Different appearance between SDCC and Keil with the same code in 8051

Question

I write a delay function:

void delay(a){
    for (int i=a;i>0;i--) 
        for (int j=0; j<200;j++)
} 

But when I compiled this code by sdcc and keil, and run in 8051 chip. The result is that the delay function compiled by sdcc runs much more slower than the function compiled by keil.

Can someone tell me why...

Answer 1

Different compilers use different realizations in machine language. There are several issues, not limited to, but what pops up in my mind:

• No standard definition of translation into machine code: Each compiler may use any solution that complies to the standard. There is more than one possible solution.
• Different compiler behavior: Each compiler has its own set of options to change specific generation variants.
• Optimization levels: An empty loop might be optimized away completely, for example.
• Variable allocation: Compilers are free to select registers or RAM cells to use for their variables.
• The bit widths of (in your case) int: Probably it's the same with SDCC and Keil, but sometimes there are differences.