llvm_ir_to_executable

HIP-Basic LLVM-IR to Executable Example

Description

This example shows how to manually compile and link a HIP application from device LLVM IR. Pre-generated LLVM-IR files are compiled into an offload bundle, a bundle of device object files, and then linked with the host object file to produce the final executable.

LLVM IR is the intermediary language used by the LLVM compiler, which hipcc is built on. Building HIP executables from LLVM IR can be useful for example to experiment with specific LLVM instructions, or can help debugging miscompilations.

Building

• Build with Makefile: to compile for specific GPU architectures, optionally provide the HIP_ARCHITECTURES variable. Provide the architectures separated by comma.

  make HIP_ARCHITECTURES="gfx803;gfx900;gfx906;gfx908;gfx90a;gfx1030;gfx1100;gfx1101;gfx1102

• Build with CMake:

  cmake -S . -B build -DCMAKE_HIP_ARCHITECTURES="gfx803;gfx900;gfx906;gfx908;gfx90a;gfx1030;gfx1100;gfx1101;gfx1102"
  cmake --build build

  On Windows the path to RC compiler may be needed: -DCMAKE_RC_COMPILER="C:/Program Files (x86)/Windows Kits/path/to/x64/rc.exe"

Generating device LLVM IR

In this example, a HIP executable is compiled from device LLVM IR code. While LLVM IR can be written completely manually, is it not advisable to do so because it is unstable between LLVM versions. Instead, in this example it is generated from main.hip, using the following commands:

$ROCM_INSTALL_DIR/bin/hipcc --cuda-device-only -c -emit-llvm ./main.hip --offload-arch=<arch> -o main_<arch>.bc -I ../../Common -std=c++17
$ROCM_INSTALL_DIR/llvm/bin/llvm-dis main_<arch>.bc -o main_<arch>.ll

Where <arch> is the architecture to generate the LLVM IR for. Note that the --cuda-device-only flag is required to instruct hipcc to only generate LLVM IR for the device part of the computation, and -c is required to prevent the compiler from linking the outputs into an executable. In the case of this example, the LLVM IR files where generated using architectures gfx803, gfx900, gfx906, gfx908, gfx90a, gfx1030, gfx1100, gfx1101, gfx1102. The user may modify the --offload-arch flag to build for other architectures and choose to either enable or disable extra device code-generation features such as xnack or sram-ecc, which can be specified as --offload-arch=<arch>:<feature>+ to enable it or --offload-arch=<arch>:<feature>- to disable it. Multiple features may be present, separated by colons.

The first of these two commands generates a bitcode module: this is a binary encoded version of LLVM IR. The second command, using llvm-dis disassembles the bitcode module into textual LLVM IR.

Build Process

A HIP binary consists of a regular host executable, which has an offload bundle containing device code embedded inside it. This offload bundle contains object files for each of the target devices that it is compiled for, and is loaded at runtime to provide the machine code for the current device. A HIP executable can be built from device LLVM IR and host HIP code according to the following process:

1. The main.hip file is compiled to an object file with hipcc that only contains host code by using the --cuda-host-only option. main.hip is a program that launches a simple kernel to compute the square of each element of a vector. The -c option is required to prevent the compiler from creating an executable, and make it create an object file containing the compiled host code instead.

   $ROCM_INSTALL_DIR/bin/hipcc -c --cuda-host-only main.hip

2. Each LLVM IR file is assembled to a device object file using clang. This requires specifying the correct architecture using -target amdgcn-amd-amdhsa, and the target architecture that should be assembled for using -mcpu:

   $ROCM_INSTALL_DIR/llvm/bin/clang -target amdgcn-amd-amdhsa -mcpu=gfx1030 main_gfx1030.ll -o main_gfx1030.o
   $ROCM_INSTALL_DIR/llvm/bin/clang -target amdgcn-amd-amdhsa -mcpu=<arch> main_<arch>.ll -o main_<arch>.o
   ...

3. The device object files are combined into an offload bundle using clang-offload-bundler. This requires specifying the target as well as the offload kind for each device, in the form <offload-kind>-<target>-<arch>. For HIP device code, <offload-kind> is hipv4. Note that this command requires an (empty) entry for the host to also be present, with <offload-kind> host. The order of targets and inputs must match. <target> is an LLVM target triple, which is specified as <isa>-<vendor>-<os>-<abi>. <abi> is left empty for AMD targets.

