MSVC编译器从if / else语句反汇编代码

Question

I have a general question about how MSVC generates machine code regarding to else statement.

A simple exmaple here:

1 bool is_zero(int num) {
2    if (num)
3        return false;
4    else
5        return true;
6 }

and its disassembly code looks like

; Listing generated by Microsoft (R) Optimizing Compiler Version 19.20.27508.1 

; Function compile flags: /Odtp
num$ = 8
bool is_zero(int) PROC ; is_zero
; File C:\Users\ContainerAdministrator\AppData\Local\Temp\compiler-explorer-compiler11943-18164-1cmj5fb.ujww\example.cpp
; Line 1
  mov DWORD PTR [rsp+8], ecx
; Line 2
  cmp DWORD PTR num$[rsp], 0
  je SHORT $LN2@is_zero
; Line 3
  xor al, al
  jmp SHORT $LN1@is_zero
; Line 4
  jmp SHORT $LN3@is_zero
$LN2@is_zero:
; Line 5
  mov al, 1
$LN3@is_zero:
$LN1@is_zero:
; Line 6
  ret 0
bool is_zero(int) ENDP ; is_zero

Question is: will the line jmp SHORT $LN3@is_zero (corresponding to the 4th line else keyword) get executed?

is there any good reason MSVC generates such code?

Answer 1

It's unoptimized code. That line 4 jmp corresponds to the jump from the if body past the else body. In this case it is never executed. Enable optimizations and it will go away.