{"items":[{"id":"44d48968-2e39-448a-aed3-7197fa28aaf9","title":"My Compiler is 1.75x smaller than C compiler","slug":"my-compiler-is-1-75x-smaller-than-c-compiler","contentHtml":"

First, I'm a begginer. QBE compiler uses libc libraries for C functions. But C libraries are heavier than my custom functions.

It doesn't mean my assembly code is perfect. Howover it's simple. Just look at this code. It's doesn't use libc.

export function w $main() {               \n    @start \n        call $print(l $str)\n        call $exit(w 0)\n        ret\n}\n\ndata $str = { b \"hello world\\n\", b 0 }

And QBE generates this assembly code.

.text\n.globl main\nmain:\n\tpushq %rbp\n\tmovq %rsp, %rbp\n\tleaq str(%rip), %rdi\n\tcallq print\n\tmovl $0, %edi\n\tcallq exit\n\tleave\n\tret\n.type main, @function\n.size main, .-main\n/* end function main */\n\n.data\n.balign 8\nstr:\n\t.ascii \"hello world\\n\"\n\t.byte 0\n/* end data */\n\n.section .note.GNU-stack,\"\",@progbits

As we can see, leaq means loads the address of str into %rdi register. From this I understood my print function must use %rdi register for the first parameter.

I've written print function and linked them together. We need to understand syscalls.

.intel_syntax noprefix\n.global print\n.text\n\nprint:\n    mov rsi, rdi   \n    mov rax, 1      \n    mov rdi, 1      \n    mov rdx, 12    \n    syscall\n    ret

If you don't understand, You can check out this website https://www.chromium.org/chromium-os/developer-library/reference/linux-constants/syscalls/

Now let's compare the C code with the custom assembly code. app--> Custom Assembly result,

main --> Compiled C Executable file.

the same result, but the sizes are different. Our Compiler 1.75x smaller than C compiler

But how? Because we don't use llvm. We are using a simple compiler called qbe. And We are writing custom assembly sometimes.

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