   $ROCM_INSTALL_DIR/llvm/bin/clang-offload-bundler -type=o -bundle-align=4096 \
           -targets=host-x86_64-unknown-linux,hipv4-amdgcn-amd-amdhsa--gfx1030,hipv4-... \
           -input=/dev/null \
           -input=main_gfx1030.o -input=... \
           -output=offload_bundle.hipfb

   Note: using -bundle-align=4096 only works on ROCm 4.0 and newer compilers. Also, the architecture must match the same --offload-arch as when compiling the source to LLVM bitcode.

4. The offload bundle is embedded inside an object file that can be linked with the object file containing the host code. The offload bundle must be placed in the .hip_fatbin section, and must be placed after the symbol __hip_fatbin. This can be done by creating an assembly file that places the offload bundle in the appropriate section using the .incbin directive:

       .type __hip_fatbin,@object
       ; Tell the assembler to place the offload bundle in the appropriate section.
       .section .hip_fatbin,"a",@progbits
       ; Make the symbol that addresses the binary public
       .globl __hip_fatbin
       ; Give the bundle the required alignment
       .p2align 12
   __hip_fatbin:
       ; Include the binary
       .incbin "offload_bundle.hipfb"

   This file can then be assembled using llvm-mc as follows:

   $ROCM_INSTALL_DIR/llvm/bin/llvm-mc -triple <host target> -o main_device.o hip_obj_gen.mcin --filetype=obj

5. Finally, using the system linker, hipcc, or clang, the host object and device objects are linked into an executable:

   <ROCM_PATH>/hip/bin/hipcc -o hip_llvm_ir_to_executable main.o main_device.o

Visual Studio 2019

The above compilation steps are implemented in Visual Studio through Custom Build Steps:

• Specifying that only host compilation should be done, is achieved by adding extra options to the source file, under main.hip -> properties -> C/C++ -> Command Line:

  Additional Options: --cuda-host-only
  

• Specifying how the LLVM IR and the offload bundle are generated, is done with a custom build step:

  Command Line:
      FOR %%a in ($(OffloadArch)) DO "$(ClangToolPath)clang++" --cuda-device-only -c -emit-llvm main.hip --offload-arch=%%a -o "$(IntDir)main_%%a.bc" -I ../../Common -std=c++17
      FOR %%a in ($(OffloadArch)) DO "$(ClangToolPath)llvm-dis" "$(IntDir)main_%%a.bc" -o "$(IntDir)main_%%a.ll"
      FOR %%a in ($(OffloadArch)) DO "$(ClangToolPath)clang++" -target amdgcn-amd-amdhsa -mcpu=%%a "$(IntDir)main_%%a.ll" -o "$(IntDir)main_%%a.o"
      SET TARGETS=host-x86_64-unknown-linux
      SET INPUTS=-input=nul
      SETLOCAL ENABLEDELAYEDEXPANSION
      FOR %%a in ($(OffloadArch)) DO SET TARGETS=!TARGETS!,hipv4-amdgcn-amd-amdhsa--%%a& SET INPUTS=!INPUTS! -input="$(IntDir)main_%%a.o"
      "$(ClangToolPath)clang-offload-bundler" -type=o -bundle-align=4096 -targets=%TARGETS%  %INPUTS% -output="$(IntDir)offload_bundle.hipfb"
      cd "$(IntDir)" & "$(ClangToolPath)llvm-mc" -triple host-x86_64-pc-windows-msvc hip_obj_gen_win.mcin -o main_device.obj --filetype=obj
  Description: Generating Device Offload Object
  Outputs: $(IntDIr)main_device.obj
  Additional Dependencies: main.hip;$(IntDir)hip_obj_gen_win.mcin
  Execute Before: ClCompile
  

• Finally, the linking step is described by passing additional inputs to the linker in project -> properties -> Linker -> Input:

  Additional Dependencies: $(IntDir)main_device.obj;%(AdditionalDependencies)
  

Used API surface

HIP runtime

• hipFree
• hipGetDeviceProperties
• hipGetLastError
• hipMalloc
• hipMemcpy