CTF-All-In-One/doc/5.6_llvm.md
2018-03-12 19:04:31 +08:00

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# 5.6 LLVM
- [简介](#简介)
- [初步使用](#初步使用)
- [参考资料](#参考资料)
## 简介
LLVM 是当今炙手可热的编译器基础框架。它从一开始就采用了模块化设计的思想使得每一个编译阶段都被独立出来形成了一系列的库。LLVM 使用面向对象的 C++ 语言开发,为编译器开发人员提供了易用而丰富的编程接口和 API。
## 初步使用
首先我们通过著名的 helloWorld 来熟悉下 LLVM 的使用。
```c
#include <stdio.h>
int main()
{
printf("hello, world\n");
}
```
将 C 源码转换成 LLVM 汇编码:
```
$ clang -emit-llvm -S hello.c -o hello.ll
```
生成的 LLVM IR 如下:
```
; ModuleID = 'hello.c'
source_filename = "hello.c"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
@.str = private unnamed_addr constant [14 x i8] c"hello, world\0A\00", align 1
; Function Attrs: noinline nounwind optnone sspstrong uwtable
define i32 @main() #0 {
%1 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([14 x i8], [14 x i8]* @.str, i32 0, i32 0))
ret i32 0
}
declare i32 @printf(i8*, ...) #1
attributes #0 = { noinline nounwind optnone sspstrong uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
!llvm.module.flags = !{!0, !1, !2}
!llvm.ident = !{!3}
!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 7, !"PIC Level", i32 2}
!2 = !{i32 7, !"PIE Level", i32 2}
!3 = !{!"clang version 5.0.1 (tags/RELEASE_501/final)"}
```
该过程从词法分析开始,将 C 源码分解成 token 流,然后传递给语法分析器,语法分析器在 CFG上下文无关文法的指导下将 token 流组织成 AST抽象语法树接下来进行语义分析检查语义正确性最后生成 IR。
LLVM bitcode 有两部分组成:位流,以及将 LLVM IR 编码成位流的编码格式。使用汇编器 llvm-as 将 LLVM IR 转换成 bitcode
```
$ llvm-as hello.ll -o hello.bc
```
结果如下:
```
$ file hello.bc
hello.bc: LLVM IR bitcode
$ xxd -g1 hello.bc | head -n5
00000000: 42 43 c0 de 35 14 00 00 05 00 00 00 62 0c 30 24 BC..5.......b.0$
00000010: 49 59 be 66 ee d3 7e 2d 44 01 32 05 00 00 00 00 IY.f..~-D.2.....
00000020: 21 0c 00 00 4d 02 00 00 0b 02 21 00 02 00 00 00 !...M.....!.....
00000030: 13 00 00 00 07 81 23 91 41 c8 04 49 06 10 32 39 ......#.A..I..29
00000040: 92 01 84 0c 25 05 08 19 1e 04 8b 62 80 10 45 02 ....%......b..E.
```
反过来将 bitcode 转回 LLVM IR 也是可以的,使用反汇编器 llvm-dis
```
$ llvm-dis hello.bc -o hello.ll
```
其实 LLVM 可以利用工具 lli 的即时编译器JIT直接执行 bitcode 格式的程序:
```
$ lli hello.bc
hello, world
```
接下来使用静态编译器 llc 命令可以将 bitcode 编译为特定架构的汇编语言:
```
$ llc -march=x86-64 hello.bc -o hello.s
```
也可以使用 clang 来生成,结果是一样的:
```
$ clang -S hello.bc -o hello.s -fomit-frame-pointer
```
结果如下:
```asm
.text
.file "hello.c"
.globl main # -- Begin function main
.p2align 4, 0x90
.type main,@function
main: # @main
.cfi_startproc
# BB#0:
pushq %rbp
.Lcfi0:
.cfi_def_cfa_offset 16
.Lcfi1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
.Lcfi2:
.cfi_def_cfa_register %rbp
movabsq $.L.str, %rdi
movb $0, %al
callq printf
xorl %eax, %eax
popq %rbp
retq
.Lfunc_end0:
.size main, .Lfunc_end0-main
.cfi_endproc
# -- End function
.type .L.str,@object # @.str
.section .rodata.str1.1,"aMS",@progbits,1
.L.str:
.asciz "hello, world\n"
.size .L.str, 14
.ident "clang version 5.0.1 (tags/RELEASE_501/final)"
.section ".note.GNU-stack","",@progbits
```
## 参考资料
- [llvm documentation](http://llvm.org/docs/index.html)