Native-code compilation (ocamlopt)

# Chapter 11 Native-code compilation (ocamlopt)

- Overview of the compiler
- Options
- Common errors
- Running executables produced by ocamlopt
- Compatibility with the bytecode compiler

This chapter describes the Objective Caml high-performance native-code compiler `ocamlopt`, which compiles Caml source files to native code object files and link these object files to produce standalone executables.

The native-code compiler is only available on certain platforms. It produces code that runs faster than the bytecode produced by `ocamlc`, at the cost of increased compilation time and executable code size. Compatibility with the bytecode compiler is extremely high: the same source code should run identically when compiled with `ocamlc` and `ocamlopt`.

It is not possible to mix native-code object files produced by `ocamlopt` with bytecode object files produced by `ocamlc`: a program must be compiled entirely with `ocamlopt` or entirely with `ocamlc`. Native-code object files produced by `ocamlopt` cannot be loaded in the toplevel system `ocaml`.

## 11.1 Overview of the compiler

The `ocamlopt` command has a command-line interface very close to that of `ocamlc`. It accepts the same types of arguments, and processes them sequentially:

- Arguments ending in `.mli` are taken to be source files for compilation unit interfaces. Interfaces specify the names exported by compilation units: they declare value names with their types, define public data types, declare abstract data types, and so on. From the file x`.mli`, the `ocamlopt` compiler produces a compiled interface in the file x`.cmi`. The interface produced is identical to that produced by the bytecode compiler `ocamlc`.
- Arguments ending in `.ml` are taken to be source files for compilation unit implementations. Implementations provide definitions for the names exported by the unit, and also contain expressions to be evaluated for their side-effects. From the file x`.ml`, the `ocamlopt` compiler produces two files: x`.o`, containing native object code, and x`.cmx`, containing extra information for linking and optimization of the clients of the unit. The compiled implementation should always be referred to under the name x`.cmx` (when given a `.o` or `.obj` file, `ocamlopt` assumes that it contains code compiled from C, not from Caml).

  The implementation is checked against the interface file x`.mli` (if it exists) as described in the manual for `ocamlc` (chapter 8).

- Arguments ending in `.cmx` are taken to be compiled object code. These files are linked together, along with the object files obtained by compiling `.ml` arguments (if any), and the Caml standard library, to produce a native-code executable program. The order in which `.cmx` and `.ml` arguments are presented on the command line is relevant: compilation units are initialized in that order at run-time, and it is a link-time error to use a component of a unit before having initialized it. Hence, a given x`.cmx` file must come before all `.cmx` files that refer to the unit x.
- Arguments ending in `.cmxa` are taken to be libraries of object code. Such a library packs in two files (lib`.cmxa` and lib`.a`/`.lib`) a set of object files (`.cmx` and `.o`/`.obj` files). Libraries are build with `ocamlopt -a` (see the description of the `-a` option below). The object files contained in the library are linked as regular `.cmx` files (see above), in the order specified when the library was built. The only difference is that if an object file contained in a library is not referenced anywhere in the program, then it is not linked in.
- Arguments ending in `.c` are passed to the C compiler, which generates a `.o`/`.obj` object file. This object file is linked with the program.
- Arguments ending in `.o`, `.a` or `.so` (`.obj`, `.lib` and `.dll` under Windows) are assumed to be C object files and libraries. They are linked with the program.

The output of the linking phase is a regular Unix or Windows executable file. It does not need `ocamlrun` to run.

## 11.2 Options

The following command-line options are recognized by `ocamlopt`. The options `-pack`, `-a`, `-shared`, `-c` and `-output-obj` are mutually exclusive.

`-a`
  Build a library (`.cmxa` and `.a`/`.lib` files) with the object files (`.cmx` and `.o`/`.obj` files) given on the command line, instead of linking them into an executable file. The name of the library must be set with the `-o` option.

  If `-cclib` or `-ccopt` options are passed on the command line, these options are stored in the resulting `.cmxa` library. Then, linking with this library automatically adds back the `-cclib` and `-ccopt` options as if they had been provided on the command line, unless the `-noautolink` option is given.

