Compiler optimization causing program to run slower

Question

I have the following piece of code that I wrote in C. Its fairly simple as it just right bit-shifts x for every loop of for.

int main() {
   int         


        

Answer 1

The optimized loop is producing an infinite loop which is a result of you depending on signed integer overflow. Signed integer overflow is undefined behavior in C and should not be depended on. Not only can it confuse developers it may also be optimized out by the compiler.

Assembly (no optimizations): gcc -std=c99 -S -O0 main.c

_main:
LFB2:
    pushq   %rbp
LCFI0:
    movq    %rsp, %rbp
LCFI1:
    movl    $1, -4(%rbp)
    movl    $0, -8(%rbp)
    jmp L2
L3:
    incl    -8(%rbp)
L2:
    cmpl    $-2, -8(%rbp)
    jg  L3
    movl    $0, %eax
    leave
    ret

Assembly (optimized level 3): gcc -std=c99 -S -O3 main.c

_main:
LFB2:
    pushq   %rbp
LCFI0:
    movq    %rsp, %rbp
LCFI1:
L2:
    jmp L2  #<- infinite loop

Answer 2

You will get the definitive answer by looking at the binary that's produced (using objdump or something).

But as others have noted, this is probably because you're relying on undefined behaviour. One possible explanation is that the compiler is free to assume that i will never be less than -2, and so will eliminate the conditional entirely, and convert this into an infinite loop.

Also, your code has no observable side effects, so the compiler is also free to optimise the entire program away to nothing, if it likes.

Answer 3

Additional information about why integer overflows are undefined can be found here:

http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html

Search for the paragraph "Signed integer overflow".