Hybridthreads Compiler

Fabrice Baijot

Jim Stevens

What we will talk about

• Give an overview of the current state of affairs in the Hybridthreads Compiler project.

• Answer the following questions:– Why are you writing a compiler?

– How do you represent C in hardware?

– What is HIF?

– How does the compilation process work?

Hthreads System

• OS kernel that runs on hybrid CPU/FPGAs

• Allows for creation of threads in both software and hardware.

• Hardware threads are custom hardware implementations of threads.

• Assume everyone here knows enough about Hthreads for us to go on.

Hthreads Diagram

HWTI

• Provides primitives that allow hardware threads to communicate with the rest of the system (read, write, pthread_mutex_lock)

• Provides a local memory for the hardware thread that is integrated into the global address space.

• Provides primitives for using the local memory to implement a function call stack and a heap.

HWTI Diagram

Why are we writing a compiler?

Reason 1: Open hthreads to Software Developers

• Allow software developers to create hardware thread cores without needing to know details of hardware design.

• It is expected by developers that all new architectures have C compilers.

Reason 2: Design space exploration

• Having a C to VHDL compilation system allows for threads to simply be recompiled to cross the software/hardware boundary.

• Developer can test more system configurations if they do not have to write VHDL for hardware threads.

Reason 3: Physical Thread Level Parallelism of C Threads

• C has limited fine-grained parallelism due to underlying Von Neumann model and there is not a lot we can do in the compiler about that.

• Other C to Hardware projects in the past have focused on creating accelerators with fine grained parallelism, often modifying the input language to increase available fine-grained parallelism.

• We are attacking course-grained parallelism that is made available by the user when threads are created.

• Input language is NOT modified!!!

• Independent, physically concurrent hardware threads are highly deterministic and potentially high performance.

Reason 4: Potential Base for Partial Reconfiguration

• User can create hardware and software images for all threads in the system.

• Use an hthread attribute or some other mechanism to pick where a thread is executed at run time.

• System reconfigures itself to multiplex different hardware threads into the FPGA fabric during the life of the system.

How to represent C in hardware

Requirements

• To represent C in hardware, you need:– Primitive arithmetic/logical operations

– Primitive control flow operations

– Memory Model

– Function Call Model

Primitive Operations

• Most of C's primitive arithmetic/logical operations can be directly represented in synthesizable VHDL– Integer add, subtract, and, or, etc.

• More complicated primitives can be implemented by using simple state machines or instantiating vendor provided IP– Integer divide, floating point ops, etc.

Control Flow

• Use a state machine to handle all control flow operations.

• If statements, all forms of loops, and unconditional branches (goto, continue, and break) can all be supported with the state machine based model.

• This is highly sequential, but we do not care since we are worried about TLP rather than ILP.

• Still allows for basic ILP optimizations (schedule multiple operations in the same state).

Memory Model• Since we are running in the hthreads system, global

memory access is done via bus transactions.

• Thread's stack and heap are in a BRAM that is part of the hardware thread.

• Since BRAM has such low latency, there is no need for the thread to have a cache.

• Hardware thread interface provides unified API for accessing both the main memory and the local BRAM.

• Pointers have been a problem in the past for C to hardware, but are trivial in this model.

Function Call Model

• Thread has a function call stack and can implement function calls using standard RISC style calling conventions.

• State machines handle everything (push parameters and return address, update stack pointer, jump to function start, etc)

Compilation Process

• Compiler is divided into two modules– hifgen – Compiles C to HIF

– hif2vhdl – Compiles HIF to VHDL

• Produces both .hif and .o files for each C file.

Flow diagram of compiler

HIF

• Defined the Hardware Intermediate Form to act as a linear representation of GCC's architecture independent GIMPLE IF.

• Acts as both an intermediate form for compilation as well as an object file.

• Has three address code style with a few additions.

Example HIF Program#### Created from factorial.c line 2@function @signed @int 32 factorial

# Argument declaration section @param @signed @int 32 hif_n

# Variable declaration section @declare @signed @int 32 hif_D.1279 @declare @signed @int 32 hif_D.1280 @declare @signed @int 32 hif_D.1281

@if hif_n != 0 @goto hif_label0 @mov hif_D.1279 1 @return hif_D.1279 @goto hif_label1

@label hif_label0 : #else @sub hif_D.1280 hif_n 1 @call factorial hif_D.1280 @castmov hif_D.1281 @returnVal @mul hif_D.1279 hif_D.1281 hif_n @return hif_D.1279 @label hif_label1 : #end if

int factorial(int n){ if (n == 0) return 1; else return factorial(n-1)*n;}

hifgen Requirements

• Must generate proper HIF to implement all of C language– We are ignoring other GCC input languages for

now. It is possible to support these.

• Module must be flexible enough to adjust to any changes in GIMPLE and GCC

• Must not interfere with normal GCC compilation

hifgen Implementation

• Added a module into GCC that implements a walk of the GIMPLE tree that generates HIF as a side effect.

• Use predefined GCC macros to access GIMPLE data structure.

• Reorders statements to match HIF syntax.

hif2vhdl Requirements

• Implement the semantics of HIF in VHDL hardware threads.

• Must generate efficient finite state machines.– Otherwise there is no point to move thread to

hardware.

• Implement HWTI protocol properly.

• Must be flexible and extensible enough to be used as a research platform.

hif2vhdl Implementation

• Implemented proof of concept version in Python– Chose Python for flexibility and development

speed.

– May implement in another language (e.g. Haskell, Ocaml, or C) in the future if there is a very good reason.

• Using PLY (Python Lex-Yacc) as the basis for the parser.

User’s View

• HTC script provides a GCC-like interface for our compiler.

• To compile from C to HIF:htc -c thread1.c

• To compile from HIF to VHDL:htc -o thread1.vhd -m mainfunction thread1.hif

Current Complete Version

• Completed for Senior Honors Project

• Works with most C constructs

• Installed on the ITTC network– To use, source the following file into a bash

environment.../projects/hybridthreads/tools/hif2vhdl_v1/init.sh

– This sets up the 'htc' command.

Limitations of Current Version

• Types missing– Only supports 32-bit signed integers.

– All integer types get mapped into 32-bit signed integers.

– Floating points not supported at all.

• Function pointers not supported

• Pointer aliasing partially implemented.

• Translation from C to HIF fails in some corner cases.

Ground Up Redesign

• Redesigned HIF to support all types that are in GIMPLE– Primitives have bit length, type modifiers, and

integer/floating point specifier

– Also has support for arrays, pointers, structs, etc.

• Redesigning compiler structure to reflect our experience and new knowledge– First version of compiler was implemented before we

had chance to study compilation in detail.

Current Development Work

• Have implemented hifgen module for new HIF language and are currently testing it.

• Have completed front end of new hif2vhdl module.

• Next step is to create the framework for the middle end of the hif2vhdl module that will later be used to implement optimizations.

– HIF CFG

– HIF PCFG

• Also working on implementation details of representing new HIF in hardware.

Future Work

• Add optimizers into hif2vhdl module– Classic compiler optimizations

– Specialized hardware thread optimizations

• Explore different implementation techniques for hardware threads.

• Partial reconfiguration research.

• Write papers!!!

Possible Hardware Thread Model

Conclusions

• Designed representations for C in hardware.

• Created the Hardware Intermediate Form.

• Developed hifgen and hif2vhdl modules.

• Currently working to create a feature complete system.

Questions?