DPACC Compiler

DPACC Compiler (PDF)

NVIDIA DOCA DPACC Compiler

This document describes DOCA DPACC Compiler and instructions about DPA toolchain setup and usage, etc.

DPACC is a high-level compiler for the DPA processor. It compiles code targeted for the DPA processor into an executable and generates a DPA program.

The DPA program is a host library with interfaces encapsulating the DPA executable. This DPA program is linked with the host application to generate a host executable. The host executable can invoke the DPA code through FlexIO runtime API.

Producing a DPA program involves steps such as input file preprocessing, validation, interface generation, compilation and linking. DPACC hides these intricate details and provides a one-step solution to enable a seamless programming experience.

DPACC uses dpa-clang to compile code targeted for DPA. dpa-clang is part of the DPA toolchain package which is an LLVM-based cross-compiling bare-metal toolchain. It provides Clang compiler, LLD linker targeting DPA architecture, and other utility tools.

1.1. Glossary

Term Definition
Device DPA simulator/hardware
Host CPU that launches the device code to run on the DPA
Device function Any C function that runs on the DPA device
Kernel Device function that is the point of entry from the host when offloading any work on DPA
Host compiler Compiler used to compile the code targeting the host CPU
Host stubs Interfaces (functions and data structures) used for argument marshalling and loading of the DPA executable
Device compiler Compiler used to compile code targeting the DPA
DPA program Host library that encapsulates the DPA device executable (ELF) and host stubs which are used to access the DPA executable with

1.2. Offloading Work on DPA

offloading.png

To offload tasks on the DPA, the following things are required:

  • DPA device code – C programs, targeted to run on the DPA. DPA device code may contain one or more kernel entry functions.
  • Host application code – the host application is responsible for initializing the device using appropriate FlexIO runtime calls and invoking kernels in the DPA executable. Kernel registration and interface with FlexIO runtime is managed by DPACC.
  • Runtime – FlexIO runtime libraries and headers supplied to DPACC through the appropriate options

The generated DPA program, when linked with host application, results in an executable containing both the host and DPA executables. Running this executable loads the DPA executable to the device memory.

1.3. Writing DPA Applications

DPA can be programmed using the FlexIO API.

DPA device code is a C code with some restrictions and special definitions.

1.3.1. Restrictions on DPA Code

Use of thread local storage is not allowed for any variables.

1.3.2. DPA RPC Functions

A remote procedure call function is a synchronous call that triggers work in DPA and waits for its completion. It is annotated with a __dpa_rpc__ attribute. For more information, please refer to NVIDIA DOCA FlexIO SDK Programming Guide.

1.3.3. DPA Kernels

A kernel function is a device function meant to be called from the host code. Kernels are annotated with a __dpa_global__ attribute.

1.3.4. Characteristics of DPA Kernels

  • Kernels cannot explicitly return a value. They must have void return type.
  • Kernels cannot accept pointers and arrays as arguments
  • Kernels cannot accept a variable number of arguments
  • Inline specifier is not allowed on kernel functions

1.3.5. Handling User-defined Data Types

User-defined data types, when used as kernel arguments, require special handling. They must be annotated with a __dpa_global__ attribute.

If the user-defined data type is typedef'd, the typedef statement must be annotated with a __dpa_global__ attribute along the data type itself.

1.3.6. Characteristics of Annotated Types

  • They must have a copy of the definition in all translation units where they are used as kernel arguments
  • They cannot have pointers, variable length arrays, and flexible arrays as members
  • Fixed-size arrays as C structure members are supported
  • These characteristics apply recursively to any user-defined/typedef'd types that are members of an annotated type

DPACC processes all kernels and annotated data structures and generates host and device interfaces to facilitate the kernel launch.

Package Instructions
Host compiler Compiler specified through hostcc option. Both gcc and clang are supported.
Device compiler Compiler which supports DPA target specified through devicecc option. The preferred device compiler is "DPA compiler". Installing DPACC package also installs DPA compiler binaries: dpa-clang, dpa-ar, dpa-nm and dpa-objdump.
FlexIO SDK and C library Available as part of the DOCA software package. DPA toolchain does not provide C library and corresponding headers. Users are expected to use the C library for DPA from the FlexIO SDK.

