The reason it doesn't work is that __asm
is a keyword, just like int
is a keyword, it cannot appear by itself and must follow the proper syntax. Take the following bit of code as an example:
int main()
{
int // here's a comment, but it's ignored by the compiler
return 0;
}
The following code will fail with a compilation error, more specifically in VS2012 you get error C2143: syntax error : missing ';' before 'return'
. This is an obvious error since we do not have the ending semi-colon to denote end of instruction; add the semi-colon and it compiles fine because we did not dis-obey the syntax of the C (or C++ in this case) language:
int main()
{
int // here's a comment, but it's ignored by the compiler
; // white space and comments are ignored by the compiler
return 0;
}
The same is true of the following code:
int main()
{
__asm ; here's a comment but it's ignored
return 0;
}
Except here we get the error error C2400: inline assembler syntax error in 'opcode'; found 'constant'
, becuase it's treating everything after the __asm
keyword as an assembler instruction and the comment is being rightfully ignored .. so the following code WOULD work:
int main()
{
__asm ; here's a comment but it's ignored
NOP ; white space and comments are ignored by the compiler
__asm {; here's an __asm 'block'
} // outside of __asm block so only C style comments work
return 0;
}
So that answers your first question: Why didn't VS allow me to add an assembly comment?
.. because it is a syntax error.
Now for your second question: Is there a different way to add an assembly comment without adding an instruction, including NOP?
Directly, no, there is not, but indirectly, yes there is. It's worth noting that the __asm
keyword gets compiled into inline assembly in your program, so comments will be removed from the compiled assembly just as if it were a standard C/C++ comment, so trying to 'force' a comment in your assembly via that method is not necessary, instead, you can use the /FAs
compiler flag and it will generate the assembly (machine code) mixed with the source, example:
Given the following (very simple) code:
int main()
{
// here's a normal comment
__asm { ; here's an asm comment and empty block
} // here's another normal comment
return 0;
}
When compiled with the /FAs
compiler flag, the file.asm
that was produced had the following output in it:
; Listing generated by Microsoft (R) Optimizing Compiler Version 18.00.31101.0
TITLE C:estfile.cpp
.686P
.XMM
include listing.inc
.model flat
INCLUDELIB LIBCMT
INCLUDELIB OLDNAMES
PUBLIC _main
; Function compile flags: /Odtp
; File c:estfile.cpp
_TEXT SEGMENT
_main PROC
; 2 : {
push ebp
mov ebp, esp
; 3 : // here's a normal comment
; 4 : __asm { ; here's an asm comment and empty block
; 5 : } // here's another normal comment
; 6 : return 0;
xor eax, eax
; 7 : }
pop ebp
ret 0
_main ENDP
_TEXT ENDS
END
Notice how it includes the source and comments. If this code did more, you would see more assembly and the source associated with that as well.
If you're wanting to put comments in the inline assembly itself, then you can use normal C/C++ style comments as well as assembly comments within the __asm
block itself:
int main()
{
// here's a C comment
__asm { ; here's an asm comment
// some other comments
NOP ; asm type comment
NOP // C style comment
} // here's another comment
return 0;
}
Hope that can help.
EDIT:
It should be noted the following bit of code also compiles without error and I'm not 100% sure why:
int main()
{
__asm
__asm ; comment
// also just doing it on a single line works too: __asm __asm
return 0;
}
Compiling this code with the single __asm ; comment
gives the compilation error, but with both it compiles fine; adding instructions to the above code and inspecting the .asm
output shows that the second __asm
is ignored for any other assembly commands preceding it. So I'm not 100% sure if this is a parsing bug or part of the __asm
keyword syntax as there's no documentation on this behavior.