2013-01-13 Terry Guo <terry.guo@arm.com>

        * gcc/Makefile.in (s-mlib): New argument MULTILIB_REUSE.
        * gcc/doc/fragments.texi: Document MULTILIB_REUSE.
        * gcc/gcc.c (multilib_reuse): New internal spec.
        (set_multilib_dir): Also search multilib from multilib_reuse.
        * gcc/genmultilib (tmpmultilib3): Refactor code.
        (tmpmultilib4): Ditto.
        (multilib_reuse): New multilib argument.



git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@195134 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 29 insertions, 0 deletions

diff --git a/gcc/doc/fragments.texi b/gcc/doc/fragments.texi
index a108ab035ba..b9a0c34b690 100644
--- a/gcc/doc/fragments.texi
+++ b/gcc/doc/fragments.texi
@@ -148,6 +148,35 @@ The @code{MULTILIB_REQUIRED} can be used together with
 @code{MULTILIB_OPTIONS} will be filtered by @code{MULTILIB_EXCEPTIONS}
 and then by @code{MULTILIB_REQUIRED}.
 
+@findex MULTILIB_REUSE
+@item MULTILIB_REUSE
+Sometimes it is desirable to reuse one existing multilib for different
+sets of options.  Such kind of reuse can minimize the number of multilib
+variants.  And for some targets it is better to reuse an existing multilib
+than to fall back to default multilib when there is no corresponding multilib.
+This can be done by adding reuse rules to @code{MULTILIB_REUSE}.
+
+A reuse rule is comprised of two parts connected by equality sign.  The left part
+is option set used to build multilib and the right part is option set that will
+reuse this multilib.  The order of options in the left part matters and should be
+same with those specified in @code{MULTILIB_REQUIRED} or aligned with order in
+@code{MULTILIB_OPTIONS}.  There is no such limitation for options in right part
+as we don't build multilib from them.  But the equality sign in both parts should
+be replaced with period.
+
+The @code{MULTILIB_REUSE} is different from @code{MULTILIB_MATCHES} in that it
+sets up relations between two option sets rather than two options.  Here is an
+example to demo how we reuse libraries built in Thumb mode for applications built
+in ARM mode:
+@smallexample
+@code{MULTILIB_REUSE} = mthumb/march.armv7-r=marm/march.armv7-r
+@end smallexample
+
+Before the advent of @code{MULTILIB_REUSE}, GCC select multilib by comparing command
+line options with options used to build multilib.  The @code{MULTILIB_REUSE} is
+complementary to that way.  Only when the original comparison matches nothing it will
+work to see if it is OK to reuse some existing multilib.
+
 @findex MULTILIB_EXTRA_OPTS
 @item MULTILIB_EXTRA_OPTS
 Sometimes it is desirable that when building multiple versions of