gcc / g ++输出类型

Question

I know this is a very basic question but when I compile my c/c++ code with gcc/g++ what exactly is the type of the intermediate output before assembler comes into play to generate the machine code ? Is it something like X86 instructions ?

Answer 1

GCC's processing chain is as follows:

• your source code

• preprocessed source code (expand macros and includes, strip comments) ( -E , .ii )

• compile to assembly ( -S , .s )

• assemble to binary ( -c , .o )

• link to executable

At each stage I've listed the relevant compiler flags that make the process stop there, as well as the corresponding file suffix.

If you compile with -flto , then object files will be embellished with GIMPLE bytecode, which is a type of low-level intermediate format, the purpose of which is to delay the actual final compilation to the linking stage, which allows for link-time optimizations.

The "compiling" stage proper is the actual heavy lifting part. The preprocessor is essentially a separate, independent tool (although its behaviour is mandated by the C and C++ standards), and the assembler and linker are acutally separate, free-standing tools that basically just implement, respectively, the hardware's binary instruction format and the operating system's loadable executable format.

Answer 2

So, compilation of executable in GCC consists of 4 parts:

1.) Preprocessing (gcc -E main.c > main.i; transforms *.c to *.i) Does include expansion, processes marcos. Removes comments.

2.) Compilation (gcc -S main.i; transforms *.i to *.s, if successful) Compiles C-code to Assembler (on target x86 architecture it is x86-assembly, on target x86_64 architecture it is x64-assembly, on target arm architecture it is arm assembly, etc.) Most of Warnings and Errors happens during this part (eg does Error and Warning reporting)

3.) Assembly (as main.s -o main.o; transforms *.i to *.o, again if successful) Assemblies generated assembler to machine code. Though there are still relative address of procedures, and such.

4.) Linking (gcc main.o) Replaces relative addresses with absolute addresses. Removes useless text. Linking errors and warnings during this phase. And in the end (if successful), we get executable file.

So, to answer your question, the intermediate output you mean is actually so called assembly language - see wiki about that Assembly language wiki .

Answer 3

Here's a graphic representation of the gcc compilation steps by courtesy of redhat magazine :

Contrary to what other answers imply, there's no assembly step - rather, generating assembler code replaces the object code generation; it doesn't make much sense to convert an in-memory representation to a textual one if what you really want is a binary representation.

Answer 4

It must be assembly code. You can get it using -S flag in command line for compilation.

Answer 5

There is no "intermediate output". The first output you get is machine code. (Although you can get C/C++ intermediate output by invoking only the preprocessor with -E .)

Answer 6

GCC tool chain, compiles the program from source code down to machine code. The compiler generates the assembly code which the assembler assembles into the machine code. Here is a good tutorial for beginners.