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133 lines
4.8 KiB
Markdown
133 lines
4.8 KiB
Markdown
# 5.6 LLVM
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- [简介](#简介)
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- [初步使用](#初步使用)
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- [参考资料](#参考资料)
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## 简介
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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。
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## 初步使用
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首先我们通过著名的 helloWorld 来熟悉下 LLVM 的使用。
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```c
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#include <stdio.h>
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int main()
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{
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printf("hello, world\n");
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}
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```
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将 C 源码转换成 LLVM 汇编码：
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```
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$ clang -emit-llvm -S hello.c -o hello.ll
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```
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生成的 LLVM IR 如下：
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```
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; ModuleID = 'hello.c'
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source_filename = "hello.c"
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target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-unknown-linux-gnu"
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@.str = private unnamed_addr constant [14 x i8] c"hello, world\0A\00", align 1
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; Function Attrs: noinline nounwind optnone sspstrong uwtable
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define i32 @main() #0 {
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%1 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([14 x i8], [14 x i8]* @.str, i32 0, i32 0))
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ret i32 0
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}
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declare i32 @printf(i8*, ...) #1
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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" }
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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" }
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!llvm.module.flags = !{!0, !1, !2}
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!llvm.ident = !{!3}
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!0 = !{i32 1, !"wchar_size", i32 4}
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!1 = !{i32 7, !"PIC Level", i32 2}
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!2 = !{i32 7, !"PIE Level", i32 2}
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!3 = !{!"clang version 5.0.1 (tags/RELEASE_501/final)"}
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```
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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。
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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：
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```
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$ llvm-as hello.ll -o hello.bc
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```
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结果如下：
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```
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$ file hello.bc
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hello.bc: LLVM IR bitcode
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$ xxd -g1 hello.bc | head -n5
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00000000: 42 43 c0 de 35 14 00 00 05 00 00 00 62 0c 30 24 BC..5.......b.0$
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00000010: 49 59 be 66 ee d3 7e 2d 44 01 32 05 00 00 00 00 IY.f..~-D.2.....
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00000020: 21 0c 00 00 4d 02 00 00 0b 02 21 00 02 00 00 00 !...M.....!.....
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00000030: 13 00 00 00 07 81 23 91 41 c8 04 49 06 10 32 39 ......#.A..I..29
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00000040: 92 01 84 0c 25 05 08 19 1e 04 8b 62 80 10 45 02 ....%......b..E.
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```
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反过来将 bitcode 转回 LLVM IR 也是可以的，使用反汇编器 llvm-dis：
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```
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$ llvm-dis hello.bc -o hello.ll
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```
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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：
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```
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$ lli hello.bc
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hello, world
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```
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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：
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```
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$ llc -march=x86-64 hello.bc -o hello.s
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```
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也可以使用 clang 来生成，结果是一样的：
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```
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$ clang -S hello.bc -o hello.s -fomit-frame-pointer
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```
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结果如下：
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```asm
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.text
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.file "hello.c"
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.globl main # -- Begin function main
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.p2align 4, 0x90
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.type main,@function
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main: # @main
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.cfi_startproc
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# BB#0:
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pushq %rbp
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.Lcfi0:
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.cfi_def_cfa_offset 16
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.Lcfi1:
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.cfi_offset %rbp, -16
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movq %rsp, %rbp
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.Lcfi2:
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.cfi_def_cfa_register %rbp
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movabsq $.L.str, %rdi
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movb $0, %al
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callq printf
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xorl %eax, %eax
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popq %rbp
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retq
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.Lfunc_end0:
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.size main, .Lfunc_end0-main
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.cfi_endproc
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# -- End function
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.type .L.str,@object # @.str
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.section .rodata.str1.1,"aMS",@progbits,1
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.L.str:
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.asciz "hello, world\n"
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.size .L.str, 14
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.ident "clang version 5.0.1 (tags/RELEASE_501/final)"
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.section ".note.GNU-stack","",@progbits
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```
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## 参考资料
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- [llvm documentation](http://llvm.org/docs/index.html)
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