This chapter describes the OCaml high-performance
native-code compiler ocamlopt, which compiles OCaml source files to
native code object files and links 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 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
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, after all options have been processed:
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 OCaml).
The implementation is checked against the interface file x.mli
(if it exists) as described in the manual for ocamlc
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 OCaml 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.
The compiler is able to emit some information on its internal stages:
.cmt files for the implementation of the compilation unit
and .cmti for signatures if the option -bin-annot is passed to it (see the
description of -bin-annot below).
Each such file contains a typed abstract syntax tree (AST), that is produced
during the type checking procedure. This tree contains all available information
about the location and the specific type of each term in the source file.
The AST is partial if type checking was unsuccessful.
These .cmt and .cmti files are typically useful for code inspection tools.
.cmir-linear files for the implementation of the compilation unit
if the option -save-ir-after scheduling is passed to it.
Each such file contains a low-level intermediate representation,
produced by the instruction scheduling pass.
An external tool can perform low-level optimisations,
such as code layout, by transforming a .cmir-linear file.
To continue compilation, the compiler can be invoked with (a possibly modified)
.cmir-linear file as an argument, instead of the corresponding source file.
The following command-line options are recognized by ocamlopt.
The options -pack, -a, -shared, -c, -output-obj and
-output-complete-obj are mutually exclusive.
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 .cmxalibrary. 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.
Force error messages to show absolute paths for file names.
Deprecated since OCaml 4.11. Please use -bin-annot instead.
Read additional newline-terminated command line arguments from filename.
Read additional null character terminated command line arguments from
Dump detailed information about the compilation (types, bindings,
tail-calls, etc) in binary format. The information for file src.ml
(resp. src.mli) is put into file src.cmt
(resp. src.cmti). In case of a type error, dump
all the information inferred by the type-checker before the error.
The *.cmt and *.cmti files produced by -bin-annot contain
more information and are much more compact than the files produced by
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.
Use ccomp as the C linker called to build the final executable
and as the C compiler for compiling .c source files.
Pass the -llibname option to the linker
This causes the given C library to be linked with the program.
Pass the given option to the C compiler and linker.
For instance, -ccopt -Ldir causes the C linker to search for C libraries in
Enable or disable colors in compiler messages (especially warnings and errors).
The following modes are supported:
use heuristics to enable colors only if the output supports them
(an ANSI-compatible tty terminal);
enable colors unconditionally;
disable color output.
The environment variable OCAML_COLOR is considered if -color is not
provided. Its values are auto/always/never as above.
If -color is not provided, OCAML_COLOR is not set and the environment
variable NO_COLOR is set, then color output is disabled. Otherwise,
the default setting is ’auto’, and the current heuristic
checks that the TERM environment variable exists and is
not empty or dumb, and that ’isatty(stderr)’ holds.
Control the way error messages and warnings are printed.
The following modes are supported:
only print the error and its location;
like short, but also display the source code snippet
corresponding to the location of the error.
The default setting is contextual.
The environment variable OCAML_ERROR_STYLE is considered if -error-style is
not provided. Its values are short/contextual as above.
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.
Print the version number of ocamlopt and a detailed
summary of its configuration, then exit.
Print the value of a specific configuration variable from the
-config output, then exit. If the variable does not exist, the exit
code is non-zero. This option is only available since OCaml 4.08,
so script authors should have a fallback for older versions.
Compute dependencies, as the ocamldep command would do. The remaining
arguments are interpreted as if they were given to the ocamldep command.
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.
Note: you can still pack a module that was compiled without
-for-pack but in this case exceptions will be printed with the wrong
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
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.
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. See also option -nostdlib.
If the given directory starts with +, it is taken relative to the
standard library directory. For instance, -I +unix adds the
subdirectory unix of the standard library to the search path.
Compile the file filename as an implementation file, even if its
extension is not .ml.
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.
Compile the file filename as an interface file, even if its
extension is not .mli.
Recognize file names ending with string as interface files
(instead of the default .mli).
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.
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 (option -a), setting the -linkall option forces all
subsequent links of programs involving that library to link all the
modules contained in the library. When compiling a module (option
-c), setting the -linkall option ensures that this module will
always be linked if it is put in a library and this library is linked.
