使用一个简单例子 孙源 链接过程ppt
将文件main.m预处理输出到main2.m.
clang -E main.m>>main2.m
将代码分析
clang -fmodules -fsyntax-only -Xclang -dump-tokens main.m
int main(int argc, const c' Loc=<main.m:9:1>
int 'int' [StartOfLine] Loc=<main.m:13:1>
identifier 'test' [LeadingSpace] Loc=<main.m:13:5>
l_paren '(' Loc=<main.m:13:9>
int 'int' Loc=<main.m:13:10>
identifier 'a' [LeadingSpace] Loc=<main.m:13:14>
comma ',' Loc=<main.m:13:15>
int 'int' Loc=<main.m:13:16>
identifier 'b' [LeadingSpace] Loc=<main.m:13:20>
r_paren ')' Loc=<main.m:13:21>
l_brace '{' Loc=<main.m:13:22>
return 'return' [StartOfLine] [LeadingSpace] Loc=<main.m:14:5>
identifier 'a' [LeadingSpace] Loc=<main.m:14:12>
plus '+' [LeadingSpace] Loc=<main.m:14:14>
identifier 'b' [LeadingSpace] Loc=<main.m:14:16>
plus '+' [LeadingSpace] Loc=<main.m:14:18>
clang -fmodules -fsyntax-only -Xclang -ast-dump main.m
TranslationUnitDecl 0x7f8ce601c608 <<invalid sloc>> <invalid sloc> <undeserialized declarations>
|-TypedefDecl 0x7f8ce601cea0 <<invalid sloc>> <invalid sloc> implicit __int128_t '__int128'
| `-BuiltinType 0x7f8ce601cba0 '__int128'
|-TypedefDecl 0x7f8ce601cf10 <<invalid sloc>> <invalid sloc> implicit __uint128_t 'unsigned __int128'
| `-BuiltinType 0x7f8ce601cbc0 'unsigned __int128'
|-TypedefDecl 0x7f8ce601cfb0 <<invalid sloc>> <invalid sloc> implicit SEL 'SEL *'
| `-PointerType 0x7f8ce601cf70 'SEL *'
| `-BuiltinType 0x7f8ce601ce00 'SEL'
|-TypedefDecl 0x7f8ce601d098 <<invalid sloc>> <invalid sloc> implicit id 'id'
| `-ObjCObjectPointerType 0x7f8ce601d040 'id'
| `-ObjCObjectType 0x7f8ce601d010 'id'
|-TypedefDecl 0x7f8ce601d178 <<invalid sloc>> <invalid sloc> implicit Class 'Class'
| `-ObjCObjectPointerType 0x7f8ce601d120 'Class'
| `-ObjCObjectType 0x7f8ce601d0f0 'Class'
|-ObjCInterfaceDecl 0x7f8ce601d1d0 <<invalid sloc>> <invalid sloc> implicit Protocol
|-TypedefDecl 0x7f8ce601d568 <<invalid sloc>> <invalid sloc> implicit __NSConstantString 'struct __NSConstantString_tag'
| `-RecordType 0x7f8ce601d340 'struct __NSConstantString_tag'
| `-Record 0x7f8ce601d2a0 '__NSConstantString_tag'
|-TypedefDecl 0x7f8ce701ce00 <<invalid sloc>> <invalid sloc> implicit __builtin_ms_va_list 'char *'
| `-PointerType 0x7f8ce601d5c0 'char *'
| `-BuiltinType 0x7f8ce601c6a0 'char'
|-TypedefDecl 0x7f8ce701d108 <<invalid sloc>> <invalid sloc> implicit __builtin_va_list 'struct __va_list_tag [1]'
| `-ConstantArrayType 0x7f8ce701d0b0 'struct __va_list_tag [1]' 1
| `-RecordType 0x7f8ce701cef0 'struct __va_list_tag'
| `-Record 0x7f8ce701ce58 '__va_list_tag'
|-ImportDecl 0x7f8ce701d930 <main.m:9:1> col:1 implicit Darwin.C.stdio
|-FunctionDecl 0x7f8ce701db10 <line:13:1, line:15:1> line:13:5 test 'int (int, int)'
| |-ParmVarDecl 0x7f8ce701d988 <col:10, col:14> col:14 used a 'int'
| |-ParmVarDecl 0x7f8ce701da08 <col:16, col:20> col:20 used b 'int'
| `-CompoundStmt 0x7f8ce701dd00 <col:22, line:15:1>
| `-ReturnStmt 0x7f8ce701dcf0 <line:14:5, col:20>
| `-BinaryOperator 0x7f8ce701dcd0 <col:12, col:20> 'int' '+'
| |-BinaryOperator 0x7f8ce701dc90 <col:12, col:16> 'int' '+'
| | |-ImplicitCastExpr 0x7f8ce701dc60 <col:12> 'int' <LValueToRValue>
| | | `-DeclRefExpr 0x7f8ce701dc20 <col:12> 'int' lvalue ParmVar 0x7f8ce701d988 'a' 'int'
| | `-ImplicitCastExpr 0x7f8ce701dc78 <col:16> 'int' <LValueToRValue>
