Add TARGET_COMPUTE_MULTILIB hook to override multi-lib result.

Create a new hook to let target could override the multi-lib result,
the motivation is RISC-V might have very complicated multi-lib re-use
rule*, which is hard to maintain and use current multi-lib scripts,
we even hit the "argument list too long" error when we tried to add more
multi-lib reuse rule.

So I think it would be great to have a target specific way to determine
the multi-lib re-use rule, then we could write those rule in C, instead
of expand every possible case in MULTILIB_REUSE.

* Here is an example for RISC-V multi-lib rules:
https://gist.github.com/kito-cheng/0289cd42d9a756382e5afeb77b42b73b

gcc/ChangeLog:

	* common/common-target.def (compute_multilib): New.
	* common/common-targhooks.h (default_compute_multilib): New.
	* common/common-targhooks.cc (default_compute_multilib): New.
	* doc/tm.texi.in (TARGET_COMPUTE_MULTILIB): New.
	* doc/tm.texi: Regen.
	* gcc.cc: Include common/common-target.h.
	(set_multilib_dir) Call targetm_common.compute_multilib.
	(SWITCH_LIVE): Move to opts.h.
	(SWITCH_FALSE): Ditto.
	(SWITCH_IGNORE): Ditto.
	(SWITCH_IGNORE_PERMANENTLY): Ditto.
	(SWITCH_KEEP_FOR_GCC): Ditto.
	(struct switchstr): Ditto.
	* opts.h (SWITCH_LIVE): Move from gcc.c.
	(SWITCH_FALSE): Ditto.
	(SWITCH_IGNORE): Ditto.
	(SWITCH_IGNORE_PERMANENTLY): Ditto.
	(SWITCH_KEEP_FOR_GCC): Ditto.
	(struct switchstr): Ditto.

1 files changed, 17 insertions, 0 deletions

diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 431b414..11be5d0 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -778,6 +778,23 @@ options are changed via @code{#pragma GCC optimize} or by using the
 Set target-dependent initial values of fields in @var{opts}.
 @end deftypefn
 
+@deftypefn {Common Target Hook} {const char *} TARGET_COMPUTE_MULTILIB (const struct switchstr *@var{switches}, int @var{n_switches}, const char *@var{multilib_dir}, const char *@var{multilib_defaults}, const char *@var{multilib_select}, const char *@var{multilib_matches}, const char *@var{multilib_exclusions}, const char *@var{multilib_reuse})
+Some targets like RISC-V might have complicated multilib reuse rules which
+are hard to implement with the current multilib scheme.  This hook allows
+targets to override the result from the built-in multilib mechanism.
+@var{switches} is the raw option list with @var{n_switches} items;
+@var{multilib_dir} is the multi-lib result which is computed by the built-in
+multi-lib mechanism;
+@var{multilib_defaults} is the default options list for multi-lib;
+@var{multilib_select} is the string containing the list of supported
+multi-libs, and the option checking list.
+@var{multilib_matches}, @var{multilib_exclusions}, and @var{multilib_reuse}
+are corresponding to @var{MULTILIB_MATCHES}, @var{MULTILIB_EXCLUSIONS},
+and @var{MULTILIB_REUSE}.
+The default definition does nothing but return @var{multilib_dir} directly.
+@end deftypefn
+
+
 @defmac SWITCHABLE_TARGET
 Some targets need to switch between substantially different subtargets
 during compilation.  For example, the MIPS target has one subtarget for