RISC-V: Support STO_RISCV_VARIANT_CC and DT_RISCV_VARIANT_CC.

This is the original discussion,
https://github.com/riscv/riscv-elf-psabi-doc/pull/190

And here is the glibc part,
https://sourceware.org/pipermail/libc-alpha/2021-August/129931.html

For binutils part, we need to support a new direcitve: .variant_cc.
The function symbol marked by .variant_cc means it need to be resolved
directly without resolver for dynamic linker.  We also add a new dynamic
entry, STO_RISCV_VARIANT_CC, to indicate there are symbols with the
special attribute in the dynamic symbol table of the object.

I heard that llvm already have supported this in their mainline, so
I think it's time to commit this.

bfd/
	* elfnn-riscv.c (riscv_elf_link_hash_table): Added variant_cc
	flag. It is used to check if relocations for variant CC symbols
	may be present.
	(allocate_dynrelocs): If the symbol has STO_RISCV_VARIANT_CC
	flag, then raise the variant_cc flag of riscv_elf_link_hash_table.
	(riscv_elf_size_dynamic_sections): Added dynamic entry for
	variant_cc.
	(riscv_elf_merge_symbol_attribute): New function, used to merge
	non-visibility st_other attributes, including STO_RISCV_VARIANT_CC.
binutils/
	* readelf.c (get_riscv_dynamic_type): New function.
	(get_dynamic_type): Called get_riscv_dynamic_type for riscv targets.
	(get_riscv_symbol_other): New function.
	(get_symbol_other): Called get_riscv_symbol_other for riscv targets.
gas/
	* config/tc-riscv.c (s_variant_cc): Marked symbol that it follows a
	variant CC convention.
	(riscv_elf_copy_symbol_attributes): Same as elf_copy_symbol_attributes,
	but without copying st_other.  If a function symbol has special st_other
	value set via directives, then attaching an IFUNC resolver to that symbol
	should not override the st_other setting.
	(riscv_pseudo_table): Support variant_cc diretive.
	* config/tc-riscv.h (OBJ_COPY_SYMBOL_ATTRIBUTES): Defined.
	* testsuite/gas/riscv/variant_cc-set.d: New testcase.
	* testsuite/gas/riscv/variant_cc-set.s: Likewise.
	* testsuite/gas/riscv/variant_cc.d: Likewise.
	* testsuite/gas/riscv/variant_cc.s: Likewise.
include/
	* elf/riscv.h (DT_RISCV_VARIANT_CC): Defined to (DT_LOPROC + 1).
	(STO_RISCV_VARIANT_CC): Defined to 0x80.
ld/
	* testsuite/ld-riscv-elf/variant_cc-1.s: New testcase.
	* testsuite/ld-riscv-elf/variant_cc-2.s: Likewise.
	* testsuite/ld-riscv-elf/variant_cc-now.d: Likewise.
	* testsuite/ld-riscv-elf/variant_cc-r.d: Likewise.
	* testsuite/ld-riscv-elf/variant_cc-shared.d: Likewise.
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.

1 files changed, 43 insertions, 1 deletions

diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
index 36cbf1e..d8c9066 100644
--- a/bfd/elfnn-riscv.c
+++ b/bfd/elfnn-riscv.c
@@ -134,6 +134,9 @@ struct riscv_elf_link_hash_table
   /* The data segment phase, don't relax the section
      when it is exp_seg_relro_adjust.  */
   int *data_segment_phase;
+
+  /* Relocations for variant CC symbols may be present.  */
+  int variant_cc;
 };
 
 /* Instruction access functions. */
@@ -1172,6 +1175,11 @@ allocate_dynrelocs (struct elf_link_hash_entry *h, void *inf)
 	      h->root.u.def.section = s;
 	      h->root.u.def.value = h->plt.offset;
 	    }
+
+	  /* If the symbol has STO_RISCV_VARIANT_CC flag, then raise the
+	     variant_cc flag of riscv_elf_link_hash_table.  */
+	  if (h->other & STO_RISCV_VARIANT_CC)
+	    htab->variant_cc = 1;
 	}
       else
 	{
@@ -1555,7 +1563,18 @@ riscv_elf_size_dynamic_sections (bfd *output_bfd, struct bfd_link_info *info)
 	return false;
     }
 
-  return _bfd_elf_add_dynamic_tags (output_bfd, info, true);
+  /* Add dynamic entries.  */
+  if (elf_hash_table (info)->dynamic_sections_created)
+    {
+      if (!_bfd_elf_add_dynamic_tags (output_bfd, info, true))
+	return false;
+
+      if (htab->variant_cc
+	  && !_bfd_elf_add_dynamic_entry (info, DT_RISCV_VARIANT_CC, 0))
+       return false;
+    }
+
+  return true;
 }
 
 #define TP_OFFSET 0
@@ -5227,6 +5246,28 @@ riscv_elf_modify_segment_map (bfd *abfd,
   return true;
 }
 
+/* Merge non-visibility st_other attributes.  */
+
+static void
+riscv_elf_merge_symbol_attribute (struct elf_link_hash_entry *h,
+				  unsigned int st_other,
+				  bool definition ATTRIBUTE_UNUSED,
+				  bool dynamic ATTRIBUTE_UNUSED)
+{
+  unsigned int isym_sto = st_other & ~ELF_ST_VISIBILITY (-1);
+  unsigned int h_sto = h->other & ~ELF_ST_VISIBILITY (-1);
+
+  if (isym_sto == h_sto)
+    return;
+
+  if (isym_sto & ~STO_RISCV_VARIANT_CC)
+    _bfd_error_handler (_("unknown attribute for symbol `%s': 0x%02x"),
+			h->root.root.string, isym_sto);
+
+  if (isym_sto & STO_RISCV_VARIANT_CC)
+    h->other |= STO_RISCV_VARIANT_CC;
+}
+
 #define TARGET_LITTLE_SYM			riscv_elfNN_vec
 #define TARGET_LITTLE_NAME			"elfNN-littleriscv"
 #define TARGET_BIG_SYM				riscv_elfNN_be_vec
@@ -5263,6 +5304,7 @@ riscv_elf_modify_segment_map (bfd *abfd,
 #define elf_backend_additional_program_headers \
   riscv_elf_additional_program_headers
 #define elf_backend_modify_segment_map		riscv_elf_modify_segment_map
+#define elf_backend_merge_symbol_attribute	riscv_elf_merge_symbol_attribute
 
 #define elf_backend_init_index_section		_bfd_elf_init_1_index_section