README.md

Book LLVM Essentials ⬆

Directory llvm-essentials\ contains LLVM code examples from the book LLVM Essentials by S. Sarda & M. Pandey (Packt Publishing, December 2015). It also includes several batch files for running the example on a Windows machine.

1.2 Getting_familiar_with_LLVM_IR

This code example consists of the two files src\add.c and build.bat.

In this first example we simply run clang.exe with option -emit-llvm to generate the assembly file add.ll¹.

> build -verbose run
Toolset: Clang/CMake, Project: add
Generate IR code to file "build\add.ll"

The generated assembly file add.ll² looks as follows :

> type target\add.ll
; ModuleID = 'L:\llvm-essentials\1_2_Getting_familiar_with_LLVM_IR\src\add.c'
source_filename = "L:\\llvm-essentials\\1_2_Getting_familiar_with_LLVM_IR\\src\\add.c"
target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc19.28.29912"

@globvar = dso_local global i32 12, align 4

; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @add(i32 %0) #0 {
  %2 = alloca i32, align 4
  store i32 %0, i32* %2, align 4
  %3 = load i32, i32* @globvar, align 4
  %4 = load i32, i32* %2, align 4
  %5 = add nsw i32 %3, %4
  ret i32 %5
}

attributes #0 = { noinline nounwind optnone [...] }

!llvm.module.flags = !{!0, !1}
!llvm.ident = !{!2}

!0 = !{i32 1, !"wchar_size", i32 2}
!1 = !{i32 7, !"PIC Level", i32 2}
!2 = !{!"clang version 14.0.4"}

3.2 Getting_address_of_element ▴

This code example consists of two files: src\toy.cpp and build.bat.

Function main in source file src\toy.cpp calls LLVM API functions to generate assembly code for the tiny function foo :

> build -verbose run
Project: toy, Configuration: Release, Platform: x64
Current directory: "L:\llvm-essentials\3_2_Getting_address_of_element\build"
Generate configuration files into directory "build"
Generate executable "toy.exe"
Execute build\Release\toy.exe
; ModuleID = 'my compiler'
source_filename = "my compiler"

define i32 @foo(<2 x i32>* %a) {
entry:
  %a1 = getelementptr i32, <2 x i32>* %a, i32 1
  ret i32 0
}

3.3 Reading_from_memory

This code example consists of two files: src\toy.cpp and build.bat.

Function main in source file src\toy.cpp calls LLVM API functions to generate assembly code for the tiny function foo :

> build -verbose run
Project: toy, Configuration: Release, Platform: x64
Current directory: L:\llvm-essentials\3_3_Reading_from_memory\build
Generate configuration files into directory "build"
Generate executable "toy.exe"
Execute build\Release\toy.exe
; ModuleID = 'my compiler'
source_filename = "my compiler"

define i32 @foo(<2 x i32>* %a) {
entry:
  %a1 = getelementptr <2 x i32>*, <2 x i32>* %a, i32 1
  %load = load <2 x i32>*, <2 x i32>** %a1, align 8
  ret <2 x i32>* %load
}

3.4 Writing to memory ▴

This code example consists of two files: src\toy.cpp and build.bat.

Function main in source file src\toy.cpp calls LLVM API functions to generate assembly code for the tiny function foo :

> build -verbose run
Project: toy, Configuration: Release, Platform: x64
Current directory: "L:\llvm-essentials\3_4_Writing_to_memory\build"
Generate configuration files into directory "build"
Generate executable "toy.exe"
Execute "build\Release\toy.exe"
; ModuleID = 'my compiler'
source_filename = "my compiler"

define i32 @foo(<2 x i32>* %a) {
entry:
  %a1 = getelementptr i32, <2 x i32>* %a, i32 1
  %load = load i32, i32* %a1, align 1
  %multmp = mul i32 %load, 16
  store i32 %multmp, i32* %a1, align 1
  ret i32 %multmp
}

Footnotes

[1] LLVM file extensions ↩

Extension	Description
.bc	bitcode format (binary)
.ll	assembly language format (text)

[2] main function not found ↩

The assembly code in file add.ll can not be executed with lli.exe since it doesn't define a main function as the program entry point :

> %LLVM_HOME%\bin\lli.exe build\add.ll
C:\opt\LLVM-15.0.6\bin\lli.exe: error: 'main' function not found in module.

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mics/August 2024 ▲