In programming, we often need to use infinite loops, two commonly used methods are while (1) and for (;;). Both methods work exactly the same, but which one is better? Let's take a look at their compiled code:
Before compiling:
while (1)；
After compilation:
mov eax，1
test eax，eax
your foo+23h
jmp foo+18h
Before compiling:
for (；；)；
After compilation:
jmp foo+23h
Obviously, for (;;) has fewer instructions, does not occupy registers, and has no judgment or jump, which is better than while (1). This is because the previous compilers have inconsistent compilation processing of these two kinds of wireless loops.
Third, the impact of modern compilers (gcc-4.1.1-52-e15) on these two wireless loops?
First look at for.c
int main()
{
for(;;);
return 0;
}
First look at while.c
intmain()
{
while(1);
return 0;
}
Compile with gcc
gcc –g –c for.c –o for.o
gcc –g –c while.c –o while.o
objdump –j .text –Sl while.o > while.txt
objdump –j .text –Sl for.o > for.txt
As a result, from the assembly point of view, the assembly codes of for(;;) and while(1) are both
4 : eb fe jmp 4
The conclusion is: their assembly instructions are the same, so the efficiency is the same
The while (1) infinite loop is often used in the programming of the single-chip microcomputer to perform the round-robin operation, but for (;;) is often seen as the condition of the infinite loop when analyzing the Linux kernel source code.
The difference between the two:
difference one
The two ;;s in the for(;;) infinite loop represent two empty statements, and the compiler will generally optimize them away and enter the loop body directly.
The 1 in the while(1) infinite loop is regarded as an expression, and every time the loop is to determine whether the constant 1 is equal to zero.
difference two
for.c source code:
include
include
Reprinted from: https://blog.csdn.net/nh5431313/article/details/68482778