### 18.2 Controlling the Compilation Driver, `gcc`

You can control the compilation driver.

Macro: DRIVER_SELF_SPECS

  A list of specs for the driver itself. It should be a suitable initializer for an array of strings, with no surrounding braces.

  The driver applies these specs to its own command line between loading default `specs` files (but not command-line specified ones) and choosing the multilib directory or running any subcommands. It applies them in the order given, so each spec can depend on the options added by earlier ones. It is also possible to remove options using ‘`%<option`’ in the usual way.

  This macro can be useful when a port has several interdependent target options. It provides a way of standardizing the command line so that the other specs are easier to write.

  Do not define this macro if it does not need to do anything.

Macro: OPTION_DEFAULT_SPECS

  A list of specs used to support configure-time default options (i.e. `--with` options) in the driver. It should be a suitable initializer for an array of structures, each containing two strings, without the outermost pair of surrounding braces.

  The first item in the pair is the name of the default. This must match the code in `config.gcc` for the target. The second item is a spec to apply if a default with this name was specified. The string ‘`%(VALUE)`’ in the spec will be replaced by the value of the default everywhere it occurs.

  The driver will apply these specs to its own command line between loading default `specs` files and processing `DRIVER_SELF_SPECS`, using the same mechanism as `DRIVER_SELF_SPECS`.

  Do not define this macro if it does not need to do anything.

Macro: CPP_SPEC

  A C string constant that tells the GCC driver program options to pass to CPP. It can also specify how to translate options you give to GCC into options for GCC to pass to the CPP.

  Do not define this macro if it does not need to do anything.

Macro: CPLUSPLUS_CPP_SPEC

  This macro is just like `CPP_SPEC`, but is used for C++, rather than C. If you do not define this macro, then the value of `CPP_SPEC` (if any) will be used instead.

Macro: CC1_SPEC

  A C string constant that tells the GCC driver program options to pass to `cc1`, `cc1plus`, `f771`, and the other language front ends. It can also specify how to translate options you give to GCC into options for GCC to pass to front ends.

  Do not define this macro if it does not need to do anything.

Macro: CC1PLUS_SPEC

  A C string constant that tells the GCC driver program options to pass to `cc1plus`. It can also specify how to translate options you give to GCC into options for GCC to pass to the `cc1plus`.

  Do not define this macro if it does not need to do anything. Note that everything defined in CC1_SPEC is already passed to `cc1plus` so there is no need to duplicate the contents of CC1_SPEC in CC1PLUS_SPEC.

Macro: ASM_SPEC

  A C string constant that tells the GCC driver program options to pass to the assembler. It can also specify how to translate options you give to GCC into options for GCC to pass to the assembler. See the file `sun3.h` for an example of this.

  Do not define this macro if it does not need to do anything.

Macro: ASM_FINAL_SPEC

  A C string constant that tells the GCC driver program how to run any programs which cleanup after the normal assembler. Normally, this is not needed. See the file `mips.h` for an example of this.

  Do not define this macro if it does not need to do anything.

Macro: AS_NEEDS_DASH_FOR_PIPED_INPUT

  Define this macro, with no value, if the driver should give the assembler an argument consisting of a single dash, `-`, to instruct it to read from its standard input (which will be a pipe connected to the output of the compiler proper). This argument is given after any `-o` option specifying the name of the output file.

  If you do not define this macro, the assembler is assumed to read its standard input if given no non-option arguments. If your assembler cannot read standard input at all, use a ‘`%{pipe:%e}`’ construct; see `mips.h` for instance.

Macro: LINK_SPEC

  A C string constant that tells the GCC driver program options to pass to the linker. It can also specify how to translate options you give to GCC into options for GCC to pass to the linker.

  Do not define this macro if it does not need to do anything.

Macro: LIB_SPEC

  Another C string constant used much like `LINK_SPEC`. The difference between the two is that `LIB_SPEC` is used at the end of the command given to the linker.

  If this macro is not defined, a default is provided that loads the standard C library from the usual place. See `gcc.c`.

Macro: LIBGCC_SPEC

  Another C string constant that tells the GCC driver program how and when to place a reference to `libgcc.a` into the linker command line. This constant is placed both before and after the value of `LIB_SPEC`.

