &child.uninstall_tls);
     child.default_step = &child.install_tls.step;
     return child;
 }
@@ -1773,10 +1773,16 @@ pub fn run(b: *Build, argv: []const []const u8) []u8 {
 /// it is desirable to use that same information without requiring the user to
 /// provide it again.
 pub fn addSearchPrefix(b: *Build, search_prefix: []const u8) void {
-    _ = b;
-    _ = search_prefix;
-    @panic("TODO");
-    //b.search_prefixes.append(b.allocator, b.dupePath(search_prefix)) catch @panic("OOM");
+    if (b.isRoot()) {
+        const graph = b.graph;
+        const wc = &graph.wip_configuration;
+        const string = wc.addString(search_prefix) catch @panic("OOM");
+        wc.search_prefixes.append(wc.gpa, string) catch @panic("OOM");
+    }
+}
+
+pub fn isRoot(b: *const Build) bool {
+    return b.pkg_hash.len == 0;
 }
 
 pub const Dependency = struct {

lib/std/Build/Configuration.zig

@@ -13,6 +13,7 @@ path_deps_sub: []String,
 unlazy_deps: []String,
 system_integrations: []SystemIntegration,
 available_options: []AvailableOption,
+search_prefixes: []String,
 extra: []u32,
 default_step: Step.Index,
 generated_files_len: u32,
@@ -26,6 +27,7 @@ pub const Header = extern struct {
     unlazy_deps_len: u32,
     system_integrations_len: u32,
     available_options_len: u32,
+    search_prefixes_len: u32,
     extra_len: u32,
 
     default_step: Step.Index,
@@ -47,6 +49,7 @@ pub const Wip = struct {
     available_options: std.ArrayList(AvailableOption) = .empty,
     steps: std.ArrayList(Step) = .empty,
     path_deps: std.MultiArrayList(Path) = .empty,
+    search_prefixes: std.ArrayList(String) = .empty,
     extra: std.ArrayList(u32) = .empty,
     next_generated_file_index: u32 = 0,
 
@@ -126,6 +129,7 @@ pub const Wip = struct {
         wip.available_options.deinit(gpa);
         wip.steps.deinit(gpa);
         wip.path_deps.deinit(gpa);
+        wip.search_prefixes.deinit(gpa);
         wip.extra.deinit(gpa);
         wip.* = undefined;
     }
@@ -143,6 +147,7 @@ pub const Wip = struct {
             .unlazy_deps_len = @intCast(wip.unlazy_deps.items.len),
             .system_integrations_len = @intCast(wip.system_integrations.items.len),
             .available_options_len = @intCast(wip.available_options.items.len),
+            .search_prefixes_len = @intCast(wip.search_prefixes.items.len),
             .extra_len = @intCast(wip.extra.items.len),
 
             .default_step = static.default_step,
@@ -157,6 +162,7 @@ pub const Wip = struct {
             @ptrCast(wip.unlazy_deps.items),
             @ptrCast(wip.system_integrations.items),
             @ptrCast(wip.available_options.items),
+            @ptrCast(wip.search_prefixes.items),
             @ptrCast(wip.extra.items),
         };
         try w.writeVecAll(&buffers);
@@ -3017,6 +3023,7 @@ pub fn load(arena: Allocator, reader: *Io.Reader) LoadError!Configuration {
         .unlazy_deps = try arena.alloc(String, header.unlazy_deps_len),
         .system_integrations = try arena.alloc(SystemIntegration, header.system_integrations_len),
         .available_options = try arena.alloc(AvailableOption, header.available_options_len),
+        .search_prefixes = try arena.alloc(String, header.search_prefixes_len),
         .extra = try arena.alloc(u32, header.extra_len),
         .default_step = header.default_step,
         .generated_files_len = header.generated_files_len,
@@ -3029,6 +3036,7 @@ pub fn load(arena: Allocator, reader: *Io.Reader) LoadError!Configuration {
         @ptrCast(result.unlazy_deps),
         @ptrCast(result.system_integrations),
         @ptrCast(result.available_options),
+        @ptrCast(result.search_prefixes),
         @ptrCast(result.extra),
     };
     try reader.readVecAll(&vecs);