2.1. Supported Platforms

Architecture Operating Systems
x86_64

Ubuntu 20.04

Ubuntu 22.04

CentOS 8.2

RHEL 8.2

arm64

Ubuntu 20.04

Ubuntu 22.04


3.1. DPACC Inputs and Outputs

DPACC can produce DPA programs in a single command by accepting all source files as input.

Additionally, DPACC offers the flexibility of producing DPA object files from individual source files.

DPA object files contain both host stub objects and device objects. These DPA object files can later be given to DPACC as input to produce the DPA library.

Phase Option Name Default Output File Name
Compile input device code files to DPA object files --compile or -c .dpa.o appended to the name of each input source file
Compile and link the input device code files/DPA object files, and produce a DPA program No specific option No default name, output file name must be specified

DPACC can accept following two file types as input:

File Extension File Type Description
.c C source file DPA device code
.dpa.o DPA object file Object file generated by DPACC, containing both host and device objects

Based on the mode of operations, DPACC can generate the two following output files:

Output File Type Input Files
DPA program C source files and/or DPA object files
DPA object file C source files

3.1.1. DPA Program

DPACC produces a DPA program in compile-and-link mode. A DPA program is a library built for the host machine. This library contains a single object file which comprises of the host stubs which facilitate invoking a kernel from the host application.

Additionally, the device executable which is built from the input device code files to DPACC is embedded into a specific section inside this library. On linking this library with the host application and running the resulting executable, the FlexIO runtime along with the host stubs will load this executable onto the DPA memory.

dpacc-output.png

3.1.2. DPA Object

DPACC produces DPA object files in compile-only mode. A DPA object is an object file for the host machine. In a DPA object, the device object generated by compiling the input device code file is placed inside a specific section of the generated host stubs object. This process is repeated for each input file.

3.2. Modes of Operation

trajectory.png

This is a one-step mode that accepts C source files or DPA object files and produces the DPA program. Specifying the output library name is mandatory in this mode. Example commands:

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$ dpacc in1.c in2.c -o myLib1.a -hostcc=gcc # Takes C sources to produce myLib1.a library $ dpacc in3.dpa.o in4.dpa.o -o myLib2.a -hostcc=gcc # Takes DPA object files to produce myLib2.a library $ dpacc in1.c in3.dpa.o -o myLib3.a -hostcc=gcc # Takes C source and DPA object to produce myLib3.a library

3.2.2. Compile-only Mode

This mode accepts C source code and produces .dpa.o object files. These files can be given to DPACC to produce the DPA program. The mode is invoked by the --compile or -c option.

The user can explicitly provide the output object file name using the --output-file or -o option. Example commands:

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$ dpacc -c input1.c -hostcc=gcc # Produces input1.dpa.o $ dpacc -c input3.c input4.c -hostcc=gcc # Produces input3.dpa.o and input4.dpa.o $ dpacc -c input2.c -o myObj.dpa.o -hostcc=gcc # Produces myObj.dpa.o

To execute DOCA DPACC Compiler:

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Usage: dpacc <list-of-input-files> -hostcc=<path> [other options] Helper Flags: -h, --help Print help information about DPACC -V, --version Print DPACC version information -v, --verbose List the compilation commands generated by this invocation while also executing every command in verbose mode -dryrun, --dryrun Only list the compilation commands generated by DPACC, without executing them -keep, --keep Keep all intermediate files that are generated during internal compilation steps in the current directory -keep-dir, --keep-dir Keep all intermediate files that are generated during internal compilation steps in the given directory -optf, --options-file <file>,... Include command line options from the specified file

4.1. Mandatory Arguments

Flag DPACC Mode Description
List of one or more input files All List of C source files or DPA object file names. Specifying at least one input file is mandatory. A file with an unknown extension is treated as a DPA object file.
-hostcc, --hostcc <path> All Specify the host compiler. This is typically the native compiler present on the host system.
-o, --output-file <file> Compile-and-link Specify name and location of the output archive (DPA program)

4.2. Commonly Used Arguments

Flag Description
-devicecc-options, --devicecc-options <options>,... Specify the list of options to pass to the device compiler
-devicelink-options, --devicelink-options <options>,... Specify the list of options to pass during device linking stage. Typically, these include FlexIO libraries and DPA linker scripts.
-I, --common-include-path <path>,... Specify include search paths common to host and device code compilation. Typically, these are FlexIO headers.
-devicecc, --devicecc <path> Specify the device compiler. By default, DPACC invokes dpa-clang.
-o, --output-file <file>

Specify name and location of the output file.