  If this macro is not defined, the GCC driver provides a default that passes the string `-lgcc` to the linker.

Macro: REAL_LIBGCC_SPEC

  By default, if `ENABLE_SHARED_LIBGCC` is defined, the `LIBGCC_SPEC` is not directly used by the driver program but is instead modified to refer to different versions of `libgcc.a` depending on the values of the command line flags `-static`, `-shared`, `-static-libgcc`, and `-shared-libgcc`. On targets where these modifications are inappropriate, define `REAL_LIBGCC_SPEC` instead. `REAL_LIBGCC_SPEC` tells the driver how to place a reference to `libgcc` on the link command line, but, unlike `LIBGCC_SPEC`, it is used unmodified.

Macro: USE_LD_AS_NEEDED

  A macro that controls the modifications to `LIBGCC_SPEC` mentioned in `REAL_LIBGCC_SPEC`. If nonzero, a spec will be generated that uses `--as-needed` or equivalent options and the shared `libgcc` in place of the static exception handler library, when linking without any of `-static`, `-static-libgcc`, or `-shared-libgcc`.

Macro: LINK_EH_SPEC

  If defined, this C string constant is added to `LINK_SPEC`. When `USE_LD_AS_NEEDED` is zero or undefined, it also affects the modifications to `LIBGCC_SPEC` mentioned in `REAL_LIBGCC_SPEC`.

Macro: STARTFILE_SPEC

  Another C string constant used much like `LINK_SPEC`. The difference between the two is that `STARTFILE_SPEC` is used at the very beginning of the command given to the linker.

  If this macro is not defined, a default is provided that loads the standard C startup file from the usual place. See `gcc.c`.

Macro: ENDFILE_SPEC

  Another C string constant used much like `LINK_SPEC`. The difference between the two is that `ENDFILE_SPEC` is used at the very end of the command given to the linker.

  Do not define this macro if it does not need to do anything.

Macro: THREAD_MODEL_SPEC

  GCC `-v` will print the thread model GCC was configured to use. However, this doesn’t work on platforms that are multilibbed on thread models, such as AIX 4.3. On such platforms, define `THREAD_MODEL_SPEC` such that it evaluates to a string without blanks that names one of the recognized thread models. `%*`, the default value of this macro, will expand to the value of `thread_file` set in `config.gcc`.

Macro: SYSROOT_SUFFIX_SPEC

  Define this macro to add a suffix to the target sysroot when GCC is configured with a sysroot. This will cause GCC to search for usr/lib, et al, within sysroot+suffix.

Macro: SYSROOT_HEADERS_SUFFIX_SPEC

  Define this macro to add a headers_suffix to the target sysroot when GCC is configured with a sysroot. This will cause GCC to pass the updated sysroot+headers_suffix to CPP, causing it to search for usr/include, et al, within sysroot+headers_suffix.

Macro: EXTRA_SPECS

  Define this macro to provide additional specifications to put in the `specs` file that can be used in various specifications like `CC1_SPEC`.

  The definition should be an initializer for an array of structures, containing a string constant, that defines the specification name, and a string constant that provides the specification.

  Do not define this macro if it does not need to do anything.

  `EXTRA_SPECS` is useful when an architecture contains several related targets, which have various `…_SPECS` which are similar to each other, and the maintainer would like one central place to keep these definitions.

  For example, the PowerPC System V.4 targets use `EXTRA_SPECS` to define either `_CALL_SYSV` when the System V calling sequence is used or `_CALL_AIX` when the older AIX-based calling sequence is used.

  The `config/rs6000/rs6000.h` target file defines:

      #define EXTRA_SPECS \
        { "cpp_sysv_default", CPP_SYSV_DEFAULT },

      #define CPP_SYS_DEFAULT ""

  The `config/rs6000/sysv.h` target file defines:

      #undef CPP_SPEC
      #define CPP_SPEC \
      "%{posix: -D_POSIX_SOURCE } \
      %{mcall-sysv: -D_CALL_SYSV } \
      %{!mcall-sysv: %(cpp_sysv_default) } \
      %{msoft-float: -D_SOFT_FLOAT} %{mcpu=403: -D_SOFT_FLOAT}"

      #undef CPP_SYSV_DEFAULT
      #define CPP_SYSV_DEFAULT "-D_CALL_SYSV"

  while the `config/rs6000/eabiaix.h` target file defines `CPP_SYSV_DEFAULT` as:

      #undef CPP_SYSV_DEFAULT
      #define CPP_SYSV_DEFAULT "-D_CALL_AIX"

Macro: LINK_LIBGCC_SPECIAL_1

  Define this macro if the driver program should find the library `libgcc.a`. If you do not define this macro, the driver program will pass the argument `-lgcc` to tell the linker to do the search.

