Libgcc without red zone: Difference between revisions

 

To get around this the <tt>red-zone</tt> can be disabled by passing <tt>-mno-red-zone</tt> to [[GCC]].

 

== Why modify libgcc? ==

 

Create the following file and save it as <tt>t-x86_64-elf</tt> inside <i><tt>gcc/config/i386/</tt></i> under your [[GCC]] sources.

# Add libgcc multilib variant without red-zone requirement

 

MULTILIB_OPTIONS += mno-red-zone

MULTILIB_DIRNAMES += no-red-zone

 

By default this new configuration will not be used by [[GCC]] unless it's explicitly told to.

Open <tt>gcc/config.gcc</tt> in your favorite editor and search for case block like this:

x86_64-*-elf*)

tm_file="${tm_file} i386/unix.h i386/att.h elfos.h newlib-stdint.h i386/i386elf.h i386/x86-64.h"

;;

 

This is the target configuration used when creating a [[GCC Cross-Compiler]] for x86_64-elf.

Modify it to include the new [[multilib]] configuration:

x86_64-*-elf*)

tmake_file="${tmake_file} i386/t-x86_64-elf" # include the new multilib configuration

tm_file="${tm_file} i386/unix.h i386/att.h elfos.h newlib-stdint.h i386/i386elf.h i386/x86-64.h"

;;

 

== Building libgcc ==

Run configure and then invoke the <tt>all-target-libgcc</tt> and <tt>install-target-libgcc</tt> as usual and [[GCC]] will build [[libgcc]] in two versions - one with <tt>red-zone</tt> enabled and one without.

You can check the successful build by checking the installed <tt>libgcc.a</tt> archives:

find $TOOLCHAIN_PREFIX/lib -name 'libgcc.a'

 

If all went well you should see an additional [[libgcc]] installed in the <tt>no-red-zone</tt> multilib directory:

./gcc/x86_64-pc-elf/4.9.1/libgcc.a

./gcc/x86_64-pc-elf/4.9.1/no-red-zone/libgcc.a

 

== Linking against the correct multilib version ==

Assuming you're using [[GCC]] to link your kernel you're probably fine. All that's needed is to make sure <tt>-mno-red-zone</tt> is in your <tt>LDFLAGS</tt> when doing the final linker call.

x86_64-elf-gcc $LDFLAGS -mno-red-zone -o kernel $SOURCES

 

If you're unsure which [[libgcc]] version is going to be used you can check by passing <tt>-mno-red-zone</tt> and <tt>-print-libgcc-file-name</tt> to [[GCC]]:

x86_64-elf-gcc -mno-red-zone -print-libgcc-file-name

lib/gcc/x86_64-pc-elf/4.9.1/no-red-zone/libgcc.a

 

==See Also==