Use linear scan register allocation. Compiling with this allocator is faster
than with the usual graph coloring allocator, sometimes quite drastically so for
long functions and modules. On the other hand, the generated code can be a bit
Set the number of rows of context used for optimization during
pattern matching compilation. The default value is 32. Lower values
cause faster compilation, but less optimized code. This advanced
option is meant for use in the event that a pattern-match-heavy
program leads to significant increases in compilation time.
Do not record dependencies for module aliases. See
section 10.8 for more information.
Deactivates the applicative behaviour of functors. With this option,
each functor application generates new types in its result and
applying the same functor twice to the same argument yields two
Deactivates the constant propagation for floating-point operations.
This option should be given if the program changes the float rounding
mode during its execution.
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.
When linking .cmxalibraries, 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.
Allow the compiler to use some optimizations that are valid only for
code that is statically linked to produce a non-relocatable
executable. The generated code cannot be linked to produce a shared
library nor a position-independent executable (PIE). Many operating
systems produce PIEs by default, causing errors when linking code
compiled with -nodynlink. Either do not use -nodynlink or pass
the option -ccopt -no-pie at link-time.
Ignore non-optional labels in types. Labels cannot be used in
applications, and parameter order becomes strict.
Do not automatically add the standard library directory to the list of
directories searched for compiled interface files (.cmi), compiled
object code files (.cmx), and libraries (.cmxa). See also option
Specify the name of the output file produced by 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 or -output-complete-obj
options are given, specify the name of the output file produced.
If the -shared option is given, specify the name of plugin
When the native compiler compiles an implementation, by default it
produces a .cmx file containing information for cross-module
optimization. It also expects .cmx files to be present for the
dependencies of the currently compiled source, and uses them for
optimization. Since OCaml 4.03, the compiler will emit a warning if it
is unable to locate the .cmx file of one of those dependencies.
The -opaque option, available since 4.04, disables cross-module
optimization information for the currently compiled unit. When
compiling .mli interface, using -opaque marks the compiled .cmi
interface so that subsequent compilations of modules that depend on it
will not rely on the corresponding .cmx file, nor warn if it is
absent. When the native compiler compiles a .ml implementation,
using -opaque generates a .cmx that does not contain any
cross-module optimization information.
Using this option may degrade the quality of generated code, but it
reduces compilation time, both on clean and incremental
builds. Indeed, with the native compiler, when the implementation of
a compilation unit changes, all the units that depend on it may need
to be recompiled – because the cross-module information may have
changed. If the compilation unit whose implementation changed was
compiled with -opaque, no such recompilation needs to occur. This
option can thus be used, for example, to get faster edit-compile-test
Opens the given module before processing the interface or
implementation files. If several -open options are given,
they are processed in order, just as if
the statements open!Module1;;...open!ModuleN;;
were added at the top of each file.
Cause the linker to produce a C object file instead of
an executable file.
This is useful to wrap OCaml code as a C library,
callable from any C program. See chapter 20,
section 20.7.5. The name of the output object file
must be set with the -o option.
This option can also be used to produce a compiled shared/dynamic library (.so extension, .dll under Windows).
Same as -output-obj options except the object file produced includes the
runtime and autolink libraries.
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:
The resulting P.cmx object file has sub-modules P.Q, P.Q.A
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.
After parsing, pipe the abstract syntax tree through the preprocessor
command. The module Ast_mapper, described in
implements the external interface of a preprocessor.
Check information path during type-checking, to make sure that all
types are derived in a principal way. When using labelled arguments
and/or polymorphic methods, this flag is required to ensure future
versions of the compiler will be able to infer types correctly, even
if internal algorithms change.
All programs accepted in -principal mode are also accepted in the
default mode with equivalent types, but different binary signatures,
and this may slow down type checking; yet it is a good idea to
use it once before publishing source code.
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.
Add the suffix string to the name of the runtime library used by
the program. Currently, only one such suffix is supported: d, and
only if the OCaml compiler was configured with option
-with-debug-runtime. This suffix gives the debug version of the
runtime, which is useful for debugging pointer problems in low-level
code such as C stubs.
Stop compilation after the given compilation pass. The currently
supported passes are:
parsing, typing, scheduling, emit.
Save intermediate representation after the given compilation pass
to a file.
