CHAPTER 14. LOOPS CHAPTER 14. LOOPS
or $at, $zero ; NOP
slti $v0, 0xA
; if it is less than 10, jump to loc_80 (loop body begin):
bnez $v0, loc_80
or $at, $zero ; branch delay slot, NOP
; finishing, return 0:
move $v0, $zero
; function epilogue:
move $sp, $fp
lw $ra, 0x28+saved_RA($sp)
lw $fp, 0x28+saved_FP($sp)
addiu $sp, 0x28
jr $ra
or $at, $zero ; branch delay slot, NOP
The instruction that’s new to us is “B”. It is actually the pseudoinstruction (BEQ).
14.1.6 One more thing.
In the generated code we can see: after initializingi, the body of the loop is not to be executed, as the condition foriis
checked first, and only after that loop body can be executed. And that is correct. Because, if the loop condition is not met
at the beginning, the body of the loop must not be executed. This is possible in the following case:
for (i=0; i<total_entries_to_process; i++)
loop_body;
Iftotal_entries_to_processis 0, the body of the loo must not be executed at all. This is why the condition checked before the
execution.
However, an optimizing compiler may swap the condition check and loop body, if it sure that the situation described here is
not possible (like in the case of our very simple example and Keil, Xcode (LLVM), MSVC in optimization mode).
14.2 Memory blocks copying routine
Real-world memory copy routines may copy 4 or 8 bytes at each iteration, useSIMD^2 , vectorization, etc. But for the sake of
simplicity, this example is the simplest possible.
#include <stdio.h>
void my_memcpy (unsigned char dst, unsigned char src, size_t cnt)
{
size_t i;
for (i=0; i<cnt; i++)
dst[i]=src[i];
};
14.2.1 Straight-forward implementation
Listing 14.10: GCC 4.9 x64 optimized for size (-Os)my_memcpy:
; RDI = destination address
; RSI = source address
; RDX = size of block
; initialize counter (i) at 0
xor eax, eax
.L2:
; all bytes copied? exit then:
cmp rax, rdx
je .L5
; load byte at RSI+i:
mov cl, BYTE PTR [rsi+rax]
(^2) Single instruction, multiple data