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LLVM架构

clang test.cpp -o test的执行步骤：

前端语法分析

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clang -Xclang -ast-dump -fsyntax-only test.cpp

输出test.cpp经过编译器前端的预处理、语法分析、语义分析之后生成的抽象语法树（AST），最后几行显示了经过分析后的AST

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`-FunctionDecl 0x55d7ecff3d90 <test.cpp:1:1, line:4:1> line:1:5 main 'int ()'
  `-CompoundStmt 0x55d7ecff3ed0 <line:2:1, line:4:1>
    `-ReturnStmt 0x55d7ecff3ec0 <line:3:5, col:12>
      `-IntegerLiteral 0x55d7ecff3ea0 <col:12> 'int' 0

前端生成中间代码

第二步骤时将内存中的AST生成LLVM IR中间代码。

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clang++ -S -emit-llvm test.cpp

生成一个test.ll文件

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; ModuleID = 'test.cpp'
source_filename = "test.cpp"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: mustprogress noinline norecurse nounwind optnone uwtable
define dso_local noundef i32 @main() #0 {
  %1 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  ret i32 0
}

attributes #0 = { mustprogress noinline norecurse nounwind optnone uwtable "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }

!llvm.module.flags = !{!0, !1, !2, !3, !4}
!llvm.ident = !{!5}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 7, !"PIC Level", i32 2}
!2 = !{i32 7, !"PIE Level", i32 2}
!3 = !{i32 7, !"uwtable", i32 2}
!4 = !{i32 7, !"frame-pointer", i32 2}
!5 = !{!"clang version 13.0.0 (https://github.com/apple/llvm-project.git f0fb631dd1a3a2988b23ba5057cd9106713cd0b4)"}

其中的define i32 @main()即源代码转化为LLVM IR的核心部分

LLVM后端优化IR

使用llvm中的opt进行O3优化

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opt test.ll -S -O3
# 或者
clang++ -S -emit-llvm -O3 test.cpp
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; ModuleID = 'test.cpp'
source_filename = "test.cpp"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone willreturn uwtable
define dso_local noundef i32 @main() local_unnamed_addr #0 {
  ret i32 0
}

attributes #0 = { mustprogress nofree norecurse nosync nounwind readnone willreturn uwtable "frame-pointer"="none" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }

!llvm.module.flags = !{!0, !1, !2, !3}
!llvm.ident = !{!4}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 7, !"PIC Level", i32 2}
!2 = !{i32 7, !"PIE Level", i32 2}
!3 = !{i32 7, !"uwtable", i32 2}
!4 = !{!"clang version 13.0.0 (https://github.com/apple/llvm-project.git f0fb631dd1a3a2988b23ba5057cd9106713cd0b4)"}

LLVM后端生成汇编代码

使用llc组件llc test.ll，生成test.s

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 .text
 .file "test.cpp"
 .globl main                            # -- Begin function main
 .p2align 4, 0x90
 .type main,@function
main:                                   # @main
 .cfi_startproc
# %bb.0:
 xorl %eax, %eax
 retq
.Lfunc_end0:
 .size main, .Lfunc_end0-main
 .cfi_endproc
                                        # -- End function
 .ident "clang version 13.0.0 (https://github.com/apple/llvm-project.git f0fb631dd1a3a2988b23ba5057cd9106713cd0b4)"
 .section ".note.GNU-stack","",@progbits

然后就是操作系统自带的汇编器、连接器。最终生成可执行程序(.s ==OS Assembler -> .o ==OS Linker -> executable)