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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).

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.

// 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:

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:

.hsa_code_object_version 1,0
.hsa_code_object_isa

.text

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 v0, v[1:2]
  s_endpgm