On 13/10/2016 08:11, Debajyoti Chatterjee wrote:
> Hello,
> I was wondering if gcc can optimize this type of for loop using loop
> rollback technique,
> for(int i=5;i--;){//body}
> in general we use,
> for(int i=0;i<5;i++){//body} , this type of loops are easy to
> understand for compilers and as 5 is constant compiler can apply loop
> rollback technique to optimize.
> I wonder which type of for loop will be good to use.
Do you read assembly code?
If so, https://gcc.godbolt.org/ is nice to examine the assembly code
output by different versions of gcc.
For example, paste the following code in the source window:
extern void foo(int);
void do_loop(void) {
int i;
for (int i=5; i--; ) {
foo(i);
}
}
Then x86-64 gcc 6.2 -O3 will output
do_loop():
push rbx
mov ebx, 4
.L2:
mov edi, ebx
sub ebx, 1
call foo(int)
cmp ebx, -1
jne .L2
pop rbx
ret
What exactly is loop rollback?
https://en.wikipedia.org/wiki/Loop_optimization#Common_loop_transformations
Regards.
