Question 134751 in Visual Studio Code Question posted in
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Visual Studio Code – jmp instruction difference in assembly code when used goto between with C and C++

I am using same code snippet in C and C++.

#include <stdio.h>

int main() {
    goto myLabel;
    printf("skippedn");
myLabel:
    printf("after myLabeln");
    return 0;
}

Using Visual Studio 2022 IDE and Compiler.

Assembly Code for C++

0000000140001000  sub         rsp,28h  
0000000140001004  jmp         0000000140001014  
0000000140001006  jmp         0000000140001014  
0000000140001008  lea         rcx,[0000000140004230h]  
000000014000100F  call        0000000140001090  
0000000140001014  lea         rcx,[0000000140004240h]  
000000014000101B  call        0000000140001090  
0000000140001020  xor         eax,eax  
0000000140001022  add         rsp,28h  
0000000140001026  ret

Assembly Code for C

0000000140001000  sub         rsp,28h  
0000000140001004  jmp         0000000140001012  
0000000140001006  lea         rcx,[0000000140006000h]  
000000014000100D  call        0000000140001090  
0000000140001012  lea         rcx,[0000000140006010h]  
0000000140001019  call        0000000140001090  
000000014000101E  xor         eax,eax  
0000000140001020  add         rsp,28h  
0000000140001024  ret

Question is why C++ assembly code uses 2 jmp instructions when C is using 1.

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2

Answers


  1. 0

    C and C++ are two completely different programming languages.

    Different compilers are used to compile them. The actual compiler might be a single, monolithic program, but functionally there are two logically distinct compilers and algorithms, that have nothing to do with each other.

    It is not entirely unexpected that different compilers will generate different compiled code from syntactically identical source. The differences in the resulting compiled code result from the different algorithms that are employed by the different compilers, when translating the source code. The differences carry no special, inherent meaning.

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  2. 0

    It is like this by design in debug builds (msvc bug database) see :
    S2019 (debug, x86) generates two identical JMP instructions for one goto statement

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