Add read-update-write benchmarks

Add benchmarks for parsing, updating, and serializing in Rust and C.
Update the fork instructions in AGENTS.md.

AGENTS.md

@@ -14,6 +14,6 @@ If you are writing code, write tests first. The tests must pass for your work to
 If the users asks you to fork off and work on something, this means that you should:
 
 1. Create a temporary directory using `mktemp -d`
-2. Create a new branch to work on
+2. Create a new branch to work on `git branch xyz`
 3. Use git worktree to extract that branch to the temporary directory (i.e., `git worktree add $TMP_DIR -- $BRANCH)
 4. Work from that directory (i.e., `cd $TMP_DIR`)

benches/benches/hackers_bench.rs

@@ -205,11 +205,39 @@ fn bench_iterate(c: &mut Criterion) {
     group.finish();
 }
 
+/// Parse, update a field, and serialize back to a buffer.
+fn bench_read_update_write(c: &mut Criterion) {
+    let cases = [
+        ("tiny", load("tiny")),
+        ("small", load("small")),
+        ("medium", load("medium")),
+    ];
+
+    let mut group = c.benchmark_group("read_update_write");
+    for (label, maybe_data) in &cases {
+        let Some(data) = maybe_data else { continue };
+        group.throughput(Throughput::Bytes(data.len() as u64));
+        group.bench_with_input(BenchmarkId::new("Campaign", label), data, |b, data| {
+            b.iter(|| {
+                let campaign = Campaign::new(data).unwrap();
+                let mut buf = vec![0u8; data.len() * 2];
+                let res = CampaignBuilder::builder(&mut buf)
+                    .name("updated")
+                    .with(&campaign)
+                    .finish();
+                black_box(res)
+            })
+        });
+    }
+    group.finish();
+}
+
 criterion_group!(
     benches,
     bench_shallow_parse,
     bench_deep_parse,
     bench_field_access,
-    bench_iterate
+    bench_iterate,
+    bench_read_update_write
 );
 criterion_main!(benches);

upb_test/hackers_bench.c

@@ -342,6 +342,34 @@ static void fn_count_all_crew(void *state) {
     upb_Arena_Free(arena);
 }
 
+static void fn_read_update_write(void *state) {
+    BenchData *d = state;
+    upb_Arena *arena = upb_Arena_New();
+    Campaign  *c     = Campaign_parse((const char *)d->data, d->len, arena);
+
+    upb_StringView name = { .data = "updated", .size = 7 };
+    Campaign_set_name(c, name);
+
+    size_t len;
+    char *out = Campaign_serialize(c, arena, &len);
+    g_sink = (uintptr_t)out;
+
+    upb_Arena_Free(arena);
+}
+
+static void bench_read_update_write(void) {
+    const char *sizes[] = {"tiny", "small", "medium", NULL};
+    printf("\n=== read_update_write ===\n");
+    for (int i = 0; sizes[i]; i++) {
+        BenchData d;
+        if (!load_bench_data(&d, sizes[i])) continue;
+        char label[80];
+        snprintf(label, sizeof(label), "Campaign_parse+set+serialize/%s  [%zu B]", sizes[i], d.len);
+        run_bench(fn_read_update_write, &d, d.len, label);
+        free_bench_data(&d);
+    }
+}
+
 static void bench_iterate(void) {
     const char *sizes[] = {"tiny", "small", "medium", NULL};
     printf("\n=== iterate ===\n");
@@ -374,6 +402,7 @@ int main(void) {
     bench_deep_parse();
     bench_field_access();
     bench_iterate();
+    bench_read_update_write();
 
     printf("\n");
     return 0;