The currently supported passes and the corresponding file extensions are:
This experimental feature enables external tools to inspect and manipulate
compiler’s intermediate representation of the program
using compiler-libs library (see
chapter 27 and
Keep the assembly code produced during the compilation. The assembly
code for the source file x.ml is saved in the file x.s.
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 OCaml
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 OCaml 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).
Enforce the separation between types string and bytes,
thereby making strings read-only. This is the default.
When a type is visible under several module-paths, use the shortest
one when printing the type’s name in inferred interfaces and error and
warning messages. Identifier names starting with an underscore _ or
containing double underscores __ incur a penalty of +10 when computing
Force the left-hand part of each sequence to have type unit.
Reject invalid formats that were accepted in legacy format
implementations. You should use this flag to detect and fix such
invalid formats, as they will be rejected by future OCaml versions.
When a type is unboxable (i.e. a record with a single argument or a
concrete datatype with a single constructor of one argument) it will
be unboxed unless annotated with [@@ocaml.boxed].
When a type is unboxable it will be boxed unless annotated with
[@@ocaml.unboxed]. This is the default.
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.
Identify the types string and bytes, thereby making strings writable.
This is intended for compatibility with old source code and should not
be used with new software.
Print the version number of the compiler and the location of the
standard library directory, then exit.
Print all external commands before they are executed,
in particular invocations of the assembler, C compiler, and linker.
Useful to debug C library problems.
-version or -vnum
Print the version number of the compiler in short form (e.g. 3.11.0),
Enable, disable, or mark as fatal the warnings specified by the argument
Each warning can be enabled or disabled, and each warning
can be fatal or non-fatal.
If a warning is disabled, it isn’t displayed and doesn’t affect
compilation in any way (even if it is fatal). If a warning is
enabled, it is displayed normally by the compiler whenever the source
code triggers it. If it is enabled and fatal, the compiler will also
stop with an error after displaying it.
The warning-list argument is a sequence of warning specifiers,
with no separators between them. A warning specifier is one of the
Enable warning number num.
Disable warning number num.
Enable and mark as fatal warning number num.
Enable warnings in the given range.
Disable warnings in the given range.
Enable and mark as fatal warnings in
the given range.
Enable the set of warnings corresponding to
letter. The letter may be uppercase or lowercase.
Disable the set of warnings corresponding to
letter. The letter may be uppercase or lowercase.
Enable and mark as fatal the set of warnings
corresponding to letter. The letter may be uppercase or
Enable the set of warnings corresponding
Disable the set of warnings corresponding
Alternatively, warning-list can specify a single warning using its
mnemonic name (see below), as follows:
Enable warning name.
Disable warning name.
Enable and mark as fatal warning name.
Warning numbers, letters and names which are not currently defined are
ignored. The warnings are as follows (the name following each number specifies
the mnemonic for that warning).
Suspicious-looking start-of-comment mark.
Suspicious-looking end-of-comment mark.
Deprecated synonym for the ’deprecated’ alert.
Fragile pattern matching: matching that will remain complete even
if additional constructors are added to one of the variant types
Partially applied function: expression whose result has function
type and is ignored.
Label omitted in function application.
Partial match: missing cases in pattern-matching.
Missing fields in a record pattern.
Expression on the left-hand side of a sequence that doesn’t have type
unit (and that is not a function, see warning number 5).
Redundant case in a pattern matching (unused match case).
Redundant sub-pattern in a pattern-matching.
Instance variable overridden.
Illegal backslash escape in a string constant.
Private method made public implicitly.
Unerasable optional argument.
Undeclared virtual method.
Type without principality.
Unused function argument.
Useless record with clause.
Bad module name: the source file name is not a valid OCaml module name.
Ignored: now part of warning 8.
Suspicious unused variable: unused variable that is bound
with let or as, and doesn’t start with an underscore (_)
Innocuous unused variable: unused variable that is not bound with
let nor as, and doesn’t start with an underscore (_)
Wildcard pattern given as argument to a constant constructor.
Unescaped end-of-line in a string constant (non-portable code).
Two labels or constructors of the same name are defined in two
mutually recursive types.
A module is linked twice in the same executable.
Unused value declaration.
Unused open statement.
Unused type declaration.
