xref: /aosp_15_r20/external/llvm/docs/AMDGPUUsage.rst (revision 9880d6810fe72a1726cb53787c6711e909410d58)

==============================
User Guide for AMDGPU Back-end
==============================

Introduction
============

The AMDGPU back-end provides ISA code generation for AMD GPUs, starting with
the R600 family up until the current Volcanic Islands (GCN Gen 3).


Conventions
===========

Address Spaces
--------------

The AMDGPU back-end uses the following address space mapping:

   ============= ============================================
   Address Space Memory Space
   ============= ============================================
   0             Private
   1             Global
   2             Constant
   3             Local
   4             Generic (Flat)
   5             Region
   ============= ============================================

The terminology in the table, aside from the region memory space, is from the
OpenCL standard.


Assembler
=========

The assembler is currently considered experimental.

For syntax examples look in test/MC/AMDGPU.

Below some of the currently supported features (modulo bugs).  These
all apply to the Southern Islands ISA, Sea Islands and Volcanic Islands
are also supported but may be missing some instructions and have more bugs:

DS Instructions
---------------
All DS instructions are supported.

FLAT Instructions
------------------
These instructions are only present in the Sea Islands and Volcanic Islands
instruction set.  All FLAT instructions are supported for these architectures

MUBUF Instructions
------------------
All non-atomic MUBUF instructions are supported.

SMRD Instructions
-----------------
Only the s_load_dword* SMRD instructions are supported.

SOP1 Instructions
-----------------
All SOP1 instructions are supported.

SOP2 Instructions
-----------------
All SOP2 instructions are supported.

SOPC Instructions
-----------------
All SOPC instructions are supported.

SOPP Instructions
-----------------

Unless otherwise mentioned, all SOPP instructions that have one or more
operands accept integer operands only.  No verification is performed
on the operands, so it is up to the programmer to be familiar with the
range or acceptable values.

s_waitcnt
^^^^^^^^^

s_waitcnt accepts named arguments to specify which memory counter(s) to
wait for.

.. code-block:: nasm

   ; Wait for all counters to be 0
   s_waitcnt 0

   ; Equivalent to s_waitcnt 0.  Counter names can also be delimited by
   ; '&' or ','.
   s_waitcnt vmcnt(0) expcnt(0) lgkcmt(0)

   ; Wait for vmcnt counter to be 1.
   s_waitcnt vmcnt(1)

VOP1, VOP2, VOP3, VOPC Instructions
-----------------------------------

All 32-bit and 64-bit encodings should work.

The assembler will automatically detect which encoding size to use for
VOP1, VOP2, and VOPC instructions based on the operands.  If you want to force
a specific encoding size, you can add an _e32 (for 32-bit encoding) or
_e64 (for 64-bit encoding) suffix to the instruction.  Most, but not all
instructions support an explicit suffix.  These are all valid assembly
strings:

.. code-block:: nasm

   v_mul_i32_i24 v1, v2, v3
   v_mul_i32_i24_e32 v1, v2, v3
   v_mul_i32_i24_e64 v1, v2, v3

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.  This value will be stored
in an entry of the .note section.

.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".

If no arguments are specified, then the assembler will derive the ISA version,
*vendor*, and *arch* from the value of the -mcpu option that is passed to the
assembler.

ISA version, *vendor*, and *arch* will all be stored in a single entry of the
.note section.

.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, see the examples in
test/CodeGen/AMDGPU/hsa.s.  For an explanation of the meanings of the different
keys, see the comments in lib/Target/AMDGPU/AmdKernelCodeT.h

Here is an example of a minimal amd_kernel_code_t specification:

.. code-block:: nasm

   .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