RISC-V: Implement TARGET_COMPUTE_MULTILIB

Use TARGET_COMPUTE_MULTILIB to search the multi-lib reuse for riscv*-*-elf*,
according following rules:

 1. Check ABI is same.
 2. Check both has atomic extension or both don't have atomic extension.
    - Because mix soft and hard atomic operation doesn't make sense and
      won't work as expect.
 3. Check current arch is superset of the target multi-lib arch.
    - It might result slower performance or larger code size, but it
      safe to run.
 4. Pick most match multi-lib set if more than one multi-lib are pass
    the above checking.

Example for how to select multi-lib:
  We build code with -march=rv32imaf and -mabi=ilp32, and we have
  following 5 multi-lib set:

    1. rv32ia/ilp32
    2. rv32ima/ilp32
    3. rv32imf/ilp32
    4. rv32imaf/ilp32f
    5. rv32imafd/ilp32

  The first and second multi-lib is safe to like, 3rd multi-lib can't
  re-use becasue it don't have atomic extension, which is mismatch according
  rule 2, and the 4th multi-lib can't re-use too due to the ABI mismatch,
  the last multi-lib can't use since current arch is not superset of the
  arch of multi-lib.

And emit error if not found suitable multi-lib set, the error message
only emit when link with standard libraries.

Example for when error will be emitted:

  $ riscv64-unknown-elf-gcc -print-multi-lib
  .;
  rv32i/ilp32;@march=rv32i@mabi=ilp32
  rv32im/ilp32;@march=rv32im@mabi=ilp32
  rv32iac/ilp32;@march=rv32iac@mabi=ilp32
  rv32imac/ilp32;@march=rv32imac@mabi=ilp32
  rv32imafc/ilp32f;@march=rv32imafc@mabi=ilp32f
  rv64imac/lp64;@march=rv64imac@mabi=lp64

  // No actual linking, so no error emitted.
  $ riscv64-unknown-elf-gcc -print-multi-directory -march=rv32ia -mabi=ilp32
  .

  // Link to default libc and libgcc, so check the multi-lib, and emit
  // error because not found suitable multilib.
  $ riscv64-unknown-elf-gcc -march=rv32ia -mabi=ilp32 ~/hello.c
  riscv64-unknown-elf-gcc: fatal error: can't found suitable multilib set for '-march=rv32ia'/'-mabi=ilp32'
  compilation terminated.

  // No error emitted, because not link to stdlib.
  $ riscv64-unknown-elf-gcc -march=rv32ia -mabi=ilp32 ~/hello.c -nostdlib

  // No error emitted, because compile only.
  $ riscv64-unknown-elf-gcc -march=rv32ia -mabi=ilp32 ~/hello.c -c

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc: Include <vector>.
	(struct riscv_multi_lib_info_t): New.
	(riscv_subset_list::match_score): Ditto.
	(find_last_appear_switch): Ditto.
	(prefixed_with): Ditto.
	(struct multi_lib_info_t): Ditto.
	(riscv_current_arch_str): Ditto.
	(riscv_current_abi_str): Ditto.
	(riscv_multi_lib_info_t::parse): Ditto.
	(riscv_check_cond): Ditto.
	(riscv_check_conds): Ditto.
	(riscv_compute_multilib): Ditto.
	(TARGET_COMPUTE_MULTILIB): Defined.
	* config/riscv/elf.h (LIB_SPEC): Call riscv_multi_lib_check if
	doing link.
	(RISCV_USE_CUSTOMISED_MULTI_LIB): New.
	* config/riscv/riscv.h (riscv_multi_lib_check): New.
	(EXTRA_SPEC_FUNCTIONS): Add riscv_multi_lib_check.
	* config/riscv/riscv-subset.h (riscv_subset_list::match_score): New.

3 files changed, 10 insertions, 2 deletions

diff --git a/gcc/config/riscv/elf.h b/gcc/config/riscv/elf.h
index f0e865d..f3d767c 100644
--- a/gcc/config/riscv/elf.h
+++ b/gcc/config/riscv/elf.h
@@ -27,10 +27,14 @@ along with GCC; see the file COPYING3.  If not see
 /* Link against Newlib libraries, because the ELF backend assumes Newlib.
    Handle the circular dependence between libc and libgloss. */
 #undef  LIB_SPEC
-#define LIB_SPEC "--start-group -lc %{!specs=nosys.specs:-lgloss} --end-group"
+#define LIB_SPEC \
+  "--start-group -lc %{!specs=nosys.specs:-lgloss} --end-group " \
+  "%{!nostartfiles:%{!nodefaultlibs:%{!nolibc:%{!nostdlib:%:riscv_multi_lib_check()}}}}"
 
 #undef  STARTFILE_SPEC
 #define STARTFILE_SPEC "crt0%O%s crtbegin%O%s"
 
 #undef  ENDFILE_SPEC
 #define ENDFILE_SPEC "crtend%O%s"
+
+#define RISCV_USE_CUSTOMISED_MULTI_LIB 1
diff --git a/gcc/config/riscv/riscv-subset.h b/gcc/config/riscv/riscv-subset.h
index c592650..0bb3a9d 100644
--- a/gcc/config/riscv/riscv-subset.h
+++ b/gcc/config/riscv/riscv-subset.h
@@ -89,6 +89,8 @@ public:
 
   const riscv_subset_t *begin () const {return m_head;};
   const riscv_subset_t *end () const {return NULL;};
+
+  int match_score (riscv_subset_list *) const;
 };
 
 extern const riscv_subset_list *riscv_current_subset_list (void);
diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
index 3ee5a93..e35aaa7 100644
--- a/gcc/config/riscv/riscv.h
+++ b/gcc/config/riscv/riscv.h
@@ -50,11 +50,13 @@ along with GCC; see the file COPYING3.  If not see
 extern const char *riscv_expand_arch (int argc, const char **argv);
 extern const char *riscv_expand_arch_from_cpu (int argc, const char **argv);
 extern const char *riscv_default_mtune (int argc, const char **argv);
+extern const char *riscv_multi_lib_check (int argc, const char **argv);
 
 # define EXTRA_SPEC_FUNCTIONS						\
   { "riscv_expand_arch", riscv_expand_arch },				\
   { "riscv_expand_arch_from_cpu", riscv_expand_arch_from_cpu },		\
-  { "riscv_default_mtune", riscv_default_mtune },
+  { "riscv_default_mtune", riscv_default_mtune },			\
+  { "riscv_multi_lib_check", riscv_multi_lib_check },
 
 /* Support for a compile-time default CPU, et cetera.  The rules are:
    --with-arch is ignored if -march or -mcpu is specified.