RISC-V: Support to add implicit extensions for G.

G is a special case, consider the ISA spec github issue as follows,
https://github.com/riscv/riscv-isa-manual/issues/575

My understand is that - i, m, a, f and d extensions are not g's implicit
extensions, they are g's expansions.  The zifencei is the implicit extension
of g, and so is zicsr, since it is implicited by f (or i2p1).  However,
we add the g with the RISCV_UNKNOWN_VERSION to the subset list, and it
will not output to the arch string, it is only used to check what implicit
extensions are need to be added.

	bfd/
	* elfxx-riscv.c (riscv_parse_add_subset): Allow to add g with
	RISCV_UNKNOWN_VERSION versions.
	(riscv_parse_std_ext): Add g to the subset list, we only use it
	to add the implicit extensions, but won't output it to arch string.
	(riscv_parse_add_implicit_subsets): Add implicit zicsr and zifencei
	for g extension.
	(riscv_arch_str1): Do not output g to the arch string.
	* elfxx-riscv.h (RISCV_UNKNOWN_VERSION): Moved to include/opcode/riscv.h.

	gas/
	* testsuite/gas/riscv/attribute-10.d: Updated.
	* testsuite/gas/riscv/march-imply-g.d: New testcase for g.
	* testsuite/gas/riscv/march-imply-unsupported.d: The zicsr and zifencei
	are not supported in the ISA spec v2.2, so don't add and output them.

	include/
	* opcode/riscv.h (RISCV_UNKNOWN_VERSION): added.

bfd/ChangeLog

@@ -1,3 +1,14 @@
+2020-12-01  Nelson Chu  <nelson.chu@sifive.com>
+
+	* elfxx-riscv.c (riscv_parse_add_subset): Allow to add g with
+	RISCV_UNKNOWN_VERSION versions.
+	(riscv_parse_std_ext): Add g to the subset list, we only use it
+	to add the implicit extensions, but won't output it to arch string.
+	(riscv_parse_add_implicit_subsets): Add implicit zicsr and zifencei
+	for g extension.
+	(riscv_arch_str1): Do not output g to the arch string.
+	* elfxx-riscv.h (RISCV_UNKNOWN_VERSION): Moved to include/opcode/riscv.h.
+
 2020-12-01  Nelson Chu  <nelson.chu@sifive.com>
 
 	* elfnn-riscv.c (riscv_merge_std_ext): Updated since

bfd/elfxx-riscv.c

@@ -1173,6 +1173,7 @@ riscv_parse_add_subset (riscv_parse_subset_t *rps,
     rps->get_default_version (subset, &major_version, &minor_version);
 
   if (!implicit
+      && strcmp (subset, "g") != 0
       && (major_version == RISCV_UNKNOWN_VERSION
 	  || minor_version == RISCV_UNKNOWN_VERSION))
     {
@@ -1354,8 +1355,6 @@ riscv_parse_std_ext (riscv_parse_subset_t *rps,
 	break;
 
       case 'g':
-	/* The g-ext shouldn't has the version, so we just
-	   skip the setting if user set a version to it.  */
 	p = riscv_parsing_subset_version (rps, march, ++p,
 					  &major_version,
 					  &minor_version, TRUE);
@@ -1363,6 +1362,11 @@ riscv_parse_std_ext (riscv_parse_subset_t *rps,
 	riscv_parse_add_subset (rps, "i",
 				RISCV_UNKNOWN_VERSION,
 				RISCV_UNKNOWN_VERSION, FALSE);
+	/* g-ext is used to add the implicit extensions, but will
+	   not be output to the arch string.  */
+	riscv_parse_add_subset (rps, "g",
+				major_version,
+				minor_version, FALSE);
 	for ( ; *std_exts != 'q'; std_exts++)
 	  {
 	    subset[0] = *std_exts;
@@ -1742,6 +1746,16 @@ riscv_parse_add_implicit_subsets (riscv_parse_subset_t *rps)
     riscv_parse_add_subset (rps, "zicsr",
 			    RISCV_UNKNOWN_VERSION,
 			    RISCV_UNKNOWN_VERSION, TRUE);
+
+  if ((riscv_lookup_subset (rps->subset_list, "g", &subset)))
+    {
+      riscv_parse_add_subset (rps, "zicsr",
+			      RISCV_UNKNOWN_VERSION,
+			      RISCV_UNKNOWN_VERSION, TRUE);
+      riscv_parse_add_subset (rps, "zifencei",
+			      RISCV_UNKNOWN_VERSION,
+			      RISCV_UNKNOWN_VERSION, TRUE);
+    }
 }
 
