## MLton 20241230

# CompileTimeOptions

MLton’s compile-time options control the name of the output file, the verbosity of compile-time messages, and whether or not certain optimizations are performed. They also can specify which intermediate files are saved and can stop the compilation process early, at some intermediate pass, in which case compilation can be resumed by passing the generated files to MLton. MLton uses the input file suffix to determine the type of input program. The possibilities are `.c`, `.mlb`, `.o`, `.s`, and `.sml`.

With no arguments, MLton prints the version number and exits. For a usage message, run MLton with an invalid switch, e.g. `mlton -z`. In the explanation below and in the usage message, for flags that take a number of choices (e.g. `{true|false}`), the first value listed is the default.

## Options

- `-align n`

  Aligns object in memory by the specified alignment (`4` or `8`). The default varies depending on architecture.

- `-as-opt option`

  Pass option to `cc` when compiling assembler code. If you wish to pass an option to the assembler, you must use `cc`’s `-Wa,` syntax.

- `-cc cc`

  Specify the executable for the C compiler. The C compiler is also invoked to compile assembly (`.s` files) to object code (`.o` files) and to link object code into an executable.

- `-cc-opt option`

  Pass option to `cc` when compiling C code.

- `-codegen {native|amd64|c|llvm|x86}`

  Generate native object code via amd64 assembly, C code, LLVM code, or x86 code or C code. With `-codegen native` (`-codegen amd64` or `-codegen x86`), MLton typically compiles more quickly and generates better code.

- `-const name value`

  Set the value of a compile-time constant. Here is a list of available constants, their default values, and what they control.

  - `Exn.keepHistory {false|true}`

    Enable `MLton.Exn.history`. See MLtonExn for details. There is a performance cost to setting this to `true`, both in memory usage of exceptions and in run time, because of additional work that must be performed at each exception construction, raise, and handle.

- `-default-ann ann`

  Specify default ML Basis annotations. For example, `-default-ann 'warnUnused true'` causes unused variable warnings to be enabled by default. A default is overridden by the corresponding annotation in an ML Basis file.

- `-default-type type`

  Specify the default binding for a primitive type. For example, `-default-type word64` causes the top-level type `word` and the top-level structure `Word` in the Basis Library to be equal to `Word64.word` and `Word64:WORD`, respectively. Similarly, `-default-type intinf` causes the top-level type `int` and the top-level structure `Int` in the Basis Library to be equal to `IntInf.int` and `IntInf:INTEGER`, respectively.

- `-disable-ann ann`

  Ignore the specified ML Basis annotation in every ML Basis file. For example, to see all match and unused warnings, compile with

      -default-ann 'warnUnused true'
      -disable-ann forceUsed
      -disable-ann nonexhaustiveMatch
      -disable-ann redundantMatch
      -disable-ann warnUnused

- `-export-header file`

  Write C prototypes to file for all of the functions in the program exported from SML to C.

- `-ieee-fp {false|true}`

  Cause the x86 native code generator to be pedantic about following the IEEE floating point standard. By default, it is not, because of the performance cost. This only has an effect with `-codegen x86`.

- `-inline n`

  Set the inlining threshold used in the optimizer. The threshold is an approximate measure of code size of a procedure. The default is `320`.

- `-keep {g|o}`

  Save intermediate files. If no `-keep` argument is given, then only the output file is saved.

  `g` | generated `.c` and `.s` files passed to `cc` and generated `.ll` files passed to `llvm-as`
  `o` | object (`.o`) files

- `-link-opt option`

  Pass option to `cc` when linking. You can use this to specify library search paths, e.g. `-link-opt -Lpath`, and libraries to link with, e.g., `-link-opt -lfoo`, or even both at the same time, e.g. `-link-opt '-Lpath -lfoo'`. If you wish to pass an option to the linker, you must use `cc`’s `-Wl,` syntax, e.g., `-link-opt '-Wl,--export-dynamic'`.

- `-llvm-as llvm-as`

  Specify the executable for the LLVM `.ll` to `.bc` assembler; can be used to select a specific version of the tool (e.g., `-llvm-as llvm-as-14`).

- `-llvm-as-opt option`

  Pass option to `llvm-as` when assembling (`.ll` to `.bc`) LLVM code.