`-annot`
  Dump detailed information about the compilation (types, bindings, tail-calls, etc). The information for file src`.ml` is put into file src`.annot`. In case of a type error, dump all the information inferred by the type-checker before the error. The src`.annot` file can be used with the emacs commands given in `emacs/caml-types.el` to display types and other annotations interactively.
`-c`
  Compile only. Suppress the linking phase of the compilation. Source code files are turned into compiled files, but no executable file is produced. This option is useful to compile modules separately.
`-cc` ccomp
  Use ccomp as the C linker called to build the final executable and as the C compiler for compiling `.c` source files.
`-cclib` `-l`libname
  Pass the `-l`libname option to the linker. This causes the given C library to be linked with the program.
`-ccopt` option
  Pass the given option to the C compiler and linker. For instance, `-ccopt -L`dir causes the C linker to search for C libraries in directory dir.
`-compact`
  Optimize the produced code for space rather than for time. This results in slightly smaller but slightly slower programs. The default is to optimize for speed.
`-config`
  Print the version number of `ocamlopt` and a detailed summary of its configuration, then exit.
`-for-pack` module-path
  Generate an object file (`.cmx` and `.o`/`.obj` files) that can later be included as a sub-module (with the given access path) of a compilation unit constructed with `-pack`. For instance, `ocamlopt -for-pack P -c A.ml` will generate `a.cmx` and `a.o` files that can later be used with `ocamlopt -pack -o P.cmx a.cmx`.
`-g`
  Add debugging information while compiling and linking. This option is required in order to produce stack backtraces when the program terminates on an uncaught exception (see section 10.2).
`-i`
  Cause the compiler to print all defined names (with their inferred types or their definitions) when compiling an implementation (`.ml` file). No compiled files (`.cmo` and `.cmi` files) are produced. This can be useful to check the types inferred by the compiler. Also, since the output follows the syntax of interfaces, it can help in writing an explicit interface (`.mli` file) for a file: just redirect the standard output of the compiler to a `.mli` file, and edit that file to remove all declarations of unexported names.
`-I` directory
  Add the given directory to the list of directories searched for compiled interface files (`.cmi`), compiled object code files (`.cmx`), and libraries (`.cmxa`). By default, the current directory is searched first, then the standard library directory. Directories added with `-I` are searched after the current directory, in the order in which they were given on the command line, but before the standard library directory.

  If the given directory starts with `+`, it is taken relative to the standard library directory. For instance, `-I +labltk` adds the subdirectory `labltk` of the standard library to the search path.

`-inline` n
  Set aggressiveness of inlining to n, where n is a positive integer. Specifying `-inline 0` prevents all functions from being inlined, except those whose body is smaller than the call site. Thus, inlining causes no expansion in code size. The default aggressiveness, `-inline 1`, allows slightly larger functions to be inlined, resulting in a slight expansion in code size. Higher values for the `-inline` option cause larger and larger functions to become candidate for inlining, but can result in a serious increase in code size.
`-intf` filename
  Compile the file filename as an interface file, even if its extension is not `.mli`.
`-intf-suffix` string
  Recognize file names ending with string as interface files (instead of the default `.mli`).
`-labels`
  Labels are not ignored in types, labels may be used in applications, and labelled parameters can be given in any order. This is the default.
`-linkall`
  Force all modules contained in libraries to be linked in. If this flag is not given, unreferenced modules are not linked in. When building a library (`-a` flag), setting the `-linkall` flag forces all subsequent links of programs involving that library to link all the modules contained in the library.
`-noassert`
  Do not compile assertion checks. Note that the special form `assert false` is always compiled because it is typed specially. This flag has no effect when linking already-compiled files.
`-noautolink`
  When linking `.cmxa` libraries, ignore `-cclib` and `-ccopt` options potentially contained in the libraries (if these options were given when building the libraries). This can be useful if a library contains incorrect specifications of C libraries or C options; in this case, during linking, set `-noautolink` and pass the correct C libraries and options on the command line.
`-nodynlink`
  Allow the compiler to use some optimizations that are valid only for code that is never dynlinked.
`-nolabels`
  Ignore non-optional labels in types. Labels cannot be used in applications, and parameter order becomes strict.
`-o` exec-file
  Specify the name of the output file produced by the linker. The default output name is `a.out` under Unix and `camlprog.exe` under Windows. If the `-a` option is given, specify the name of the library produced. If the `-pack` option is given, specify the name of the packed object file produced. If the `-output-obj` option is given, specify the name of the output file produced. If the `-shared` option is given, specify the name of plugin file produced.
`-output-obj`
  Cause the linker to produce a C object file instead of an executable file. This is useful to wrap Caml code as a C library, callable from any C program. See chapter 18, section 18.7.5. The name of the output object file is `camlprog.o` by default; it can be set with the `-o` option. This option can also be used to produce a compiled shared/dynamic library (`.so` extension, `.dll` under Windows).
`-p`
  Generate extra code to write profile information when the program is executed. The profile information can then be examined with the analysis program `gprof`. (See chapter 17 for more information on profiling.) The `-p` option must be given both at compile-time and at link-time. Linking object files not compiled with `-p` is possible, but results in less precise profiling.