Unused for-loop index.
Unused ancestor variable.
Unused extension constructor.
Unused rec flag.
Constructor or label name used out of scope.
Ambiguous constructor or label name.
Disambiguated constructor or label name (compatibility warning).
Nonoptional label applied as optional.
Open statement shadows an already defined identifier.
Open statement shadows an already defined label or constructor.
Error in environment variable.
Illegal attribute payload.
Implicit elimination of optional arguments.
Absent cmi file when looking up module alias.
Unexpected documentation comment.
Function call annotated with an incorrect @tailcall attribute
The default setting is -w +a-4-6-7-9-27-29-32..42-44-45-48-50-60.
It is displayed by ocamlopt -help.
Note that warnings 5 and 10 are not always triggered, depending on
the internals of the type checker.
Mark as fatal the warnings specified in the argument warning-list.
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: a + sign (or an uppercase letter) marks the
corresponding warnings as fatal, a -
sign (or a lowercase letter) turns them back into non-fatal warnings,
and a @ sign both enables and marks as fatal the corresponding
Note: it is not recommended to use warning sets (i.e. letters) as
arguments to -warn-error
in production code, because this can break your build when future versions
of OCaml add some new warnings.
The default setting is -warn-error -a+31 (only warning 31 is fatal).
Show the description of all available warning numbers.
Print the location of the standard library, then exit.
Include the runtime system in the generated program. This is the default.
The compiler does not include the runtime system (nor a reference to it) in the
generated program; it must be supplied separately.
Process file as a file name, even if it starts with a dash (-)
The 32-bit code generator for Intel/AMD x86 processors (i386
architecture) supports the
following additional option:
Use the processor 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
The compiler command line can be modified “from the outside”
with the following mechanisms. These are experimental
and subject to change. They should be used only for experimental and
development work, not in released packages.
A set of arguments that will be inserted before or after the arguments from
the command line. Arguments are specified in a comma-separated list
of name=value pairs. A _ is used to specify the position of
the command line arguments, i.e. a=x,_,b=y means that a=x should be
executed before parsing the arguments, and b=y after. Finally,
an alternative separator can be specified as the
first character of the string, within the set :|; ,.
ocaml_compiler_internal_params(file in the stdlib directory)
A mapping of file names to lists of arguments that
will be added to the command line (and OCAMLPARAM) arguments.
Alternative executable to use on native
Windows for flexlink instead of the
configured value. Primarily used for bootstrapping.
Executables generated by ocamlopt are native, stand-alone executable
files that can be invoked directly. They do
not depend on the ocamlrun bytecode runtime system nor on
dynamically-loaded C/OCaml stub libraries.
During execution of an ocamlopt-generated executable,
the following environment variables are also consulted:
Same usage as in ocamlrun
(see section 13.2), except that option l
is ignored (the operating system’s stack size limit
is used instead).
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.
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.
On ARM and PowerPC processors (32 and 64 bits), fused
multiply-add (FMA) instructions can be generated for a
floating-point multiplication followed by a floating-point addition
or subtraction, as in x *. y +. z. The FMA instruction avoids
rounding the intermediate result x *. y, which is generally
beneficial, but produces floating-point results that differ slightly
from those produced by the bytecode interpreter.
On Intel/AMD x86 processors 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 slightly between bytecode and native code.
The native-code compiler performs a number of optimizations that
the bytecode compiler does not perform, especially when the Flambda
optimizer is active. In particular, the native-code compiler
identifies and eliminates “dead code”, i.e. computations that do
not contribute to the results of the program. For example,
let _ = ignore M.f
contains a reference to compilation unit M when compiled to
bytecode. This reference forces M to be linked and its
initialization code to be executed. The native-code compiler
eliminates the reference to M, hence the compilation unit M may
not be linked and executed. A workaround is to compile M with the
-linkall flag so that it will always be linked and executed, even if
not referenced. See also the Sys.opaque_identity function from the
Sys standard library module.
Before 4.10, stack overflows, typically caused by excessively
deep recursion, are not always turned into a Stack_overflow
exception like with the bytecode compiler. The runtime system makes
a best effort to trap stack overflows and raise the Stack_overflow
exception, but sometimes it fails and a “segmentation fault” or
another system fault occurs instead.