 /* Function for parsing arch string.
@@ -1911,10 +1925,11 @@ riscv_arch_str1 (riscv_subset_t *subset,
 
   strncat (attr_str, buf, bufsz);
 
-  /* Skip 'i' extension after 'e'.  */
-  if ((strcasecmp (subset->name, "e") == 0)
-      && subset->next
-      && (strcasecmp (subset->next->name, "i") == 0))
+  /* Skip 'i' extension after 'e', and skip 'g' extension.  */
+  if (subset->next
+      && ((strcmp (subset->name, "e") == 0
+	   && strcmp (subset->next->name, "i") == 0)
+	  || strcmp (subset->next->name, "g") == 0))
     riscv_arch_str1 (subset->next->next, attr_str, buf, bufsz);
   else
     riscv_arch_str1 (subset->next, attr_str, buf, bufsz);

bfd/elfxx-riscv.h

@@ -33,8 +33,6 @@ riscv_reloc_type_lookup (bfd *, bfd_reloc_code_real_type);
 extern reloc_howto_type *
 riscv_elf_rtype_to_howto (bfd *, unsigned int r_type);
 
-#define RISCV_UNKNOWN_VERSION -1
-
 /* The information of architecture attribute.  */
 struct riscv_subset_t
 {

gas/ChangeLog

@@ -1,3 +1,10 @@
+2020-12-01  Nelson Chu  <nelson.chu@sifive.com>
+
+	* testsuite/gas/riscv/attribute-10.d: Updated.
+	* testsuite/gas/riscv/march-imply-g.d: New testcase for g.
+	* testsuite/gas/riscv/march-imply-unsupported.d: The zicsr and zifencei
+	are not supported in the ISA spec v2.2, so don't add and output them.
+
 2020-12-01  Nelson Chu  <nelson.chu@sifive.com>
 
 	* config/tc-riscv.c (riscv_subset_supports): Updated.

gas/testsuite/gas/riscv/attribute-10.d

@@ -3,4 +3,4 @@
 #source: empty.s
 Attribute Section: riscv
 File Attributes
-  Tag_RISCV_arch: "rv32i2p1_m2p0_a2p1_f2p2_d2p2_c2p0_zicsr2p0"
+  Tag_RISCV_arch: "rv32i2p1_m2p0_a2p1_f2p2_d2p2_c2p0_zicsr2p0_zifencei2p0"

gas/testsuite/gas/riscv/march-imply-g.d

@@ -0,0 +1,6 @@
+#as: -march=rv32g -march-attr -misa-spec=20191213
+#readelf: -A
+#source: empty.s
+Attribute Section: riscv
+File Attributes
+  Tag_RISCV_arch: "rv32i2p1_m2p0_a2p1_f2p2_d2p2_zicsr2p0_zifencei2p0"

gas/testsuite/gas/riscv/march-imply-unsupported.d

@@ -0,0 +1,6 @@
+#as: -march=rv32g -march-attr -misa-spec=2.2
+#readelf: -A
+#source: empty.s
+Attribute Section: riscv
+File Attributes
+  Tag_RISCV_arch: "rv32i2p0_m2p0_a2p0_f2p0_d2p0"

include/ChangeLog

@@ -1,3 +1,7 @@
+2020-12-01  Nelson Chu  <nelson.chu@sifive.com>
+
+	* opcode/riscv.h (RISCV_UNKNOWN_VERSION): added.
+
 2020-12-01  Nelson Chu  <nelson.chu@sifive.com>
 
 	* opcode/riscv.h (riscv_ext_version):

include/opcode/riscv.h

@@ -354,6 +354,8 @@ enum riscv_isa_spec_class
   ISA_SPEC_CLASS_20191213
 };
 
+#define RISCV_UNKNOWN_VERSION -1
+
 /* This structure holds version information for specific ISA.  */
 
 struct riscv_ext_version