# 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 SVN or snapshots and enabling the `treelang` front end, or if you modify *.y files, you need the Bison parser generator installed. If you do not modify *.y files, releases contain the Bison-generated files and you do not need Bison installed to build them. Note that most front ends now use hand-written parsers, which can be modified with no need for Bison.

Similarly, when building from SVN or snapshots, or if you modify *.l files, you need the Flex lexical analyzer generator installed. There is still one Flex-based lexical analyzer (part of the build machinery, not of GCC itself) that is used even if you only build the C front end.

When building from SVN or snapshots, or if you modify Texinfo documentation, you need version 4.4 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.

### 0.1 Building a native compiler

For a native build, the default configuration is to perform a 3-stage bootstrap of the compiler when make is invoked. This will build the entire GCC system and ensure that it compiles itself correctly. It can be disabled with the --disable-bootstrap parameter to configure, but bootstrapping is suggested because the compiler will be tested more completely and could also have better performance.

The bootstrapping process will complete the following steps:

- Build tools necessary to build the compiler.
- Perform a 3-stage bootstrap of the compiler. This includes building three times the 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 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. The sequence of compilation is the same described above, but 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. 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.

Note that using non-standard `CFLAGS` can cause bootstrap to fail if these trigger a warning with the new compiler. For example using -O2 -g -mcpu=i686 on `i686-pc-linux-gnu` will cause bootstrap failure as -mcpu= is deprecated in 3.4.0 and above.

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 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.)

If you do not want to bootstrap your compiler, you can configure with --disable-bootstrap. In particular cases, you may want to bootstrap your compiler even if the target system is not the same as the one you are building on: for example, you could build a `powerpc-unknown-linux-gnu` toolchain on a `powerpc64-unknown-linux-gnu` host. In this case, pass --enable-bootstrap to the configure script.

### 0.2 Building a cross compiler

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.

If the cross compiler is to be built with support for the Java programming language and the ability to compile .java source files is desired, the installed native compiler used to build the cross compiler needs to be the same GCC version as the cross compiler. In addition the cross compiler needs to be configured with --with-ecj-jar=....

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.
- 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.

### 0.3 Building in parallel

GNU Make 3.79 and above, which is necessary to build GCC, support building in parallel. To activate this, you can use make -j 2 instead of make. 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.

### 0.4 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). This includes GNAT tools such as gnatmake and gnatlink, since the Ada front end is written in Ada and uses some GNAT-specific extensions.

In order to build a cross compiler, it is suggested to install the new compiler as native first, and then use it to build the cross compiler.

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.

ADA_INCLUDE_PATH and ADA_OBJECT_PATH environment variables must not be set when building the Ada compiler, the Ada tools, or the Ada runtime libraries. You can check that your build environment is clean by verifying that gnatls -v lists only one explicit path in each section.

### 0.5 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 the 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 standard 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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