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LLVM Cookbook

LLVM Cookbook

By : Mayur Pandey, Suyog Sarda
2 (9)
Buy this Book
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LLVM Cookbook

LLVM Cookbook

2 (9)
By: Mayur Pandey, Suyog Sarda
Buy this Book

Overview of this book

The book is for compiler programmers who are familiar with concepts of compilers and want to indulge in understanding, exploring, and using LLVM infrastructure in a meaningful way in their work. This book is also for programmers who are not directly involved in compiler projects but are often involved in development phases where they write thousands of lines of code. With knowledge of how compilers work, they will be able to code in an optimal way and improve performance with clean code.
Table of Contents (11 chapters)
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Preface
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Preface
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What this book covers
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What you need for this book
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Who this book is for
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Sections
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Conventions
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Reader feedback
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Customer support
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1
1. LLVM Design and Use
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1. LLVM Design and Use
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Introduction
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Understanding modular design
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Cross-compiling Clang/LLVM
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Converting a C source code to LLVM assembly
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Converting IR to LLVM bitcode
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Converting LLVM bitcode to target machine assembly
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Converting LLVM bitcode back to LLVM assembly
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Transforming LLVM IR
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Linking LLVM bitcode
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Executing LLVM bitcode
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Using the C frontend Clang
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Using the GO frontend
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Using DragonEgg
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2. Steps in Writing a Frontend
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2. Steps in Writing a Frontend
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Introduction
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Defining a TOY language
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Implementing a lexer
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Defining Abstract Syntax Tree
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Implementing a parser
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Parsing simple expressions
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Parsing binary expressions
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Invoking a driver for parsing
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Running lexer and parser on our TOY language
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Defining IR code generation methods for each AST class
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Generating IR code for expressions
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Generating IR code for functions
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Adding IR optimization support
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3. Extending the Frontend and Adding JIT Support
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3. Extending the Frontend and Adding JIT Support
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Introduction
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Handling decision making paradigms – if/then/else constructs
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Generating code for loops
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Handling user-defined operators – binary operators
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Handling user-defined operators – unary operators
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Adding JIT support
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4. Preparing Optimizations
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4. Preparing Optimizations
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Introduction
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Various levels of optimization
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Writing your own LLVM pass
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Running your own pass with the opt tool
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Using another pass in a new pass
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Registering a pass with pass manager
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Writing an analysis pass
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Writing an alias analysis pass
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Using other analysis passes
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5. Implementing Optimizations
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5. Implementing Optimizations
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Introduction
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Writing a dead code elimination pass
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Writing an inlining transformation pass
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Writing a pass for memory optimization
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Combining LLVM IR
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Transforming and optimizing loops
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Reassociating expressions
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Vectorizing IR
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Other optimization passes
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6. Target-independent Code Generator
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6. Target-independent Code Generator
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Introduction
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The life of an LLVM IR instruction
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Visualizing LLVM IR CFG using GraphViz
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Describing targets using TableGen
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Defining an instruction set
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Adding a machine code descriptor
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Implementing the MachineInstrBuilder class
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Implementing the MachineBasicBlock class
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Implementing the MachineFunction class
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Writing an instruction selector
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Legalizing SelectionDAG
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Optimizing SelectionDAG
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Selecting instruction from the DAG
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Scheduling instructions in SelectionDAG
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7. Optimizing the Machine Code
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7. Optimizing the Machine Code
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Introduction
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Eliminating common subexpression from machine code
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Analyzing live intervals
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Allocating registers
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Inserting the prologue-epilogue code
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Code emission
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Tail call optimization
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Sibling call optimisation
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8. Writing an LLVM Backend
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8. Writing an LLVM Backend
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Introduction
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Defining registers and registers sets
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Defining the calling convention
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Defining the instruction set
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Implementing frame lowering
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Printing an instruction
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Selecting an instruction
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Adding instruction encoding
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Supporting a subtarget
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Lowering to multiple instructions
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Registering a target
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9. Using LLVM for Various Useful Projects
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9. Using LLVM for Various Useful Projects
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Introduction
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Exception handling in LLVM
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Using sanitizers
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Writing the garbage collector with LLVM
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Converting LLVM IR to JavaScript
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Using the Clang Static Analyzer
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Using bugpoint
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Using LLDB
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Using LLVM utility passes
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Index
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Index

Converting LLVM bitcode to target machine assembly

In this recipe, you will learn how to convert the LLVM bitcode file to target specific assembly code.

Getting ready

The LLVM static compiler llc should be in installed from the LLVM toolchain.

How to do it...

Do the following steps:

  1. The bitcode file created in the previous recipe, test.bc, can be used as input to llc here. Using the following command, we can convert LLVM bitcode to assembly code:
    $ llc test.bc –o test.s
  2. The output is generated in the test.s file, which is the assembly code. To have a look at that, use the following command lines:
    $ cat test.s
.text
.file "test.bc"
.globl mult
.align 16, 0x90
.type mult,@function
mult:                                   # @mult
.cfi_startproc
# BB#0:
Pushq  %rbp
.Ltmp0:
.cfi_def_cfa_offset 16
.Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
.Ltmp2:
.cfi_def_cfa_register %rbp
imull %esi, %edi
movl %edi, %eax
popq %rbp
retq
.Ltmp3:
.size mult, .Ltmp3-mult
.cfi_endproc
  3. You can also use Clang to dump assembly code from the bitcode file format. By passing the –S option to Clang, we get test.s in assembly format when the test.bc file is in bitstream file format:
    $ clang -S test.bc -o test.s –fomit-frame-pointer # using the clang front end

    The test.s file output is the same as that of the preceding example. We use the additional option fomit-frame-pointer, as Clang by default does not eliminate the frame pointer whereas llc eliminates it by default.

How it works...

The llc command compiles LLVM input into assembly language for a specified architecture. If we do not mention any architecture as in the preceding command, the assembly will be generated for the host machine where the llc command is being used. To generate executable from this assembly file, you can use assembler and linker.

There's more...

By specifying -march=architecture flag in the preceding command, you can specify the target architecture for which the assembly needs to be generated. Using the -mcpu=cpu flag setting, you can specify a CPU within the architecture to generate code. Also by specifying -regalloc=basic/greedy/fast/pbqp, you can specify the type of register allocation to be used.

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