Macro: LINK_GCC_C_SEQUENCE_SPEC

  The sequence in which libgcc and libc are specified to the linker. By default this is `%G %L %G`.

Macro: POST_LINK_SPEC

  Define this macro to add additional steps to be executed after linker. The default value of this macro is empty string.

Macro: LINK_COMMAND_SPEC

  A C string constant giving the complete command line need to execute the linker. When you do this, you will need to update your port each time a change is made to the link command line within `gcc.c`. Therefore, define this macro only if you need to completely redefine the command line for invoking the linker and there is no other way to accomplish the effect you need. Overriding this macro may be avoidable by overriding `LINK_GCC_C_SEQUENCE_SPEC` instead.

Common Target Hook: bool TARGET_ALWAYS_STRIP_DOTDOT

  True if `..` components should always be removed from directory names computed relative to GCC’s internal directories, false (default) if such components should be preserved and directory names containing them passed to other tools such as the linker.

Macro: MULTILIB_DEFAULTS

  Define this macro as a C expression for the initializer of an array of string to tell the driver program which options are defaults for this target and thus do not need to be handled specially when using `MULTILIB_OPTIONS`.

  Do not define this macro if `MULTILIB_OPTIONS` is not defined in the target makefile fragment or if none of the options listed in `MULTILIB_OPTIONS` are set by default. See Target Fragment.

Macro: RELATIVE_PREFIX_NOT_LINKDIR

  Define this macro to tell `gcc` that it should only translate a `-B` prefix into a `-L` linker option if the prefix indicates an absolute file name.

Macro: MD_EXEC_PREFIX

  If defined, this macro is an additional prefix to try after `STANDARD_EXEC_PREFIX`. `MD_EXEC_PREFIX` is not searched when the compiler is built as a cross compiler. If you define `MD_EXEC_PREFIX`, then be sure to add it to the list of directories used to find the assembler in `configure.ac`.

Macro: STANDARD_STARTFILE_PREFIX

  Define this macro as a C string constant if you wish to override the standard choice of `libdir` as the default prefix to try when searching for startup files such as `crt0.o`. `STANDARD_STARTFILE_PREFIX` is not searched when the compiler is built as a cross compiler.

Macro: STANDARD_STARTFILE_PREFIX_1

  Define this macro as a C string constant if you wish to override the standard choice of `/lib` as a prefix to try after the default prefix when searching for startup files such as `crt0.o`. `STANDARD_STARTFILE_PREFIX_1` is not searched when the compiler is built as a cross compiler.

Macro: STANDARD_STARTFILE_PREFIX_2

  Define this macro as a C string constant if you wish to override the standard choice of `/lib` as yet another prefix to try after the default prefix when searching for startup files such as `crt0.o`. `STANDARD_STARTFILE_PREFIX_2` is not searched when the compiler is built as a cross compiler.

Macro: MD_STARTFILE_PREFIX

  If defined, this macro supplies an additional prefix to try after the standard prefixes. `MD_EXEC_PREFIX` is not searched when the compiler is built as a cross compiler.

Macro: MD_STARTFILE_PREFIX_1

  If defined, this macro supplies yet another prefix to try after the standard prefixes. It is not searched when the compiler is built as a cross compiler.

Macro: INIT_ENVIRONMENT

  Define this macro as a C string constant if you wish to set environment variables for programs called by the driver, such as the assembler and loader. The driver passes the value of this macro to `putenv` to initialize the necessary environment variables.

Macro: LOCAL_INCLUDE_DIR

  Define this macro as a C string constant if you wish to override the standard choice of `/usr/local/include` as the default prefix to try when searching for local header files. `LOCAL_INCLUDE_DIR` comes before `NATIVE_SYSTEM_HEADER_DIR` (set in `config.gcc`, normally `/usr/include`) in the search order.

