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We can instruction select exactly one instruction 'ret void'. Wow. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4284 91177308-0d34-0410-b5e6-96231b3b80d8
144 lines
5.7 KiB
Plaintext
144 lines
5.7 KiB
Plaintext
//===- README.txt - Information about the X86 backend and related files ---===//
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//
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// This file contains random notes and points of interest about the X86 backend.
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//
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// Snippets of this document will probably become the final report for CS497
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//
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//===----------------------------------------------------------------------===//
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===========
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I. Overview
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===========
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This directory contains a machine description for the X86 processor. Currently
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this machine description is used for a high performance code generator used by a
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LLVM JIT. One of the main objectives that we would like to support with this
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project is to build a nice clean code generator that may be extended in the
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future in a variety of ways: new targets, new optimizations, new
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transformations, etc.
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This document describes the current state of the LLVM JIT, along with
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implementation notes, design decisions, and other stuff.
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===================================
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II. Architecture / Design Decisions
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===================================
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We designed the infrastructure for the machine specific representation to be as
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light-weight as possible, while also being able to support as many targets as
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possible with our framework. This framework should allow us to share many
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common machine specific transformations (register allocation, instruction
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scheduling, etc...) among all of the backends that may eventually be supported
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by the JIT, and unify the JIT and static compiler backends.
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At the high-level, LLVM code is translated to a machine specific representation
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formed out of MFunction, MBasicBlock, and MInstruction instances (defined in
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include/llvm/CodeGen). This representation is completely target agnostic,
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representing instructions in their most abstract form: an opcode, a destination,
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and a series of operands. This representation is designed to support both SSA
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representation for machine code, as well as a register allocated, non-SSA form.
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Because the M* representation must work regardless of the target machine, it
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contains very little semantic information about the program. To get semantic
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information about the program, a layer of Target description datastructures are
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used, defined in include/llvm/Target.
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Currently the Sparc backend and the X86 backend do not share a common
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representation. This is an intentional decision, and will be rectified in the
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future (after the project is done).
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=======================
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III. Source Code Layout
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=======================
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The LLVM-JIT is composed of source files primarily in the following locations:
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include/llvm/CodeGen
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--------------------
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This directory contains header files that are used to represent the program in a
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machine specific representation. It currently also contains a bunch of stuff
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used by the Sparc backend that we don't want to get mixed up in.
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include/llvm/Target
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-------------------
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This directory contains header files that are used to interpret the machine
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specific representation of the program. This allows us to write generic
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transformations that will work on any target that implements the interfaces
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defined in this directory. Again, this also contains a bunch of stuff from the
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Sparc Backend that we don't want to deal with.
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lib/CodeGen
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-----------
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This directory will contain all of the target independant transformations (for
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example, register allocation) that we write. These transformations should only
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use information exposed through the Target interface, it should not include any
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target specific header files.
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lib/Target/X86
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--------------
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This directory contains the machine description for X86 that is required to the
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rest of the compiler working. It contains any code that is truely specific to
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the X86 backend, for example the instruction selector and machine code emitter.
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tools/jello
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-----------
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This directory contains the top-level code for the JIT compiler.
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test/Regression/Jello
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---------------------
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This directory contains regression tests for the JIT. Initially it contains a
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bunch of really trivial testcases that we should build up to supporting.
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==========================
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IV. TODO / Future Projects
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==========================
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There are a large number of things remaining to do. Here is a partial list:
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Critial path:
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-------------
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0. Finish providing SSA form. This involves keeping track of some information
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when instructions are added to the function, but should not affect that API
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for creating new MInstructions or adding them to the program. There are
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also various FIXMEs in the M* files that need to get taken care of in the
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near term.
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1. Finish dumb instruction selector
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2. Write dumb register allocator
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3. Write assembly language emitter
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4. Write machine code emitter
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Next Phase:
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-----------
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1. Implement linear time optimal instruction selector
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2. Implement smarter (linear scan?) register allocator
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After this project:
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-------------------
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1. Implement lots of nifty runtime optimizations
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2. Implement a static compiler backend for x86
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3. Migrate Sparc backend to new representation
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4. Implement new spiffy targets: IA64? X86-64? M68k? Who knows...
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Infrastructure Improvements:
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----------------------------
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1. Bytecode is designed to be able to read particular functions from the
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bytecode without having to read the whole program. Bytecode reader should be
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extended to allow on demand loading of functions.
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2. PassManager needs to be able to run just a single function through a pipeline
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of FunctionPass's. When this happens, all of our code will become
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FunctionPass's for real.
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3. llvmgcc needs to be modified to output 32-bit little endian LLVM files.
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Preferably it will be parameterizable so that multiple binaries need not
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exist. Until this happens, we will be restricted to using type safe
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programs (most of the Olden suite and many smaller tests), which should be
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sufficient for our 497 project.
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