AMDHSA: Put old assembler docs back

[kzhuravl]

Summary

Until we switch to code object v3 by default. Follow up for https://reviews.llvm.org/D47736.

[scott.linder]

LGTM

[artem.tamazov]

LGTM, thanks.

docs/AMDGPUUsage.rst

.. code-block:: nasm
   v_mov_b32 v1, v2 dst_sel:BYTE_0 dst_unused:UNUSED_PRESERVE src0_sel:DWORD
   v_min_u32 v200, v200, v1 dst_sel:WORD_1 dst_unused:UNUSED_PAD src0_sel:BYTE_1 src1_sel:DWORD
   v_sin_f32 v0, v0 dst_unused:UNUSED_PAD src0_sel:WORD_1
   v_fract_f32 v0, |v0| dst_sel:DWORD dst_unused:UNUSED_PAD src0_sel:WORD_1
   v_cmpx_le_u32 vcc, v1, v2 src0_sel:BYTE_2 src1_sel:WORD_0
 
 For full list of supported instructions, refer to "Vector ALU instructions".
 
-Predefined Symbols
-~~~~~~~~~~~~~~~~~~
+.. TODO
+   Remove once we switch to code object v3 by default.
+
+HSA Code Object Directives
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+AMDGPU ABI defines auxiliary data in output code object. In assembly source,
+one can specify them with assembler directives.
+
+.hsa_code_object_version major, minor
++++++++++++++++++++++++++++++++++++++
+
+*major* and *minor* are integers that specify the version of the HSA code
+object that will be generated by the assembler.
+
+.hsa_code_object_isa [major, minor, stepping, vendor, arch]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+
+*major*, *minor*, and *stepping* are all integers that describe the instruction
+set architecture (ISA) version of the assembly program.
+
+*vendor* and *arch* are quoted strings.  *vendor* should always be equal to
+"AMD" and *arch* should always be equal to "AMDGPU".
+
+By default, the assembler will derive the ISA version, *vendor*, and *arch*
+from the value of the -mcpu option that is passed to the assembler.
+
+.amdgpu_hsa_kernel (name)
++++++++++++++++++++++++++
+
+This directives specifies that the symbol with given name is a kernel entry point
+(label) and the object should contain corresponding symbol of type STT_AMDGPU_HSA_KERNEL.
+
+.amd_kernel_code_t
+++++++++++++++++++
+
+This directive marks the beginning of a list of key / value pairs that are used
+to specify the amd_kernel_code_t object that will be emitted by the assembler.
+The list must be terminated by the *.end_amd_kernel_code_t* directive.  For
+any amd_kernel_code_t values that are unspecified a default value will be
+used.  The default value for all keys is 0, with the following exceptions:
+
+- *kernel_code_version_major* defaults to 1.
+- *machine_kind* defaults to 1.
+- *machine_version_major*, *machine_version_minor*, and
+  *machine_version_stepping* are derived from the value of the -mcpu option
+  that is passed to the assembler.
+- *kernel_code_entry_byte_offset* defaults to 256.
+- *wavefront_size* defaults to 6.
+- *kernarg_segment_alignment*, *group_segment_alignment*, and
+  *private_segment_alignment* default to 4. Note that alignments are specified
+  as a power of two, so a value of **n** means an alignment of 2^ **n**.
+
+The *.amd_kernel_code_t* directive must be placed immediately after the
+function label and before any instructions.
+
+For a full list of amd_kernel_code_t keys, refer to AMDGPU ABI document,
+comments in lib/Target/AMDGPU/AmdKernelCodeT.h and test/CodeGen/AMDGPU/hsa.s.
+
+Here is an example of a minimal amd_kernel_code_t specification:
+
+.. code-block:: none
+
+   .hsa_code_object_version 1,0
+   .hsa_code_object_isa
+
+   .hsatext
+   .globl  hello_world
+   .p2align 8
+   .amdgpu_hsa_kernel hello_world
+
+   hello_world:
+
+      .amd_kernel_code_t
+         enable_sgpr_kernarg_segment_ptr = 1
+         is_ptr64 = 1
+         compute_pgm_rsrc1_vgprs = 0
+         compute_pgm_rsrc1_sgprs = 0
+         compute_pgm_rsrc2_user_sgpr = 2
+         kernarg_segment_byte_size = 8
+         wavefront_sgpr_count = 2
+         workitem_vgpr_count = 3
+     .end_amd_kernel_code_t
+
+     s_load_dwordx2 s[0:1], s[0:1] 0x0
+     v_mov_b32 v0, 3.14159
+     s_waitcnt lgkmcnt(0)
+     v_mov_b32 v1, s0
+     v_mov_b32 v2, s1
+     flat_store_dword v[1:2], v0
+     s_endpgm
+   .Lfunc_end0:
+        .size   hello_world, .Lfunc_end0-hello_world
+
+Predefined Symbols (-mattr=+code-object-v3)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The AMDGPU assembler defines and updates some symbols automatically. These
 symbols do not affect code generation.
 
 .amdgcn.gfx_generation_number
 +++++++++++++++++++++++++++++
 
 Set to the GFX generation number of the target being assembled for. For

 referenced within that instruction then the symbol value is updated to equal
 that SGPR number plus one.
 
 May be used to set the `.amdhsa_next_free_spgr` directive in
 :ref:`amdhsa-kernel-directives-table`.
 
 May be set at any time, e.g. manually set to zero at the start of each kernel.
 
-Code Object Directives
-~~~~~~~~~~~~~~~~~~~~~~
+Code Object Directives (-mattr=+code-object-v3)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Directives which begin with ``.amdgcn`` are valid for all ``amdgcn``
 architecture processors, and are not OS-specific. Directives which begin with
 ``.amdhsa`` are specific to ``amdgcn`` architecture processors when the
 ``amdhsa`` OS is specified. See :ref:`amdgpu-target-triples` and
 :ref:`amdgpu-processors`.
 
 .amdgcn_target <target>
.. table:: AMDHSA Kernel Assembler Directives
      ``.amdhsa_exception_fp_ieee_underflow``                  0                GFX6-GFX9    Controls ENABLE_EXCEPTION_IEEE_754_FP_UNDERFLOW in
                                                                                             :ref:`amdgpu-amdhsa-compute_pgm_rsrc2-gfx6-gfx9-table`.
      ``.amdhsa_exception_fp_ieee_inexact``                    0                GFX6-GFX9    Controls ENABLE_EXCEPTION_IEEE_754_FP_INEXACT in
                                                                                             :ref:`amdgpu-amdhsa-compute_pgm_rsrc2-gfx6-gfx9-table`.
      ``.amdhsa_exception_int_div_zero``                       0                GFX6-GFX9    Controls ENABLE_EXCEPTION_INT_DIVIDE_BY_ZERO in
                                                                                             :ref:`amdgpu-amdhsa-compute_pgm_rsrc2-gfx6-gfx9-table`.
      ======================================================== ================ ============ ===================
 
-Example HSA Source Code
-~~~~~~~~~~~~~~~~~~~~~~~
+Example HSA Source Code (-mattr=+code-object-v3)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Here is an example of a minimal assembly source file, defining one HSA kernel:
 
 .. code-block:: nasm
 
   .amdgcn_target "amdgcn-amd-amdhsa--gfx900+xnack" // optional
 
   .text