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Native-code compilation (ocamlopt)

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 ocamlc with bytecode object files produced by ocamlopt: 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.

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:

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


The following command-line options are recognized by ocamlopt.

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

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.

-cclib -llibname
Pass the -llibname 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 -Ldir causes the C linker to search for C libraries in directory dir.

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.

Cause the compiler to print all defined names (with their inferred types or their definitions) when compiling an implementation (.ml file). 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) and compiled object code files (.cmx). 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.

-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.

Forces 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.

-o exec-file
Specify the name of the output file produced by the linker. The default output name is a.out, in keeping with the Unix tradition. If the -a option is given, specify the name of the library produced. If the -output-obj option is given, specify the name of the output file produced.

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 15, section 15.6. The name of the output object file is camlprog.o by default; it can be set with the -o option.

-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. The name of this file is built from the basename of the source file with the extension .ppi for an interface (.mli) file and .ppo for an implementation (.ml) file.

Keep the assembly code produced during the compilation. The assembly code for the source file is saved in the file x.s.

Compile or link multithreaded programs, in combination with the threads library described in chapter 21. What this option actually does is select a special, thread-safe version of the standard library.

Turn bound checking off on 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.

Print the version number of the compiler.

Common errors

The error messages are almost identical to those of ocamlc. See section 7.4.

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.

The best way to avoid running into those incompatibilities is to never trap the Invalid_argument, Division_by_zero, and Stack_overflow exceptions, thus also treating them as fatal errors with the bytecode compiler as well as with the native-code compiler. Test the divisor or array/string index before performing the operation instead of trapping the exception afterwards.

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