  Cross compilers do not search either `/usr/local/include` or its replacement.

Macro: NATIVE_SYSTEM_HEADER_COMPONENT

  The “component” corresponding to `NATIVE_SYSTEM_HEADER_DIR`. See `INCLUDE_DEFAULTS`, below, for the description of components. If you do not define this macro, no component is used.

Macro: INCLUDE_DEFAULTS

  Define this macro if you wish to override the entire default search path for include files. For a native compiler, the default search path usually consists of `GCC_INCLUDE_DIR`, `LOCAL_INCLUDE_DIR`, `GPLUSPLUS_INCLUDE_DIR`, and `NATIVE_SYSTEM_HEADER_DIR`. In addition, `GPLUSPLUS_INCLUDE_DIR` and `GCC_INCLUDE_DIR` are defined automatically by `Makefile`, and specify private search areas for GCC. The directory `GPLUSPLUS_INCLUDE_DIR` is used only for C++ programs.

  The definition should be an initializer for an array of structures. Each array element should have four elements: the directory name (a string constant), the component name (also a string constant), a flag for C++-only directories, and a flag showing that the includes in the directory don’t need to be wrapped in `extern ‘`C`’` when compiling C++. Mark the end of the array with a null element.

  The component name denotes what GNU package the include file is part of, if any, in all uppercase letters. For example, it might be ‘`GCC`’ or ‘`BINUTILS`’. If the package is part of a vendor-supplied operating system, code the component name as ‘`0`’.

  For example, here is the definition used for VAX/VMS:

      #define INCLUDE_DEFAULTS \
      {                                       \
        { "GNU_GXX_INCLUDE:", "G++", 1, 1},   \
        { "GNU_CC_INCLUDE:", "GCC", 0, 0},    \
        { "SYS$SYSROOT:[SYSLIB.]", 0, 0, 0},  \
        { ".", 0, 0, 0},                      \
        { 0, 0, 0, 0}                         \
      }

Here is the order of prefixes tried for exec files:

1. Any prefixes specified by the user with `-B`.
2. The environment variable `GCC_EXEC_PREFIX` or, if `GCC_EXEC_PREFIX` is not set and the compiler has not been installed in the configure-time prefix, the location in which the compiler has actually been installed.
3. The directories specified by the environment variable `COMPILER_PATH`.
4. The macro `STANDARD_EXEC_PREFIX`, if the compiler has been installed in the configured-time prefix.
5. The location `/usr/libexec/gcc/`, but only if this is a native compiler.
6. The location `/usr/lib/gcc/`, but only if this is a native compiler.
7. The macro `MD_EXEC_PREFIX`, if defined, but only if this is a native compiler.

Here is the order of prefixes tried for startfiles:

1. Any prefixes specified by the user with `-B`.
2. The environment variable `GCC_EXEC_PREFIX` or its automatically determined value based on the installed toolchain location.
3. The directories specified by the environment variable `LIBRARY_PATH` (or port-specific name; native only, cross compilers do not use this).
4. The macro `STANDARD_EXEC_PREFIX`, but only if the toolchain is installed in the configured prefix or this is a native compiler.
5. The location `/usr/lib/gcc/`, but only if this is a native compiler.
6. The macro `MD_EXEC_PREFIX`, if defined, but only if this is a native compiler.
7. The macro `MD_STARTFILE_PREFIX`, if defined, but only if this is a native compiler, or we have a target system root.
8. The macro `MD_STARTFILE_PREFIX_1`, if defined, but only if this is a native compiler, or we have a target system root.
9. The macro `STANDARD_STARTFILE_PREFIX`, with any sysroot modifications. If this path is relative it will be prefixed by `GCC_EXEC_PREFIX` and the machine suffix or `STANDARD_EXEC_PREFIX` and the machine suffix.
10. The macro `STANDARD_STARTFILE_PREFIX_1`, but only if this is a native compiler, or we have a target system root. The default for this macro is `/lib/`.
11. The macro `STANDARD_STARTFILE_PREFIX_2`, but only if this is a native compiler, or we have a target system root. The default for this macro is `/usr/lib/`.