| | `-DeclRefExpr 0x7f8ce701dc40 <col:16> 'int' lvalue ParmVar 0x7f8ce701da08 'b' 'int'
| `-IntegerLiteral 0x7f8ce701dcb0 <col:20> 'int' 3
`-FunctionDecl 0x7f8ce910edb0 <line:18:1, line:22:1> line:18:5 main 'int (int, const char **)'
|-ParmVarDecl 0x7f8ce701dd30 <col:10, col:14> col:14 argc 'int'
|-ParmVarDecl 0x7f8ce910ec60 <col:20, col:38> col:33 argv 'const char **':'const char **'
`-CompoundStmt 0x7f8ce910f690 <col:41, line:22:1>
|-DeclStmt 0x7f8ce910f418 <line:19:5, col:20>
| |-VarDecl 0x7f8ce910eeb8 <col:5, col:13> col:9 used a 'int' cinit
| | `-IntegerLiteral 0x7f8ce910ef20 <col:13> 'int' 10
| `-VarDecl 0x7f8ce910ef58 <col:5, col:18> col:16 used b 'int' cinit
| `-IntegerLiteral 0x7f8ce910efc0 <col:18> 'int' 11
|-CallExpr 0x7f8ce910f600 <line:20:5, col:22> 'int'
| |-ImplicitCastExpr 0x7f8ce910f5e8 <col:5> 'int (*)(const char *, ...)' <FunctionToPointerDecay>
| | `-DeclRefExpr 0x7f8ce910f430 <col:5> 'int (const char *, ...)' Function 0x7f8ce910efe8 'printf' 'int (const char *, ...)'
| |-ImplicitCastExpr 0x7f8ce910f648 <col:12> 'const char *' <NoOp>
| | `-ImplicitCastExpr 0x7f8ce910f630 <col:12> 'char *' <ArrayToPointerDecay>
| | `-StringLiteral 0x7f8ce910f488 <col:12> 'char [3]' lvalue "%d"
| `-BinaryOperator 0x7f8ce910f588 <col:17, line:10:11> 'int' '+'
| |-BinaryOperator 0x7f8ce910f548 <line:20:17, col:19> 'int' '+'
| | |-ImplicitCastExpr 0x7f8ce910f518 <col:17> 'int' <LValueToRValue>
| | | `-DeclRefExpr 0x7f8ce910f4a8 <col:17> 'int' lvalue Var 0x7f8ce910eeb8 'a' 'int'
| | `-ImplicitCastExpr 0x7f8ce910f530 <col:19> 'int' <LValueToRValue>
| | `-DeclRefExpr 0x7f8ce910f4e0 <col:19> 'int' lvalue Var 0x7f8ce910ef58 'b' 'int'
| `-IntegerLiteral 0x7f8ce910f568 <line:10:11> 'int' 30
`-ReturnStmt 0x7f8ce910f680 <line:21:5, col:12>
`-IntegerLiteral 0x7f8ce910f660 <col:12> 'int' 0
语法分析只是将完整的句子断开，不管对不对，词法分析负责是否对错。
如果有头文件，需要导入skd才可以。
clang -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/ iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator12.2.sdk(自己sdkpath) -fmodules -fsyntax-only -Xclang -ast-dump main.m
做完上边的步骤可以生成中间代码IR了，代码生成器会将词法树自顶向下遍历逐步翻译成LLVM IR，通过以下命令生成.ll文本文件，查看IR代码。
clang -S -fobjc-arc -emit-llvm main.m
LLVM的优化级别分别是-O0 -O1 -O2 -O3 -Os(大写字母O)
clang -Os -S -fobjc-arc -emit-llvm main.m -o main.ll
Xcode7 以后开启bitCode 苹果作进一步优化，生成.bc中间代码，我们通过优化后的IR代码生成.bc代码。
clang -emit-llvm -c main.ll -o main.bc
最终通过.ll或者.bc生成汇编代码。
clang -S -fobjc-arc main.bc -o main.s
clang -S -fobjc-arc main.ll -o main.s
生成了汇编代码也可以进行优化
clang -Os -S -fobjc-arc main.m -o main.s
目标文件的生成，是汇编机器已汇编代码作为输入，将汇编代码转换成机器代码，最终输出目标文件(.o文件)
clang -fmodules -c main.s -o main.o
通过nm命令，查看main.o中的符号。
$xcrun nm -nm main.o
(undefined) external _printf
0000000000000000 (__TEXT,__text) external _test
000000000000000a (__TEXT,__text) external _main
printf是一个undefined external的。
undefined 表示在当前文件暂时找不到符号_printf
连接器把编译产生的.o文件和.dylib .a 文件，生成一个mach-o文件。
clang main.o -o main
查看链接之后的符号
(undefined) external _printf (from libSystem)
(undefined) external dyld_stub_binder (from libSystem)
0000000100000000 (__TEXT,__text) [referenced dynamically] external __mh_execute_header
0000000100003f6d (__TEXT,__text) external _test
0000000100003f77 (__TEXT,__text) external _main
0000000100008008 (__DATA,__data) non-external __dyld_private