  > Unix: See the Unix manual page for `gprof(1)` for more information about the profiles.

  > Full support for `gprof` is only available for certain platforms (currently: Intel x86/Linux and Alpha/Digital Unix). On other platforms, the `-p` option will result in a less precise profile (no call graph information, only a time profile).

  > Windows: The `-p` option does not work under Windows.

`-pack`
  Build an object file (`.cmx` and `.o`/`.obj` files) and its associated compiled interface (`.cmi`) that combines the `.cmx` object files given on the command line, making them appear as sub-modules of the output `.cmx` file. The name of the output `.cmx` file must be given with the `-o` option. For instance,

              ocamlopt -pack -o P.cmx A.cmx B.cmx C.cmx

  generates compiled files `P.cmx`, `P.o` and `P.cmi` describing a compilation unit having three sub-modules `A`, `B` and `C`, corresponding to the contents of the object files `A.cmx`, `B.cmx` and `C.cmx`. These contents can be referenced as `P.A`, `P.B` and `P.C` in the remainder of the program.

  The `.cmx` object files being combined must have been compiled with the appropriate `-for-pack` option. In the example above, `A.cmx`, `B.cmx` and `C.cmx` must have been compiled with `ocamlopt -for-pack P`.

  Multiple levels of packing can be achieved by combining `-pack` with `-for-pack`. Consider the following example:

              ocamlopt -for-pack P.Q -c A.ml
              ocamlopt -pack -o Q.cmx -for-pack P A.cmx
              ocamlopt -for-pack P -c B.ml
              ocamlopt -pack -o P.cmx Q.cmx B.cmx

  The resulting `P.cmx` object file has sub-modules `P.Q`, `P.Q.A` and `P.B`.

`-pp` command
  Cause the compiler to call the given command as a preprocessor for each source file. The output of command is redirected to an intermediate file, which is compiled. If there are no compilation errors, the intermediate file is deleted afterwards.
`-principal`
  Check information path during type-checking, to make sure that all types are derived in a principal way. All programs accepted in `-principal` mode are also accepted in default mode with equivalent types, but different binary signatures.
`-rectypes`
  Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. Note that once you have created an interface using this flag, you must use it again for all dependencies.
`-S`
  Keep the assembly code produced during the compilation. The assembly code for the source file x`.ml` is saved in the file x`.s`.
`-shared`
  Build a plugin (usually `.cmxs`) that can be dynamically loaded with the `Dynlink` module. The name of the plugin must be set with the `-o` option. A plugin can include a number of Caml modules and libraries, and extra native objects (`.o`, `.obj`, `.a`, `.lib` files). Building native plugins is only supported for some operating system. Under some systems (currently, only Linux AMD 64), all the Caml code linked in a plugin must have been compiled without the `-nodynlink` flag. Some constraints might also apply to the way the extra native objects have been compiled (under Linux AMD 64, they must contain only position-independent code).
`-thread`
  Compile or link multithreaded programs, in combination with the system `threads` library described in chapter 24.
`-unsafe`
  Turn bound checking off for array and string accesses (the `v.(i)` and `s.[i]` constructs). Programs compiled with `-unsafe` are therefore faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds. Additionally, turn off the check for zero divisor in integer division and modulus operations. With `-unsafe`, an integer division (or modulus) by zero can halt the program or continue with an unspecified result instead of raising a `Division_by_zero` exception.
`-v`
  Print the version number of the compiler and the location of the standard library directory, then exit.
`-verbose`
  Print all external commands before they are executed, in particular invocations of the assembler, C compiler, and linker.
`-version`
  Print the version number of the compiler in short form (e.g. `3.11.0`), then exit.
`-w` warning-list
  Enable or disable warnings according to the argument warning-list. The argument is a set of letters. If a letter is uppercase, it enables the corresponding warnings; lowercase disables the warnings. The correspondence is the following:
  A
    all warnings
  C
    start of comments that look like mistakes
  D
    use of deprecated features
  E
    fragile pattern matchings (matchings that will remain complete even if additional constructors are added to one of the variant types matched)
  F
    partially applied functions (expressions whose result has function type and is ignored)
  L
    omission of labels in applications
  M
    overriding of methods
  P
    missing cases in pattern matchings (i.e. partial matchings)
  S
    expressions in the left-hand side of a sequence that don't have type `unit` (and that are not functions, see `F` above)
  U
    redundant cases in pattern matching (unused cases)
  V
    overriding of instance variables
  Y
    unused variables that are bound with `let` or `as`, and don't start with an underscore (_) character
  Z
    all other cases of unused variables that don't start with an underscore (_) character
  X
    warnings that don't fit in the above categories (except A)
  The default setting is `Aelz`, enabling all warnings except fragile pattern matchings, omitted labels, and innocuous unused variables. Note that warnings `F` and `S` are not always triggered, depending on the internals of the type checker.
`-warn-error` warning-list
  Turn the warnings indicated in the argument warning-list into errors. The compiler will stop with an error when one of these warnings is emitted. The warning-list has the same meaning as for the `-w` option: an uppercase character turns the corresponding warning into an error, a lowercase character leaves it as a warning. The default setting is `-warn-error a` (none of the warnings is treated as an error).
`-where`
  Print the location of the standard library, then exit.
`-` file
  Process file as a file name, even if it starts with a dash (-) character.
`-help` or `–help`
  Display a short usage summary and exit.

