TodayToday’’s Topics Topic

Breakdown of GCC scriptBreakdown of GCC script Description of how different system Description of how different system

programs work together to build (and programs work together to build (and run) a C program.run) a C program.

System Software (System Software (for todayfor today))

AssemblerAssembler CompilerCompiler LinkerLinker LoaderLoader Operating systemOperating system PreprocessorPreprocessor

TodayToday’’s Objectivess Objectives

Within context of Within context of ““compilingcompiling”” a C program a C program Define actions of linker, compiler, assembler, Define actions of linker, compiler, assembler,

OS, macro preprocessor, loaderOS, macro preprocessor, loader Draw block diagram showing order of linker, Draw block diagram showing order of linker,

compiler, assembler, macro preprocessor, compiler, assembler, macro preprocessor, loaderloader

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Preprocessed

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Preprocessor Compiler

Assembler

LinkerLoader

Executable

a.out

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Program

In Memory

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Preprocessor Compiler

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LinkerLoader

Executable

a.out

Executable

Program

In Memory

C Preprocessor (cpp)C Preprocessor (cpp)

Pass over sourcePass over source Insert included filesInsert included files Perform macro substitutionsPerform macro substitutions

Define macrosDefine macros #define NUM 100#define NUM 100 #define xx(v,name,op,metrics) \ #define xx(v,name,op,metrics) \

v=xxinit(op,name,IR->metrics)v=xxinit(op,name,IR->metrics)

gcc –E example.c sends preprocessor gcc –E example.c sends preprocessor output to stdoutoutput to stdout

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Assembler

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Executable

a.out

Executable

Program

In Memory

CompilerCompiler

gcc actually name of a scriptgcc actually name of a script Compiler translates one language to Compiler translates one language to

another (or the same???)another (or the same???) gcc compiler translates C to assemblergcc compiler translates C to assembler gcc –S example.c gcc –S example.c ““savessaves”” example.s example.s Compiler consists ofCompiler consists of

ParserParser Code generationCode generation MysticismMysticism

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AssemblerAssembler

Another translator ??? (Another translator ??? (asas example.s)example.s)

Assembler to (binary) objectAssembler to (binary) object Why not compile straight to binary?Why not compile straight to binary? gcc –c example.c to gcc –c example.c to ““savesave”” object object NM to look at object (nm example.o)NM to look at object (nm example.o)

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LinkerLinker

Combines objects, both user .o files Combines objects, both user .o files and libraries; makes an executable and libraries; makes an executable file file

gcc *.o –lm yields a.outgcc *.o –lm yields a.out gcc –o myExec *.o –lm gcc –o myExec *.o –lm Use nm to look at executableUse nm to look at executable

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a.out

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Program

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LoaderLoader

Runs when you type ./a.outRuns when you type ./a.out

Gets an address to place program Gets an address to place program (from the operating system)(from the operating system)

Changes necessary addresses (if Changes necessary addresses (if any)any)

Places code into memory Places code into memory

Operating SystemOperating System

OS is ALWAYS runningOS is ALWAYS running Oversees whole compilation processOversees whole compilation process

““RecognizesRecognizes”” gcc example.c command gcc example.c command Parses flags and argumentsParses flags and arguments Invokes gcc scriptInvokes gcc script Performs memory management (malloc)Performs memory management (malloc) Chooses Chooses ““addressaddress”” to place program to place program

Compiler OptimizationCompiler Optimization

What does optimization mean?What does optimization mean? What does compiler optimization mean?What does compiler optimization mean? ““Optimization” takes more time to Optimization” takes more time to

compile in hopes of generating faster compile in hopes of generating faster executableexecutable

-O# is the flag to ask for optimization-O# is the flag to ask for optimization gcc –O3 tsp.c gives “highest” level gcc –O3 tsp.c gives “highest” level