GCC compilation

1 GCC The build process

First let's look at GCC Compilation process .hello.c The contents of the document are as follows .

#include <stdio.h>

int main()
{
    
	printf("Hello World!");
	return 0;
}

gcc hello.c -o hello -save-temps --verbose

Add two compilation options when compiling , The first option is to save the intermediate files generated during compilation , The second is for viewing GCC Detailed workflow of compilation .

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: …/src/configure -v --with-pkgversion=‘Ubuntu 9.3.0-10ubuntu2’ --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)
COLLECT_GCC_OPTIONS=‘-o’ ‘hello’ ‘-save-temps’ ‘-v’ ‘-mtune=generic’ ‘-march=x86-64’
/usr/lib/gcc/x86_64-linux-gnu/9/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu hello.c -mtune=generic -march=x86-64 -fpch-preprocess -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -fcf-protection -o hello.i
ignoring nonexistent directory “/usr/local/include/x86_64-linux-gnu”
ignoring nonexistent directory “/usr/lib/gcc/x86_64-linux-gnu/9/include-fixed”
ignoring nonexistent directory “/usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/…/x86_64-linux-gnu/include”
#include “…” search starts here:
#include <…> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/9/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS=‘-o’ ‘hello’ ‘-save-temps’ ‘-v’ ‘-mtune=generic’ ‘-march=x86-64’
/usr/lib/gcc/x86_64-linux-gnu/9/cc1 -fpreprocessed hello.i -quiet -dumpbase hello.c -mtune=generic -march=x86-64 -auxbase hello -version -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -fcf-protection -o hello.s
GNU C17 (Ubuntu 9.3.0-10ubuntu2) version 9.3.0 (x86_64-linux-gnu)
compiled by GNU C version 9.3.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.22.1-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C17 (Ubuntu 9.3.0-10ubuntu2) version 9.3.0 (x86_64-linux-gnu)
compiled by GNU C version 9.3.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.22.1-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 18dc4c39b54390aa2b5013fb4339d43f
COLLECT_GCC_OPTIONS=‘-o’ ‘hello’ ‘-save-temps’ ‘-v’ ‘-mtune=generic’ ‘-march=x86-64’
as -v --64 -o hello.o hello.s
GNU Assembly version 2.34 (x86_64-linux-gnu) Use BFD edition (GNU Binutils for Ubuntu) 2.34
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/9/:/usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/…/lib/:/lib/x86_64-linux-gnu/:/lib/…/lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/…/lib/:/usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS=‘-o’ ‘hello’ ‘-save-temps’ ‘-v’ ‘-mtune=generic’ ‘-march=x86-64’
/usr/lib/gcc/x86_64-linux-gnu/9/collect2 -plugin /usr/lib/gcc/x86_64-linux-gnu/9/liblto_plugin.so -plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper -plugin-opt=-fresolution=hello.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie -z now -z relro -o hello /usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/9/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/9 -L/usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/…/lib -L/lib/x86_64-linux-gnu -L/lib/…/lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/…/lib -L/usr/lib/gcc/x86_64-linux-gnu/9/…/…/… hello.o -lgcc --push-state --as-needed -lgcc_s --pop-state -lc -lgcc --push-state --as-needed -lgcc_s --pop-state /usr/lib/gcc/x86_64-linux-gnu/9/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/9/…/…/…/x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS=‘-o’ ‘hello’ ‘-save-temps’ ‘-v’ ‘-mtune=generic’ ‘-march=x86-64’

In this message , We can find out .GCC The compilation process mainly includes four processes , Pretreatment stage （Preprocess）, Compile phase （Compile）, assembly （Assemble） And link （Link）, Used separately ccll,as and collect2.
among ,ccl It's a compiler , Used to convert source files hello.c Compiled into hello.s;as It's an assembler , take hello.s Compiled as hello.o, The linker collect It's right ld Encapsulation of commands , Is used to C The target file in the language runtime library and the required dynamic link library are linked to the executable hello.

