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//===- 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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//===----------------------------------------------------------------------===//
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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 family.
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Currently this machine description is used for a high performance code generator
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used by the LLVM JIT and static code generators. One of the main objectives
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that we would like to support with this project is to build a nice clean code
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generator that may be extended in the future in a variety of ways: new targets,
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new optimizations, new transformations, etc.
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This document describes the current state of the X86 code generator, 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 into the generic LLVM machine specific
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representation, which allows us to support as many targets as possible with our
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framework. This framework should allow us to share many common machine specific
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transformations (register allocation, instruction scheduling, etc...) among all
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of the backends that may eventually be supported by LLVM, and ensures that the
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JIT and static compiler backends are largely shared.
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At the high-level, LLVM code is translated to a machine specific representation
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formed out of MachineFunction, MachineBasicBlock, and MachineInstr instances
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(defined in include/llvm/CodeGen). This representation is completely target
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agnostic, representing instructions in their most abstract form: an opcode and a
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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 Machine* representation must work regardless of the target machine,
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it 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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Note that there is some amount of complexity that the X86 backend contains due
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to the Sparc backend's legacy requirements. These should eventually fade away
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as the project progresses.
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SSA Instruction Representation
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------------------------------
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Target machine instructions are represented as instances of MachineInstr, and
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all specific machine instruction types should have an entry in the
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X86InstrInfo.td file. In the X86 backend, there are two particularly
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interesting forms of machine instruction: those that produce a value (such as
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add), and those that do not (such as a store).
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Instructions that produce a value use Operand #0 as the "destination" register.
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When printing the assembly code with the built-in machine instruction printer,
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these destination registers will be printed to the left side of an '=' sign, as
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in: %reg1027 = add %reg1026, %reg1025
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This `add' MachineInstruction contains three "operands": the first is the
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destination register (#1027), the second is the first source register (#1026)
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and the third is the second source register (#1025). Never forget the
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destination register will show up in the MachineInstr operands vector. The code
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to generate this instruction looks like this:
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BuildMI(BB, X86::ADD32rr, 2, 1027).addReg(1026).addReg(1025);
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The first argument to BuildMI is the basic block to append the machine
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instruction to, the second is the opcode, the third is the number of operands,
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the fourth is the destination register. The two addReg calls specify operands
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in order.
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MachineInstrs that do not produce a value do not have this implicit first
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operand, they simply have #operands = #uses. To create them, simply do not
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specify a destination register to the BuildMI call.
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======================
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IV. Source Code Layout
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======================
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The LLVM code generator is composed of source files primarily in the following
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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, such as
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register allocation internals.
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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. The only classes used by the X86 backend so far are
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the TargetMachine, TargetData, MachineInstrInfo, and MRegisterInfo classes.
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lib/CodeGen
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-----------
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This directory will contain all of the target independent 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, they should not include
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any 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 truly specific to
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the X86 backend, for example the instruction selector and machine code emitter.
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lib/ExecutionEngine/JIT
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-----------------------
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This directory contains the top-level code for the JIT compiler. This code
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basically boils down to a call to TargetMachine::addPassesToJITCompile, and
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handles the compile-dispatch-recompile cycle.
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test/Regression/CodeGen/X86
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---------------------------
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This directory contains regression tests for the X86 code generator.
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==================================================
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V. Strange Things, or, Things That Should Be Known
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==================================================
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Representing memory in MachineInstrs
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------------------------------------
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The x86 has a very, uhm, flexible, way of accessing memory. It is capable of
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forming memory addresses of the following expression directly in integer
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instructions (which use ModR/M addressing):
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Base+[1,2,4,8]*IndexReg+Disp32
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Wow, that's crazy. In order to represent this, LLVM tracks no less that 4
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operands for each memory operand of this form. This means that the "load" form
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of 'mov' has the following "Operands" in this order:
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Index: 0 | 1 2 3 4
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Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement
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OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm
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Stores and all other instructions treat the four memory operands in the same
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way, in the same order.
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======================
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VI. Instruction naming
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======================
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An instruction name consists of the base name, a default operand size
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followed by a character per operand with an optional special size. For
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example:
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ADD8rr -> add, 8-bit register, 8-bit register
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IMUL16rmi -> imul, 16-bit register, 16-bit memory, 16-bit immediate
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IMUL16rmi8 -> imul, 16-bit register, 16-bit memory, 8-bit immediate
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MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
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==========================
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VII. TODO / Future Projects
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==========================
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Ideas for Improvements:
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-----------------------
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1. Implement an *optimal* linear time instruction selector
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2. Implement lots of nifty runtime optimizations
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3. Implement new targets: IA64? X86-64? M68k? MMIX? Who knows...
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Infrastructure Improvements:
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----------------------------
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1. X86/Printer.cpp and Sparc/EmitAssembly.cpp both have copies of what is
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roughly the same code, used to output constants in a form the assembler
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can understand. These functions should be shared at some point. They
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should be rewritten to pass around iostreams instead of strings. The
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list of functions is as follows:
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isStringCompatible
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toOctal
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ConstantExprToString
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valToExprString
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getAsCString
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printSingleConstantValue (with TypeToDataDirective inlined)
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printConstantValueOnly
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