Define new tag for atomic ABI.

Alternative version of #383

Defined 4 atomic abi variant.

- unknown, nonatomic, A6C and A6S

Unknown is compatilbe with exising object file.
Nonatomic is used when A-extension is absent
A6C is the table A.6 mapping defined in ISA spec.
And the A6S is the mapping defined in psABI spec.

riscv-elf.adoc

@@ -194,9 +194,9 @@ below.
 [cols="1,2,1,1,3,5"]
 [width=80%]
 |===
-| Bit 0 | Bits 1 - 2 | Bit 3 | Bit 4 | Bits 5 - 23 | Bits 24 - 31
+| Bit 0 | Bits 1 - 2 | Bit 3 | Bit 4 - 6 | Bits 7 - 23 | Bits 24 - 31
 
-| RVC   | Float ABI  | RVE   | TSO   | *Reserved*  | *Non-standard extensions*
+| RVC   | Float ABI  | RVE   | Atomic ABI | *Reserved*  | *Non-standard extensions*
 |===
 
 +
@@ -231,8 +231,20 @@ below.
 [[EF_RISCV_RVE]]
   EF_RISCV_RVE (0x0008)::: This bit is set when the binary targets the E ABI.
 
-  EF_RISCV_TSO (0x0010)::: This bit is set when the binary requires the RVTSO
-  memory consistency model.
+  EF_RISCV_ATOMIC_ABI_UNKNOWN (0x0000):::
+  EF_RISCV_ATOMIC_ABI_TSO (0x0010):::
+  EF_RISCV_ATOMIC_ABI_A6C (0x0020):::
+  EF_RISCV_ATOMIC_ABI_A6S (0x0030):::
+  These flags identify the atomic ABI in use for this ELF file. For detailed
+  mapping, refer to the _RISC-V atomics mappings_, 0x0040, 0x0050, 0x0060 and
+  0x0070 are reserved for future atomic ABIs.
+
+  EF_RISCV_ATOMIC_ABI (0x0070)::: This macro is used as a mask to test for one
+  of the above floating-point ABIs, e.g.,
+  `(e_flags & EF_RISCV_ATOMIC_ABI) == EF_RISCV_ATOMIC_ABI_TSO`.
+
+  EF_RISCV_TSO (0x0010)::: Legacy name for EF_RISCV_ATOMIC_ABI_TSO, the behavior
+  is compatible with EF_RISCV_ATOMIC_ABI_TSO.
 
 Until such a time that the *Reserved* bits (0x00ffffe0) are allocated by future
 versions of this specification, they shall not be set by standard software.
@@ -266,8 +278,12 @@ raise an error.
   RVE::: Linker should report errors if object files of different value
   for RVE field.
 
-  TSO::: Linker should report errors if object files of different value
-  for TSO field.
+  Atomic ABI:::  The linker should report errors if the object files have
+  incompatible values for atomic ABI fields. The compatibility rules are as
+  follows:
+  EF_RISCV_ATOMIC_ABI_UNKNOWN is compatible with EF_RISCV_ATOMIC_ABI_A6C and
+  EF_RISCV_ATOMIC_ABI_A6S; EF_RISCV_ATOMIC_ABI_A6C is compatible with
+  EF_RISCV_ATOMIC_ABI_A6S.
 
 NOTE: The static linker may ignore the compatibility checks if all fields in the
 `e_flags` are zero and all sections in the input file are non-executable
@@ -1072,6 +1088,7 @@ non-standard ABI extensions.
 | Tag_RISCV_priv_spec                 |        8 | uleb128        | *Deprecated*, indicates the major version of the privileged specification.
 | Tag_RISCV_priv_spec_minor           |       10 | uleb128        | *Deprecated*, indicates the minor version of the privileged specification.
 | Tag_RISCV_priv_spec_revision        |       12 | uleb128        | *Deprecated*, indicates the revision version of the privileged specification.
+| Tag_RISCV_atomic_abi                |       14 | uleb128        | Indicates for the atomic ABI.
 | Reserved for non-standard attribute | >= 32768 | -              | -
 |===
 
@@ -1176,6 +1193,27 @@ Merge policy:::
 The linker should report errors if object files of different privileged
 specification versions are merged.
 
+===== Tag_RISCV_atomic_abi, 14, uleb128=version
+
+Tag_RISCV_atomic_abi denotes the atomic for this object
+file. Its values are defined as follows:
+
+.RISC-V attributes
+[cols="4,>2,2,5"]
+[width=100%]
+|===
+| Value | Symbolic Name       | Compatibility                        | Description
+
+| 0     | UNKNOWN_ATOMIC_ABI  | Compatible with all other value.     | This object use unknown atomic ABI.
+| 1     | NON_ATOMIC_ABI      | Incompatible with all other atomic ABI other than unknown. | Atomic opertiaon all using plain load store operaion, this ABI should only used when A-extension is absent.
+| 2     | A6C_ATOMIC_ABI      | Compatible with unknown and A6S_ATOMIC_ABI. | This object use the A6 classical atomic ABI, which defined in table A.6 in <<riscv-unpriv-20191213>>.
+| 3     | A6S_ATOMIC_ABI      | Compatible with unknown and A6C_ATOMIC_ABI. | This object use the strengthened A6 ABI, which defined in <<Mappings from C/C++ primitives to RISC-V primitives>>.
+|===
+
+Merge policy:::
+The linker should report errors if object files of incompatible atomic ABI
+beginning merged, the compatibility rules for atomic ABIs can be found in
+compatibility column in the above table.
 
 === Mapping Symbol
 
@@ -1635,5 +1673,9 @@ https://www.akkadia.org/drepper/tls.pdf, Ulrich Drepper
 ISA, Document", Editors Andrew Waterman and Krste Asanovi´c,
 RISC-V International.
 
+* [[[riscv-unpriv-20191213]]] "The RISC-V Instruction Set Manual, Volume I: User-Level
+ISA, Document release 20191213", Editors Andrew Waterman and Krste Asanovi´c,
+RISC-V International.
+
 * [[[riscv-zc-extension-group]]] "ZC* extension specification"
 https://github.com/riscv/riscv-code-size-reduction