LLVM Compiler (2 of 3) Jason Dangel

Lectures High-level overview of LLVM (Katie) Walkthrough of LLVM in context of our project (Jason) –Input requirements –Configuration needs Additional configuration and global context (Dave) –Optimizations –Other file formats 2

Overview High level overview LLVM Applied to Project Optimizer Inputs Explained Variable types Instruction Overview Code Examples Dissection of Clang How to use LLVM Command Line Examples 3

LLVM Compiler Design 4

LLVM Inputs RISC like instruction set Strongly typed –Simple types: i32 and i32** Some details are abstracted away –Example ‘call’ and ‘ret’ with explicit arguments Does not use fixed set of registers –Uses an infinite set of temporaries starting with % 5

Language Reference –Details on all variable types –Details and examples of instructions Memory, Binary, Terminator(flow control) –Very detailed and should probably be treated as a dictionary not a users manual. 6

Variable Tyes Integer (i1,i8, i32) –Any bit width from 1 bit to ) Floats (half, float, double) Pointers Array – x > 7

Declaring and Assign Values Identifiers: Global declaration: = global i32 17 = global i32 42 = global [2 x i32*] [ ] –Local declaration –%X = i32 17 –%Y = i32 42 –%1 = 8

Declaring Functions define [linkage] [visibility] [DLLStorageClass] [cconv] [ret ([argument list]) [unnamed_addr] [fn Attrs] [section "name"] [comdat [($name)]] [align N] [gc] [prefix Constant] [prologue Constant] {... } *Don’t worry I will explain later with an example and this will make more sense. 9

Instructions Flow Control –br, ret, switch, resume,… Binary Operational Add, fadd, sub, mul, fmul,…. Bitwise Binary Operational shl, shr, and, xor, …. Memory, Call, Compare and Conversion 10 *Only a small subset, see reference manual for all

Provided Hello World Example 11

Code Examples 12 unsigned add1(unsigned a, unsigned b) { return a+b; } // Perhaps not the most efficient way to add two numbers. unsigned add2(unsigned a, unsigned b) { if (a == 0) return b; return add2(a-1, b+1); } define %a, i32 %b) { entry: %tmp1 = add i32 %a, %b ret i32 %tmp1 } define %a, i32 %b) { entry: %tmp1 = icmp eq i32 %a, 0 br i1 %tmp1, label %done, label %recurse recurse: %tmp2 = sub i32 %a, 1 %tmp3 = add i32 %b, 1 %tmp4 = call %tmp2, i32 %tmp3) ret i32 %tmp4 done: ret i32 %b }

Clang Three block process performed automatically Clang has optimized the ‘add2()’ instruction through the LLVM optimizer –Now looks exactly like ‘add1()’ Needed to disassemble the bitcode to read LLVM IR 13

Clang ‘hello world’ C-Code Example 14

Clang Output Results After Disassembling 15

How to Use LLVM Use clang with similar ‘C’ code to help –clang -O3 -emit-llvm hello.c -c -o hello.bc To Assemble use llvm-as to create bitcode –llvm-as hello.o –o hello.bc To compile for use on local computer use llc –llc hello.bc –o hello.exe To run on local computer with interpreter use lli –lli hello.bc 16

Command Line Example ‘llvm-as’ will convert RISC-like IR to bit code –Compressed to fit on disk and run with interpreter ‘lli’ is the interpreter –Like java interpreter Bitcode can be analyzed with disassembler 17

Questions? 18