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
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
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
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
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
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
Keep the assembly code produced during the compilation. The assembly
code for the source file x.ml 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
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.
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 following operations abort the program (either by printing
an error message or just via an hardware trap or fatal Unix signal)
instead of raising an exception:
In particular, notice that stack overflow caused by excessively deep
recursion is reported by most Unix kernels as a ``segmentation
- out-of bounds accesses to arrays and strings;
- integer division by zero, modulus by zero;
- stack overflow;
- on the Alpha processor only, floating-point operations involving
infinite or denormalized numbers (all other processors supported by
ocamlopt treat these numbers correctly, as per the IEEE 754 standard).
- The following library functions print a fatal error message and
abort the program instead of raising the Invalid_argument exception:
- structural comparisons (=), (<>), etc., when encountering a
- Array.create and String.create when the requested size is
negative or exceeds the memory manager limits;
- output_value when encountering a functional value or pointers
outside the Caml heap (such as input-output buffers).
- 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.
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.