In x = x + 1
, is x
evaluated twice? If so, does that mean in x += 1
, x
is only evaluated once? How are the two expressions evaluated in terms of compiler intermediate code?
For example, x++
could mean : take the location of x
, load the contents of x
into a register, and increment the value of x
in memory.
Also I have read that x += 1
is useful when x
is not a simple variable, but an expression involving an array. Any ideas why this is the case?
Just to give you a "real-world" example, consider this program:
int main()
{
int i = 0;
i += 1;
i++;
i = i + 1;
return 0;
}
Compiling it with GCC, in Darwin 11 with the following flags:
-S
stop in assembler-m32
to 32-bit platform, just to simplify things a bitWill generate the following program, except for the comments and blank lines which I added. Take a look specially in the comments.
.section __TEXT,__text,regular,pure_instructions
.globl _main
.align 4, 0x90
_main:
pushl %ebp # cdecl function stuff
movl %esp, %ebp #
subl $12, %esp # get room for variables
movl $0, -12(%ebp) # i = 0;
; i += 1
movl -12(%ebp), %eax # load i in register a
addl $1, %eax # add 1 to register a
movl %eax, -12(%ebp) # store it back in memory
; i++
movl -12(%ebp), %eax #
addl $1, %eax # just the same
movl %eax, -12(%ebp) #
; i = i + 1
movl -12(%ebp), %eax #
addl $1, %eax # just the same
movl %eax, -12(%ebp) #
movl $0, -8(%ebp)
movl -8(%ebp), %eax
movl %eax, -4(%ebp)
movl -4(%ebp), %eax
addl $12, %esp
popl %ebp
ret
.subsections_via_symbols