# Chapter 12 Native-code compilation (ocamlopt)

- 12.1 Overview of the compiler
- 12.2 Options
- 12.3 Common errors
- 12.4 Running executables produced by ocamlopt
- 12.5 Compatibility with the bytecode compiler

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 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.

## 12.1 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 (chapter 9).

- 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.

## 12.2 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 .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.

-absname
  Force error messages to show absolute paths for file names.
-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.
-args filename
  Read additional newline-terminated command line arguments from filename.
-args0 filename
  Read additional null character terminated command line arguments from filename.
-bin-annot
  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 -annot.
-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.
-color mode
  Enable or disable colors in compiler messages (especially warnings and errors). The following modes are supported:
  auto
    use heuristics to enable colors only if the output supports them (an ANSI-compatible tty terminal);
  always
    enable colors unconditionally;
  never
    disable color output.
  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.

  The environment variable OCAML_COLOR is considered if -color is not provided. Its values are auto/always/never as above.

-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.
-depend ocamldep-args
  Compute dependencies, as the ocamldep command would do. The remaining arguments are interpreted as if they were given to the ocamldep command.
-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. Note: you can still pack a module that was compiled without -for-pack but in this case exceptions will be printed with the wrong names.
-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 11.2).
-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. 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.

-impl filename
  Compile the file filename as an implementation file, even if its extension is not .ml.
-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 (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.
-no-alias-deps
  Do not record dependencies for module aliases. See section 8.12 for more information.
-no-app-funct
  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 incompatible structures.
-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 .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.
-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.
-nostdlib
  Do not automatically add the standard library directory the list of directories searched for compiled interface files (.cmi), compiled object code files (.cmx), and libraries (.cmxa). See also option -I.
-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.
-opaque
  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 feedback loops.

-open Module
  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.
-output-obj
  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).
-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 18 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 32 and 64 bits under Linux, BSD and MacOS X). 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.

-plugin plugin
  Dynamically load the code of the given plugin (a .cmo, .cma or .cmxs file) in the compiler. plugin must exist in the same kind of code as the compiler (ocamlopt.byte must load bytecode plugins, while ocamlopt.opt must load native code plugins), and extension adaptation is done automatically for .cma files (to .cmxs files if the compiler is compiled in native code).
-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.
-ppx command
  After parsing, pipe the abstract syntax tree through the preprocessor command. The module Ast_mapper, described in chapter 27: Ast_mapper , implements the external interface of a preprocessor.
-principal
  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.
-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.
-runtime-variant suffix
  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.
-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 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).
-safe-string
  Enforce the separation between types string and bytes, thereby making strings read-only. This will become the default in a future version of OCaml.
-short-paths
  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 their length.
-strict-sequence
  Force the left-hand part of each sequence to have type unit.
-strict-formats
  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.
-thread
  Compile or link multithreaded programs, in combination with the system threads library described in chapter 31.
-unboxed-types
  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].
-no-unboxed-types
  When a type is unboxable it will be boxed unless annotated with [@@ocaml.unboxed]. This is the default.
-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.
-unsafe-string
  Identify the types string and bytes, thereby making strings writable. For reasons of backward compatibility, this is the default setting for the moment, but this will change in a future version of OCaml.
-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. Useful to debug C library problems.
-version or -vnum
  Print the version number of the compiler in short form (e.g. 3.11.0), then exit.
-w warning-list
  Enable, disable, or mark as fatal the warnings specified by the argument warning-list. 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 following:

  +num
    Enable warning number num.
  -num
    Disable warning number num.
  @num
    Enable and mark as fatal warning number num.
  +num1..num2
    Enable warnings in the given range.
  -num1..num2
    Disable warnings in the given range.
  @num1..num2
    Enable and mark as fatal warnings in the given range.
  +letter
    Enable the set of warnings corresponding to letter. The letter may be uppercase or lowercase.
  -letter
    Disable the set of warnings corresponding to letter. The letter may be uppercase or lowercase.
  @letter
    Enable and mark as fatal the set of warnings corresponding to letter. The letter may be uppercase or lowercase.
  uppercase-letter
    Enable the set of warnings corresponding to uppercase-letter.
  lowercase-letter
    Disable the set of warnings corresponding to lowercase-letter.

  Warning numbers and letters which are out of the range of warnings that are currently defined are ignored. The warnings are as follows.

