nobbele / POT

C Preprocessor and Optimizer Thingy

C Preprocessor and Optimizer Thing

Run clang -O3 -S -mllvm --x86-asm-syntax=intel -o- MYFILE to get the result.

If you have Powershell you can use (clang -O3 -S -mllvm --x86-asm-syntax=intel -o- MYFILE | Select -Index 10).Split(",")[1].Trim() to get the result.

The result is the number thats in the selected cell at the end of the code.

Examples:

example 1

#include "POT.h"

POTTEMPLATE(
  INC INC INC INC INC
  BEGINLOOP
    RIGHT INC INC INC LEFT DEC
  ENDLOOP
  RIGHT
)

Gives the output:

        .text
        .intel_syntax noprefix
        .def     test;
        .scl    2;
        .type   32;
        .endef
        .globl  test                    # -- Begin function test
        .p2align        4, 0x90
test:                                   # @test
# %bb.0:
        mov     dword ptr [rip + out], 15
        xor     eax, eax
        ret
                                        # -- End function

When using clang -O3 -S -mllvm --x86-asm-syntax=intel -o- test.c.

You can see the "15" at the bottom which is the result.

How It Works

The C code that gets produced for Example 1 looks like

int test() { 
  char _cells[64] = {0};
  char *const cells = _cells; 
  int cell_index = 0; 
  
  ++cells[cell_index]; 
  ++cells[cell_index]; 
  ++cells[cell_index]; 
  ++cells[cell_index]; 
  ++cells[cell_index]; 
  for(;cells[cell_index];) { 
    ++cell_index; 
    ++cells[cell_index]; 
    ++cells[cell_index]; 
    ++cells[cell_index]; 
    --cell_index; 
    --cells[cell_index];
  }
  
  ++cell_index; 
  out = cells[cell_index]; 
  return 0;
}

When formatted, if you know C you should understand how this works. This will get compiled into assembly which looks like

https://pastebin.com/d6vDBAEy

But the magic happens after the optimizations, this code will become

https://pastebin.com/zQPNPmSv

And there is the result, 15.