# Installing GCC: Building

Now that GCC is configured, you are ready to build the compiler and runtime libraries.

Some commands executed when making the compiler may fail (return a nonzero status) and be ignored by `make`. These failures, which are often due to files that were not found, are expected, and can safely be ignored.

It is normal to have compiler warnings when compiling certain files. Unless you are a GCC developer, you can generally ignore these warnings unless they cause compilation to fail. Developers should attempt to fix any warnings encountered, however they can temporarily continue past warnings-as-errors by specifying the configure flag `--disable-werror`.

On certain old systems, defining certain environment variables such as `CC` can interfere with the functioning of `make`.

If you encounter seemingly strange errors when trying to build the compiler in a directory other than the source directory, it could be because you have previously configured the compiler in the source directory. Make sure you have done all the necessary preparations.

If you build GCC on a BSD system using a directory stored in an old System V file system, problems may occur in running `fixincludes` if the System V file system doesn't support symbolic links. These problems result in a failure to fix the declaration of `size_t` in `sys/types.h`. If you find that `size_t` is a signed type and that type mismatches occur, this could be the cause.

The solution is not to use such a directory for building GCC.

When building from CVS or snapshots, or if you modify parser sources, you need the Bison parser generator installed. Any version 1.25 or later should work; older versions may also work. If you do not modify parser sources, releases contain the Bison-generated files and you do not need Bison installed to build them.

When building from CVS or snapshots, or if you modify Texinfo documentation, you need version 4.2 or later of Texinfo installed if you want Info documentation to be regenerated. Releases contain Info documentation pre-built for the unmodified documentation in the release.

### Building a native compiler

For a native build issue the command `make bootstrap`. This will build the entire GCC system, which includes the following steps:

- Build host tools necessary to build the compiler such as texinfo, bison, gperf.
- Build target tools for use by the compiler such as binutils (bfd, binutils, gas, gprof, ld, and opcodes) if they have been individually linked or moved into the top level GCC source tree before configuring.
- Perform a 3-stage bootstrap of the compiler.
- Perform a comparison test of the stage2 and stage3 compilers.
- Build runtime libraries using the stage3 compiler from the previous step.

If you are short on disk space you might consider `make bootstrap-lean` instead. This is identical to `make bootstrap` except that object files from the stage1 and stage2 of the 3-stage bootstrap of the compiler are deleted as soon as they are no longer needed.

If you want to save additional space during the bootstrap and in the final installation as well, you can build the compiler binaries without debugging information as in the following example. This will save roughly 40% of disk space both for the bootstrap and the final installation. (Libraries will still contain debugging information.)

         make CFLAGS='-O' LIBCFLAGS='-g -O2' \
           LIBCXXFLAGS='-g -O2 -fno-implicit-templates' bootstrap

If you wish to use non-default GCC flags when compiling the stage2 and stage3 compilers, set `BOOT_CFLAGS` on the command line when doing `make bootstrap`. Non-default optimization flags are less well tested here than the default of `-g -O2`, but should still work. In a few cases, you may find that you need to specify special flags such as `-msoft-float` here to complete the bootstrap; or, if the native compiler miscompiles the stage1 compiler, you may need to work around this, by choosing `BOOT_CFLAGS` to avoid the parts of the stage1 compiler that were miscompiled, or by using `make bootstrap4` to increase the number of stages of bootstrap.

If you used the flag `--enable-languages=...` to restrict the compilers to be built, only those you've actually enabled will be built. This will of course only build those runtime libraries, for which the particular compiler has been built. Please note, that re-defining `LANGUAGES` when calling `make bootstrap` does not work anymore!

If the comparison of stage2 and stage3 fails, this normally indicates that the stage2 compiler has compiled GCC incorrectly, and is therefore a potentially serious bug which you should investigate and report. (On a few systems, meaningful comparison of object files is impossible; they always appear "different". If you encounter this problem, you will need to disable comparison in the `Makefile`.)

### Building a cross compiler

We recommend reading the crossgcc FAQ for information about building cross compilers.

When building a cross compiler, it is not generally possible to do a 3-stage bootstrap of the compiler. This makes for an interesting problem as parts of GCC can only be built with GCC.

To build a cross compiler, we first recommend building and installing a native compiler. You can then use the native GCC compiler to build the cross compiler. The installed native compiler needs to be GCC version 2.95 or later.

