What is the difference? - cpu-architecture

Question

I was wondering,is there a difference in execution time between ++i and i++ in the for loop increment?
1)for(int i=0;i<100;++i)
2)for(int i=0;i<100;i++)
I have heard that the one with the pre-increment uses less registers, and hence, it is faster.
Is it correct?

Answer 1

Not these days no; if you can optimise it in your head then you can bet your bottom dollar that a compiler can also optimise it.
If you are in any doubt, check the generated assembly / bytecode &c.

Answer 2

This seems to be C code. In this case, there isn't any difference: try compiling those two sources with a compiler and ask it to keep intermediate assembler code.
Then use diff to see if the assembler files differ.
On FreeBSD with clang, on an x86_64 architecture, you can check that both of the files are strictly the same. The pre-increment is done the same way as the post-increment: addl $1, %eax.
Create x.c:
int main() {
for(int i=0;i<100;++i){}
}
Compile it:
% cc -S x.c
%
look at the code:
.file "x.c"
.text
.globl main
.align 16, 0x90
.type main,#function
main: # #main
.cfi_startproc
# BB#0:
pushq %rbp
.Ltmp2:
.cfi_def_cfa_offset 16
.Ltmp3:
.cfi_offset %rbp, -16
movq %rsp, %rbp
.Ltmp4:
.cfi_def_cfa_register %rbp
movl $0, -4(%rbp)
movl $0, -8(%rbp)
.LBB0_1: # =>This Inner Loop Header: Depth=1
cmpl $100, -8(%rbp)
jge .LBB0_4
# BB#2: # in Loop: Header=BB0_1 Depth=1
jmp .LBB0_3
.LBB0_3: # in Loop: Header=BB0_1 Depth=1
movl -8(%rbp), %eax
addl $1, %eax
movl %eax, -8(%rbp)
jmp .LBB0_1
.LBB0_4:
movl -4(%rbp), %eax
popq %rbp
ret
.Ltmp5:
.size main, .Ltmp5-main
.cfi_endproc
.ident "FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512"
.section ".note.GNU-stack","",#progbits
Do the same with --i instead of ++i.