  • Compile-only mode: Name of the output DPA object file. If not specified, .dpa.o is generated for each .c file.
  • Compiler-and-link mode: Name of the output archive. This is a mandatory option in compiler-and-link mode
-hostcc-options, --hostcc-options <options>,... Specify the list of options to pass to the host compiler

4.3. Incorrect Usage

  • The devicecc-options option allows passing any option to the device compiler. However, passing options that prevent compilation of the input file may lead to unexpected behavior.

    For example: -devicecc-options="-version" makes the device compiler print the version and not process input files.

  • Incompatible options which affect the kernel argument sizes during DPACC invocation and host application compilation may lead to undefined behavior during execution

    For example: Passing -hostcc-options="-fshort-enums" to DPACC and missing this option when building the host application

4.4. Examples

This section provides some common use cases of DPACC and showcases the dpacc command.

This example specifies the names and paths of the libraries using devicelink-options:

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dpacc input.c -hostcc=gcc -o libInput.a -devicelink-options="-L <path-to-library> -l<libName>"

4.4.2. Include Headers

This example includes headers for device compilation using devicecc-options and host compilation using hostcc-options. You can also specify headers for any compilation on both the host and device side using the -I option.

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dpacc input.c -hostcc=gcc -o libInput.a -I <common-headers-path> -devicecc-options="-I <device-headers-path>" -hostcc-options="-I <host-headers-path>"

4.5. DPA Compiler Usage

dpa-clang is a compiler driver for accessing the Clang/LLVM compiler, assembler, and linker. The user is expected to invoke these tools only using the dpa-clang compiler driver.

dpa-clang is also a compiler frontend for C language.

Refer to the following resources for detailed user guide and command line references:

4.5.1. Compiler Driver Command Line Options

DPA compiler provides a Clang compiler binary, dpa-clang. It accepts C code files or object files and generates an output according to different usage modes.

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dpa-clang <list-of-input-files> [other options]

4.5.2. Linker Command Line Options

Note:

Link time optimization (LTO) is not supported by DPA toolchain LLD.

LLD is the default linker provided in the DPA toolchain. Linker is invoked through the compiler driver binary,dpa-clang. Invoking the linker directly may lead to unexpected errors.

Linker related options are passed to through the compiler driver.

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dpa-clang <list-of-input-files> [other options]


For more information, please refer to the LLD command line reference.

4.5.3. Objdump Command Line Options

The dpa-objdump utility prints the contents of object files and final linked images named on the command line.

For more information, please refer to the Objdump command line reference.

4.5.4. Archiver Command Line Options

dpa-ar is a Unix ar compatible archiver.

For more information, please refer to the Archiver command line reference.

4.5.5. NM Tool Command Line Options

The dpa-nm utility lists the names of symbols from object files, and archives.

For more information, please refer to the NM tool command line reference.

4.5.6. Common Compiler Options

Flag Description
-mcpu=nv-dpa-bf3 Option to choose micro-architecture and ABI for DPA processor. This is the default option.
-mrelax/-mno-relax Option to enable/disable linker relaxations
-isystem <dir> Option to include Libc header files present in <dir> in FlexIO SDK as system headers
-I <dir> Option to include header files present in <dir>

4.5.7. Common Linker Options

Flag Description
-L <path-to-library> -l<library-name> Option to link against libraries


Note:

Linker options are provided through the compiler driver dpa-clang.

Note:

The LLD linker script is honored in addition to the default configuration rather than replacing the whole configuration like in GNU ld. Hence, additional options may be required to override some default behaviors.

4.5.8. Debugging Options

Flag Description
-fdebug-macro Option to emit macro debugging information. This option enables macro-debugging similar to GCC option -g3.

4.5.9. Miscellaneous Notes

  • DPA Toolchain provides the ability to invoke GNU linker instead of the default LLD linker via --fuse-ld=<arg> option in command line reference. However, it is discouraged to do so and not tested.
  • Objects produced by LLD are not compatible with those generated by any other linker.
  • DPA processor does not have native floating-point support and dpa-clang generates software emulated routines for floating point operations. Note that using floating point operations will have severe performance impact. Code generation for these routines will disabled by default in the next release with a command line option to enable it if needed.

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