Can someone explain in what order a CPU would process the following arithmetic problem: 5 - (-9) = 14?
Would the CPU recognize that the 'minus a minus' combination simply represents 5 + 9, and proceed with that addition, or would the CPU have to first calculate the 2's complement of -9, and then proceed to take the 2's complement of that first result in order to complete the calculation of the addition of the 'double negative'?
This:
As you can see the piece of code that you are talking about does not go beyond compilation.Code:#include <stdio.h> int main() { int a = 5 -(-6); printf("%d", a); return 0; }
As a side-note, the CPU executes what is in the binary without any bias meaning if the binary dictates the process as a 'double minus' it will do a double minus, if the binary dictates the process as an 'addition' it will do an addition.Code:.file "main.c" .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "%d" .section .text.startup,"ax",@progbits .p2align 4,,15 .globl main .type main, @function main: .LFB11: .cfi_startproc subq $8, %rsp .cfi_def_cfa_offset 16 movl $11, %esi movl $.LC0, %edi xorl %eax, %eax call printf xorl %eax, %eax addq $8, %rsp .cfi_def_cfa_offset 8 ret .cfi_endproc .LFE11: .size main, .-main .ident "GCC: (SUSE Linux) 4.8.1 20130909 [gcc-4_8-branch revision 202388]" .section .note.GNU-stack,"",@progbits
Compilers typically implement an optimisation known as constant folding, so an expression like 5 - (-9) would be replaced by 14 at compile time, hence at run time no further evaluation is done. If not, your latter scenario would be done, i.e., -9 would be evaluated, then the overall expression would be evaluated. (Order of evaluation is generally unspecified, but here it is obvious.)
Note that two's complement is not the only possible method of representation for signed integers permitted in C.
Look up a C++ Reference and learn How To Ask Questions The Smart WayOriginally Posted by Bjarne Stroustrup (2000-10-14)
