x86 ISA Compiler Baojian Hua

Front End source code abstract syntax tree lexical analyzer parser tokens IR semantic analyzer

Code Generation Before discussing code generation, we must understand what we are trying to generate virtual machines bare architecture … This course uses x86 So you’d learn how to program at the x86 level There is an online manual covering every details relatively old, but enough for understanding Linux, Windows, gcc, …

x86 Complex Instruction Set Computer (CISC) Instructions can operate on memory values e.g., add [eax], ebx Complex, multi-cycle instructions e.g., string-copy, call Many ways to do the same thing e.g., add eax,1 inc eax, sub eax,-1 Instructions are variable-length (1-10 bytes) Registers are not orthogonal

Capsule History 1978, 8086 First x86 microprocessor, 16-bit 1985, bit, protected mode 1989, , Pentium MMX 2000, Pentium 4 Deeply pipelined, high frequency 2006, Intel Core 2 Low power, multi-core

x86 ISA Instruction Set Architecture another programming language (instructions set) encoding decoding assemble, compile to … different implementations say Intel vs AMD Basis for OS, compilers, etc. hardware-software interface

x86 ISA What ’ s important here? OS and library Note: assembly program are NOT portable language syntax another CFG, read the manual assembler directives etc. think “ compiler ”, read the gas manual

OS and Library OS simplifies programming model e.g., Linux and Windows disable segmentation the so-called “ flat ” model in the manual so all segment-related details may be ignored when reading the manual OS provides protection mode e.g., Linux and Windows run user programs on ring3 so you cannot change the page table! etc.

OS and Library OS provides system calls hide many crazy details but may be still annoying Libraries another level of indirection on top of OS system calls In particular, we ’ d use C library

Syntax Syntax = data + instructions Data Immediate 4, 3.14, “ hello ” Register general-purpose eax, ebx, … segment remember? we don ’ t care

Data Memory different usage: globl stack heap but same behavior

Data Memory addressing mode seg:[base+index*scale+disp] any part can be null complex! right? e.g., int a[5][10], to read a[3][2] moveax, 30 movebx, 2 movecx, [eax+ebx*4+a] Problems with this strategy?

Instructions Manual covers all instructions in details: Data movement Arithmetic Control transfer … Rather than explain all these bit-by-bit, I ’ ll give an example next

Assembler Assembler is more than just a compiler: it costumes assembly syntax it also offers the so-called directives Two main branches: Intel syntax assembler on Windows: masm, nasm, … the Intel manual! AT&T syntax Linux assembler: gas the good news is that recent version of gas supports Intel syntax! the GCC output! This course uses as with Intel syntax So reading the Intel manual is relatively easy

Example # Sum up an array of integers # compiled by GCC: # $ gcc test.s.intel_syntax noprefix.data a:.int 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.globl main.text main: comments start with “ # ”, also supports C/C++ style directive: telling that we prefer Intel syntax directive: assemble the following to data section label: the current address directive: store 10 integers start from the address “ a ” directive: globl symbol directive: assemble the following to text section label: another address

Example, cont ’ push ebp mov esp, ebp # convention: eax: the sum, ebx: index xor eax, eax mov ebx, eax L_start: add eax, dword ptr [ebx*4+a] inc ebx comp ebx, 10 jl L_start leave ret

Summary Assembly programming is fun and simple conceptually but CISC architecture is … and a compound knowledge of OS, architecture and compiler Read the online manual Essential for code generation