OCaml -bin-annot flag configuration for minimizing binary size in low‑traffic services

Direct answer

Disabling -bin-annot does not change the runtime performance of the produced native binary and does not affect the size of the executable itself; it only stops the compiler from writing .annot files alongside the build artifacts.

Confirmed facts

• The -bin-annot flag only controls generation of source‑level annotation files (.annot) that are used by tools such as ocamlcp or custom static analysers; these files are not linked into the executable.
• Because the flag does not emit any data into the native code, CPU usage, memory allocation, and overall speed remain identical whether the flag is present or not.
• Debug information that powers gdb and ocamlrun -g comes from DWARF sections, which are independent of .annot. Therefore breakpoints, stack traces, and source‑level debugging work unchanged.
• The OCaml profiler ocamlprof can still collect samples and produce a profile, but without the .annot files it cannot map each program counter to a precise source line; the output will show less detailed line‑number information (often just the module name).
• No mainstream OCaml library relies on .annot for its runtime behaviour or for any form of reflection; type information needed at runtime is already embedded in the compiled binary or in the compiler‑generated interface files (.cmi, .cmo, .cmx).

Likely explanation

Some developers assume that omitting -bin-annot might strip useful metadata from the binary, potentially hurting performance or debugging. In reality the flag only affects auxiliary files; the compiler’s code‑generation pipeline is unchanged, so the native image stays the same.

Steps to verify in your project

1. Build a representative service twice: once with -bin-annot and once without (e.g., ocamlopt -bin-annot ... vs ocamlopt ...).
2. Compare the sizes of the resulting executables with ls -l or du -b; they should be identical.
3. Run a micro‑benchmark (e.g., a tight loop or a realistic request handler) on both binaries and measure CPU time with time or perf stat; the numbers should match within noise.
4. Execute ocamlprof on each binary and inspect the generated profile; note whether line numbers are present.
5. Launch gdb on each binary, set a breakpoint at a known source line, and confirm it resolves correctly.
6. Search your source tree for any direct references to .annot files (e.g., open Bin_annot or custom parsers). If none are found, you can safely drop the flag.

Missing diagnostic detail

If your workflow depends on ocamlprof for fine‑grained, line‑level profiling (for example, to attribute hotspots to specific expressions), keeping -bin-annot may be worthwhile. Do you rely on that level of detail from the profiler?