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.

3 files changed, 51 insertions, 0 deletions

diff --git a/gcc/common/common-target.def b/gcc/common/common-target.def
index c4d1144..c4c6230 100644
--- a/gcc/common/common-target.def
+++ b/gcc/common/common-target.def
@@ -84,6 +84,31 @@ The result will be pruned to cases with PREFIX if not NULL.",
  vec<const char *>, (int option_code, const char *prefix),
  default_get_valid_option_values)
 
+DEFHOOK
+(compute_multilib,
+ "Some targets like RISC-V might have complicated multilib reuse rules which\n\
+are hard to implement with the current multilib scheme.  This hook allows\n\
+targets to override the result from the built-in multilib mechanism.\n\
+@var{switches} is the raw option list with @var{n_switches} items;\n\
+@var{multilib_dir} is the multi-lib result which is computed by the built-in\n\
+multi-lib mechanism;\n\
+@var{multilib_defaults} is the default options list for multi-lib;\n\
+@var{multilib_select} is the string containing the list of supported\n\
+multi-libs, and the option checking list.\n\
+@var{multilib_matches}, @var{multilib_exclusions}, and @var{multilib_reuse}\n\
+are corresponding to @var{MULTILIB_MATCHES}, @var{MULTILIB_EXCLUSIONS},\n\
+and @var{MULTILIB_REUSE}.\n\
+The default definition does nothing but return @var{multilib_dir} directly.",
+ const char *, (const struct switchstr *switches,
+		int n_switches,
+		const char *multilib_dir,
+		const char *multilib_defaults,
+		const char *multilib_select,
+		const char *multilib_matches,
+		const char *multilib_exclusions,
+		const char *multilib_reuse),
+ default_compute_multilib)
+
 /* Leave the boolean fields at the end.  */
 
 /* True if unwinding tables should be generated by default.  */
diff --git a/gcc/common/common-targhooks.cc b/gcc/common/common-targhooks.cc
index 46f5c61..7499be2 100644
--- a/gcc/common/common-targhooks.cc
+++ b/gcc/common/common-targhooks.cc
@@ -90,3 +90,18 @@ const struct default_options empty_optimization_table[] =
   {
     { OPT_LEVELS_NONE, 0, NULL, 0 }
   };
+
+/* Default version of TARGET_COMPUTE_MULTILIB.  */
+const char *
+default_compute_multilib(
+  const struct switchstr *,
+  int,
+  const char *multilib,
+  const char *,
+  const char *,
+  const char *,
+  const char *,
+  const char *)
+{
+  return multilib;
+}
diff --git a/gcc/common/common-targhooks.h b/gcc/common/common-targhooks.h
index cd49c7d..1f03495 100644
--- a/gcc/common/common-targhooks.h
+++ b/gcc/common/common-targhooks.h
@@ -32,4 +32,15 @@ extern vec<const char *> default_get_valid_option_values (int, const char *);
 
 extern const struct default_options empty_optimization_table[];
 
+const char *
+default_compute_multilib(
+  const struct switchstr *,
+  int,
+  const char *multilib,
+  const char *,
+  const char *,
+  const char *,
+  const char *,
+  const char *);
+
 #endif