Assuming you have already installed a native copy of GCC and configured your cross compiler, issue the command `make`, which performs the following steps:

- Build host tools necessary to build the compiler such as texinfo, bison, gperf.
- Build target tools for use by the compiler such as binutils (bfd, binutils, gas, gprof, ld, and opcodes) if they have been individually linked or moved into the top level GCC source tree before configuring.
- Build the compiler (single stage only).
- Build runtime libraries using the compiler from the previous step.

Note that if an error occurs in any step the make process will exit.

If you are not building GNU binutils in the same source tree as GCC, you will need a cross-assembler and cross-linker installed before configuring GCC. Put them in the directory `prefix/target/bin`. Here is a table of the tools you should put in this directory:

`as`
  This should be the cross-assembler.
`ld`
  This should be the cross-linker.
`ar`
  This should be the cross-archiver: a program which can manipulate archive files (linker libraries) in the target machine's format.
`ranlib`
  This should be a program to construct a symbol table in an archive file.

The installation of GCC will find these programs in that directory, and copy or link them to the proper place to for the cross-compiler to find them when run later.

The easiest way to provide these files is to build the Binutils package. Configure it with the same `--host` and `--target` options that you use for configuring GCC, then build and install them. They install their executables automatically into the proper directory. Alas, they do not support all the targets that GCC supports.

If you are not building a C library in the same source tree as GCC, you should also provide the target libraries and headers before configuring GCC, specifying the directories with `--with-sysroot` or `--with-headers` and `--with-libs`. Many targets also require "start files" such as `crt0.o` and `crtn.o` which are linked into each executable. There may be several alternatives for `crt0.o`, for use with profiling or other compilation options. Check your target's definition of `STARTFILE_SPEC` to find out what start files it uses.

### Building in parallel

You can use `make bootstrap MAKE="make -j 2" -j 2`, or just `make -j 2 bootstrap` for GNU Make 3.79 and above, instead of `make bootstrap` to build GCC in parallel. You can also specify a bigger number, and in most cases using a value greater than the number of processors in your machine will result in fewer and shorter I/O latency hits, thus improving overall throughput; this is especially true for slow drives and network filesystems.

### Building the Ada compiler

In order to build GNAT, the Ada compiler, you need a working GNAT compiler (GNAT version 3.14 or later, or GCC version 3.1 or later), including GNAT tools such as `gnatmake` and `gnatlink`, since the Ada front end is written in Ada (with some GNAT-specific extensions), and GNU make.

`configure` does not test whether the GNAT installation works and has a sufficiently recent version; if too old a GNAT version is installed, the build will fail unless `--enable-languages` is used to disable building the Ada front end.

At the moment, the GNAT library and several tools for GNAT are not built by `make bootstrap`. For a native build, you have to invoke `make gnatlib_and_tools` in the `objdir/gcc` subdirectory before proceeding with the next steps. For a cross build, you need to invoke `make gnatlib cross-gnattools ada.all.cross`. For a canadian cross you only need to invoke `make cross-gnattools`; the GNAT library would be the same as the one built for the cross compiler.

For example, you can build a native Ada compiler by issuing the following commands (assuming `make` is GNU make):

        cd objdir
        srcdir/configure --enable-languages=c,ada
        cd objdir
        make bootstrap
        cd gcc
        make gnatlib_and_tools
        cd ..

Currently, when compiling the Ada front end, you cannot use the parallel build feature described in the previous section.

### Building with profile feedback

It is possible to use profile feedback to optimize the compiler itself. This should result in a faster compiler binary. Experiments done on x86 using gcc 3.3 showed approximately 7 percent speedup on compiling C programs. To bootstrap compiler with profile feedback, use `make profiledbootstrap`.

When `make profiledbootstrap` is run, it will first build a `stage1` compiler. This compiler is used to build a `stageprofile` compiler instrumented to collect execution counts of instruction and branch probabilities. Then runtime libraries are compiled with profile collected. Finally a `stagefeedback` compiler is built using the information collected.

Unlike `make bootstrap` several additional restrictions apply. The compiler used to build `stage1` needs to support a 64-bit integral type. It is recommended to only use GCC for this. Also parallel make is currently not supported since collisions in profile collecting may occur.

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