  1
    Suspicious-looking start-of-comment mark.
  2
    Suspicious-looking end-of-comment mark.
  3
    Deprecated feature.
  4
    Fragile pattern matching: matching that will remain complete even if additional constructors are added to one of the variant types matched.
  5
    Partially applied function: expression whose result has function type and is ignored.
  6
    Label omitted in function application.
  7
    Method overridden.
  8
    Partial match: missing cases in pattern-matching.
  9
    Missing fields in a record pattern.
  10
    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).
  11
    Redundant case in a pattern matching (unused match case).
  12
    Redundant sub-pattern in a pattern-matching.
  13
    Instance variable overridden.
  14
    Illegal backslash escape in a string constant.
  15
    Private method made public implicitly.
  16
    Unerasable optional argument.
  17
    Undeclared virtual method.
  18
    Non-principal type.
  19
    Type without principality.
  20
    Unused function argument.
  21
    Non-returning statement.
  22
    Preprocessor warning.
  23
    Useless record with clause.
  24
    Bad module name: the source file name is not a valid OCaml module name.
  25
    Deprecated: now part of warning 8.
  26
    Suspicious unused variable: unused variable that is bound with let or as, and doesn’t start with an underscore (_) character.
  27
    Innocuous unused variable: unused variable that is not bound with let nor as, and doesn’t start with an underscore (_) character.
  28
    Wildcard pattern given as argument to a constant constructor.
  29
    Unescaped end-of-line in a string constant (non-portable code).
  30
    Two labels or constructors of the same name are defined in two mutually recursive types.
  31
    A module is linked twice in the same executable.
  32
    Unused value declaration.
  33
    Unused open statement.
  34
    Unused type declaration.
  35
    Unused for-loop index.
  36
    Unused ancestor variable.
  37
    Unused constructor.
  38
    Unused extension constructor.
  39
    Unused rec flag.
  40
    Constructor or label name used out of scope.
  41
    Ambiguous constructor or label name.
  42
    Disambiguated constructor or label name (compatibility warning).
  43
    Nonoptional label applied as optional.
  44
    Open statement shadows an already defined identifier.
  45
    Open statement shadows an already defined label or constructor.
  46
    Error in environment variable.
  47
    Illegal attribute payload.
  48
    Implicit elimination of optional arguments.
  49
    Absent cmi file when looking up module alias.
  50
    Unexpected documentation comment.
  51
    Warning on non-tail calls if @tailcall present.
  52 (see 9.5.2)
    Fragile constant pattern.
  53
    Attribute cannot appear in this context
  54
    Attribute used more than once on an expression
  55
    Inlining impossible
  56
    Unreachable case in a pattern-matching (based on type information).
  57 (see 9.5.3)
    Ambiguous or-pattern variables under guard
  58
    Missing cmx file
  59
    Assignment to non-mutable value
  60
    Unused module declaration
  61
    Unboxable type in primitive declaration
  62
    Type constraint on GADT type declaration
  A
    all warnings
  C
    warnings 1, 2.
  D
    Alias for warning 3.
  E
    Alias for warning 4.
  F
    Alias for warning 5.
  K
    warnings 32, 33, 34, 35, 36, 37, 38, 39.
  L
    Alias for warning 6.
  M
    Alias for warning 7.
  P
    Alias for warning 8.
  R
    Alias for warning 9.
  S
    Alias for warning 10.
  U
    warnings 11, 12.
  V
    Alias for warning 13.
  X
    warnings 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 30.
  Y
    Alias for warning 26.
  Z
    Alias for warning 27.

  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.

-warn-error warning-list
  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 warnings.

  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).

-warn-help
  Show the description of all available warning numbers.
-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.

##### Contextual control of command-line options

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.

OCAMLPARAM (environment variable)
  Arguments that will be inserted before or after the arguments from the command line.
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.
OCAML_FLEXLINK (environment variable)
  Alternative executable to use on native Windows for flexlink instead of the configured value. Primarily used for bootstrapping.

## 12.3 Common errors

The error messages are almost identical to those of ocamlc. See section 9.4.

## 12.4 Running executables produced by ocamlopt

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:

OCAMLRUNPARAM
  Same usage as in ocamlrun (see section 11.2), 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.

## 12.5 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 not always turned into a Stack_overflow exception like the bytecode compiler does. 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.
- 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 IA32 processors only (Intel and 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.