##### Options for the IA32 architecture

The IA32 code generator (Intel Pentium, AMD Athlon) supports the following additional option:

`-ffast-math`
  Use the IA32 instructions to compute trigonometric and exponential functions, instead of calling the corresponding library routines. The functions affected are: `atan`, `atan2`, `cos`, `log`, `log10`, `sin`, `sqrt` and `tan`. The resulting code runs faster, but the range of supported arguments and the precision of the result can be reduced. In particular, trigonometric operations `cos`, `sin`, `tan` have their range reduced to [−264, 264].

##### Options for the AMD64 architecture

The AMD64 code generator (64-bit versions of Intel Pentium and AMD Athlon) supports the following additional options:

`-fPIC`
  Generate position-independent machine code. This is the default.
`-fno-PIC`
  Generate position-dependent machine code.

##### Options for the Sparc architecture

The Sparc code generator supports the following additional options:

`-march=v8`
  Generate SPARC version 8 code.
`-march=v9`
  Generate SPARC version 9 code.

The default is to generate code for SPARC version 7, which runs on all SPARC processors.

## 11.3 Common errors

The error messages are almost identical to those of `ocamlc`. See section 8.4.

## 11.4 Running executables produced by ocamlopt

Executables generated by `ocamlopt` are native, statically-linked, stand-alone executable files that can be invoked directly. They do not depend on the `ocamlrun` bytecode runtime system.

During execution of an `ocamlopt`-generated executable, the following environment variables are also consulted:

`OCAMLRUNPARAM`
  Same usage as in `ocamlrun` (see section 10.2), except that option `l` is ignored (the operating system's stack size limit is used instead).
`CAMLRUNPARAM`
  If `OCAMLRUNPARAM` is not found in the environment, then `CAMLRUNPARAM` will be used instead. If `CAMLRUNPARAM` is not found, then the default values will be used.

## 11.5 Compatibility with the bytecode compiler

This section lists the known incompatibilities between the bytecode compiler and the native-code compiler. Except on those points, the two compilers should generate code that behave identically.

- Signals are detected only when the program performs an allocation in the heap. That is, if a signal is delivered while in a piece of code that does not allocate, its handler will not be called until the next heap allocation.
- Stack overflow, typically caused by excessively deep recursion, is handled in one of the following ways, depending on the platform used:
  - By raising a `Stack_overflow` exception, like the bytecode compiler does. (IA32/Linux, AMD64/Linux, PowerPC/MacOSX, MS Windows 32-bit ports).
  - By aborting the program on a “segmentation fault” signal. (All other Unix systems.)
  - By terminating the program silently. (MS Windows 64 bits).
- On IA32 processors only (Intel Pentium, AMD Athlon, etc, in 32-bit mode), some intermediate results in floating-point computations are kept in extended precision rather than being rounded to double precision like the bytecode compiler always does. Floating-point results can therefore differ between bytecode and native code; in general, the results obtained with native code are “more exact” (less affected by rounding errors and loss of precision).
- On the Alpha processor only, floating-point operations involving infinite or denormalized numbers can abort the program on a “floating-point exception” signal.