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Wow this is out of date. When we have _real_ code generator documentation,

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this should be folded into it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11705 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2004-02-22 05:53:54 +00:00
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lib/Target/X86/README.txt
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@ -8,14 +8,14 @@
I. Overview
===========
This directory contains a machine description for the X86 processor. Currently
this machine description is used for a high performance code generator used by a
LLVM JIT. One of the main objectives that we would like to support with this
project is to build a nice clean code generator that may be extended in the
future in a variety of ways: new targets, new optimizations, new
transformations, etc.
This directory contains a machine description for the X86 processor family.
Currently this machine description is used for a high performance code generator
used by the LLVM JIT and static code generators. One of the main objectives
that we would like to support with this project is to build a nice clean code
generator that may be extended in the future in a variety of ways: new targets,
new optimizations, new transformations, etc.
This document describes the current state of the LLVM JIT, along with
This document describes the current state of the X86 code generator, along with
implementation notes, design decisions, and other stuff.
@ -33,10 +33,9 @@ JIT and static compiler backends are largely shared.
At the high-level, LLVM code is translated to a machine specific representation
formed out of MachineFunction, MachineBasicBlock, and MachineInstr instances
(defined in include/llvm/CodeGen). This representation is completely target
agnostic, representing instructions in their most abstract form: an opcode, a
destination, and a series of operands. This representation is designed to
support both SSA representation for machine code, as well as a register
allocated, non-SSA form.
agnostic, representing instructions in their most abstract form: an opcode and a
series of operands. This representation is designed to support both SSA
representation for machine code, as well as a register allocated, non-SSA form.
Because the Machine* representation must work regardless of the target machine,
it contains very little semantic information about the program. To get semantic
@ -52,16 +51,16 @@ SSA Instruction Representation
------------------------------
Target machine instructions are represented as instances of MachineInstr, and
all specific machine instruction types should have an entry in the
InstructionInfo table defined through X86InstrInfo.def. In the X86 backend,
there are two particularly interesting forms of machine instruction: those that
produce a value (such as add), and those that do not (such as a store).
X86InstrInfo.td file. In the X86 backend, there are two particularly
interesting forms of machine instruction: those that produce a value (such as
add), and those that do not (such as a store).
Instructions that produce a value use Operand #0 as the "destination" register.
When printing the assembly code with the built-in machine instruction printer,
these destination registers will be printed to the left side of an '=' sign, as
in: %reg1027 = addl %reg1026, %reg1025
in: %reg1027 = add %reg1026, %reg1025
This 'addl' MachineInstruction contains three "operands": the first is the
This `add' MachineInstruction contains three "operands": the first is the
destination register (#1027), the second is the first source register (#1026)
and the third is the second source register (#1025). Never forget the
destination register will show up in the MachineInstr operands vector. The code
@ -83,7 +82,8 @@ specify a destination register to the BuildMI call.
IV. Source Code Layout
======================
The LLVM-JIT is composed of source files primarily in the following locations:
The LLVM code generator is composed of source files primarily in the following
locations:
include/llvm/CodeGen
--------------------
@ -113,16 +113,15 @@ This directory contains the machine description for X86 that is required to the
rest of the compiler working. It contains any code that is truly specific to
the X86 backend, for example the instruction selector and machine code emitter.
tools/lli/JIT
-------------
lib/ExecutionEngine/JIT
-----------------------
This directory contains the top-level code for the JIT compiler. This code
basically boils down to a call to TargetMachine::addPassesToJITCompile. As we
progress with the project, this will also contain the compile-dispatch-recompile
loop.
basically boils down to a call to TargetMachine::addPassesToJITCompile, and
handles the compile-dispatch-recompile cycle.
test/Regression/Jello
---------------------
This directory contains regression tests for the JIT.
test/Regression/CodeGen/X86
---------------------------
This directory contains regression tests for the X86 code generator.
==================================================
@ -154,26 +153,16 @@ way, in the same order.
VI. TODO / Future Projects
==========================
There are a large number of things remaining to do. Here is a partial list:
Next Phase:
-----------
1. Implement linear time optimal instruction selector
2. Implement smarter (linear scan?) register allocator
After this project:
-------------------
1. Implement lots of nifty runtime optimizations
2. Implement new targets: IA64? X86-64? M68k? MMIX? Who knows...
Ideas for Improvements:
-----------------------
1. Implement an *optimal* linear time instruction selector
2. Implement lots of nifty runtime optimizations
3. Implement new targets: IA64? X86-64? M68k? MMIX? Who knows...
Infrastructure Improvements:
----------------------------
1. Bytecode is designed to be able to read particular functions from the
bytecode without having to read the whole program. Bytecode reader should be
extended to allow on-demand loading of functions.
2. X86/Printer.cpp and Sparc/EmitAssembly.cpp both have copies of what is
1. X86/Printer.cpp and Sparc/EmitAssembly.cpp both have copies of what is
roughly the same code, used to output constants in a form the assembler
can understand. These functions should be shared at some point. They
should be rewritten to pass around iostreams instead of strings. The