- `-llvm-llc llc`

  Specify the executable for the LLVM `.bc` to `.o` system compiler; can be used to select a specific version of the tool (e.g., `-llvm-as llc-14`).

- `-llvm-llc-opt option`

  Pass option to `llc` when compiling (`.bc` to `.o`) LLVM code.

- `-llvm-opt opt`

  Specify the executable for the LLVM `.bc` to `.bc` optimizer; can be used to select a specific version of the tool (e.g., `-llvm-opt opt-14`).

- `-llvm-opt-opt option`

  Pass option to `opt` when optimizing (`.bc` to `.bc`) LLVM code.

- `-mlb-path-map file`

  Use file as an ML Basis path map to define additional MLB path variables. Multiple uses of `-mlb-path-map` and `-mlb-path-var` are allowed, with variable definitions in later path maps taking precedence over earlier ones.

- `-mlb-path-var name value`

  Define an additional MLB path variable. Multiple uses of `-mlb-path-map` and `-mlb-path-var` are allowed, with variable definitions in later path maps taking precedence over earlier ones.

- `-output file`

  Specify the name of the final output file. The default name is the input file name with its suffix removed and an appropriate, possibly empty, suffix added.

- `-profile {no|alloc|count|time}`

  Produce an executable that gathers profiling data. When such an executable is run, it produces an `mlmon.out` file.

- `-profile-branch {false|true}`

  If true, the profiler will separately gather profiling data for each branch of a function definition, `case` expression, and `if` expression.

- `-profile-stack {false|true}`

  If `true`, the executable will gather profiling data for all functions on the stack, not just the currently executing function. See ProfilingTheStack.

- `-profile-val {false|true}`

  If `true`, the profiler will separately gather profiling data for each (expansive) `val` declaration.

- `-runtime arg`

  Pass argument to the runtime system via `@MLton`. See RunTimeOptions. The argument will be processed before other `@MLton` command line switches. Multiple uses of `-runtime` are allowed, and will pass all the arguments in order. If the same runtime switch occurs more than once, then the last setting will take effect. There is no need to supply the leading `@MLton` or the trailing `--`; these will be supplied automatically.

  An argument to `-runtime` may contain spaces, which will cause the argument to be treated as a sequence of words by the runtime. For example the command line:

      mlton -runtime 'ram-slop 0.4' foo.sml

  will cause `foo` to run as if it had been called like:

      foo @MLton ram-slop 0.4 --

  An executable created with `-runtime stop` doesn’t process any `@MLton` arguments. This is useful to create an executable, e.g., `echo`, that must treat `@MLton` like any other command-line argument.

      % mlton -runtime stop echo.sml
      % echo @MLton --
      @MLton --

- `-show-basis file`

  Pretty print to file the basis defined by the input program. See ShowBasis.

- `-show-def-use file`

  Output def-use information to file. Each identifier that is defined appears on a line, followed on subsequent lines by the position of each use.

- `-stop {f|g|o|tc}`

  Specify when to stop.

  `f` | list of files on stdout (only makes sense when input is `foo.mlb`)
  `g` | generated `.c` and `.s` files
  `o` | object (`.o`) files
  `tc` | after type checking

  If you compile with `-stop g` or `-stop o`, you can resume compilation by running MLton on the generated `.c` and `.s` or `.o` files.

- `-target {self|…​}`

  Generate an executable that runs on the specified platform. The default is `self`, which means to compile for the machine that MLton is running on. To use any other target, you must first install a cross compiler.

- `-target-as-opt target option`

  Like `-as-opt`, this passes option to `cc` when compliling assembler code, except it only passes option when the target architecture, operating system, or arch-os pair is target.

- `-target-cc-opt target option`

  Like `-cc-opt`, this passes option to `cc` when compiling C code, except it only passes option when the target architecture, operating system, or arch-os pair is target.

- `-target-link-opt target option`

  Like `-link-opt`, this passes option to `cc` when linking, except it only passes option when the target architecture, operating system, or arch-os pair is target.

- `-verbose {0|1|2|3}`

  How verbose to be about what passes are running. The default is `0`.

  `0` | silent
  `1` | calls to compiler, assembler, and linker
  `2` | 1, plus intermediate compiler passes
  `3` | 2, plus some data structure sizes