2 Pretreatment stage

It mainly deals with # Preprocessing instructions at the beginning , Insert it directly into the program text after conversion , Get another C Program , Usually, the i As extension , Add compilation options to the command e Preprocessing can be performed separately .

gcc -E hello.c -o hello.i

Some processing rules ：

• Recursive processing “#include” Preprocessing instruction , Copy the corresponding file contents to the location of the instruction .
• Delete all “”define# Instructions , And recursively expand all macro definitions at the referenced position
• Process all conditional preprocessing instructions
• Delete all comments
• Add line number and filename ID

3 Compile phase

Execute instructions separately

gcc -S hello.i -o -masm=intel -fno-asynchronous-unwind-tables

The first compilation option is to specify the file as we are familiar with intel, The second compilation option is used to generate a non cfi Macro assembly instructions , To improve readability .
Generated hello.s Contents of the file ：

	.file	"hello.c"
	.text
	.section	.rodata
.LC0:
	.string	"Hello World!"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	endbr64
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	leaq	.LC0(%rip), %rdi
	movl	$0, %eax
	call	[email protected]
	movl	$0, %eax
	popq	%rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 9.3.0-10ubuntu2) 9.3.0"
	.section	.note.GNU-stack,"",@progbits
	.section	.note.gnu.property,"a"
	.align 8
	.long	 1f - 0f
	.long	 4f - 1f
	.long	 5
0:
	.string	 "GNU"
1:
	.align 8
	.long	 0xc0000002
	.long	 3f - 2f
2:
	.long	 0x3
3:
	.align 8
4:

4 Assembly stage

gcc -c hello.c -o hello.o
gcc -c hello.s -o hello.o

Both instructions are ok . The target file at this point hello.o Is a relocatable file , have access to objdump Command to view its contents .

Documents$ objdump -sd hello.o -M intel

hello.o：      File format  elf64-x86-64

Contents of section .text:
 0000 f30f1efa 554889e5 488d3d00 000000b8  ....UH..H.=.....
 0010 00000000 e8000000 00b80000 00005dc3  ..............].
Contents of section .rodata:
 0000 48656c6c 6f20576f 726c6421 00        Hello World!.   
Contents of section .comment:
 0000 00474343 3a202855 62756e74 7520392e  .GCC: (Ubuntu 9.
 0010 332e302d 31307562 756e7475 32292039  3.0-10ubuntu2) 9
 0020 2e332e30 00                          .3.0.           
Contents of section .note.gnu.property:
 0000 04000000 10000000 05000000 474e5500  ............GNU.
 0010 020000c0 04000000 03000000 00000000  ................
Contents of section .eh_frame:
 0000 14000000 00000000 017a5200 01781001  .........zR..x..
 0010 1b0c0708 90010000 1c000000 1c000000  ................
 0020 00000000 20000000 00450e10 8602430d  .... ....E....C.
 0030 06570c07 08000000                    .W......        

Disassembly of section .text:

0000000000000000 <main>:
   0:	f3 0f 1e fa          	endbr64 
   4:	55                   	push   rbp
   5:	48 89 e5             	mov    rbp,rsp
   8:	48 8d 3d 00 00 00 00 	lea    rdi,[rip+0x0]        # f <main+0xf>
   f:	b8 00 00 00 00       	mov    eax,0x0
  14:	e8 00 00 00 00       	call   19 <main+0x19>
  19:	b8 00 00 00 00       	mov    eax,0x0
  1e:	5d                   	pop    rbp
  1f:	c3                   	ret

At this time, because there is no link , The virtual address of the symbol in the object file cannot be determined , So we see the string “hello World” The address for 0x0000, Passing a string address as a parameter rdi The register is set to 0x0, and “call puts” Function in instruction puts（） The address of is set to the address of the next instruction 0xe.

5 Link phase

GCC Dynamic links are used by default , Add compile options “static” Can be specified as a static link .

gcc hello.o -o hello -static