How to make the GNU Assembler use a slash / for comments?
GNU GAS docs
Under the "Machine Dependencies" section, go into each arch, and then into "Syntax" and "Chars".
This documents what the comments are for each arch.
x86
https://sourceware.org/binutils/docs-2.26/as/i386_002dChars.html#i386_002dChars
The presence of a '#' appearing anywhere on a line indicates the start of a comment that extends to the end of that line.
If a '#' appears as the first character of a line then the whole line is treated as a comment, but in this case the line can also be a logical line number directive (see Comments) or a preprocessor control command (see Preprocessing).
If the --divide command line option has not been specified then the '/' character appearing anywhere on a line also introduces a line comment.
However, I either I'm missing something, or there is a bug, since my tests don't match the documentation.
OK:
/ mycomment
# mycomment
nop # mycomment
Fail:
nop / mycomment
This suggests that / only works if it is the first character.
And --divide didn't make any difference.
arm
https://sourceware.org/binutils/docs-2.26/as/ARM_002dInstruction_002dSet.html#ARM_002dInstruction_002dSet
The presence of a '@' anywhere on a line indicates the start of a comment that extends to the end of that line.
If a '#' appears as the first character of a line then the whole line is treated as a comment, but in this case the line could also be a logical line number directive (see Comments) or a preprocessor control command (see Preprocessing).
My tests with arm-linux-gnuabihf-as confirm what the documentation says.
OK:
# mycomment
@ mycomment
nop @ mycomment
Fail:
nop # mycomment
aarch64
https://sourceware.org/binutils/docs-2.26/as/AArch64_002dChars.html#AArch64_002dChars
The presence of a '//' on a line indicates the start of a comment that extends to the end of the current line. If a `#' appears as the first character of a line, the whole line is treated as a comment.
Furthermore, this is also encouraged by the ARMv8-fb manual has at C1.2 "Structure of the A64 assembler language" itself:
In Example C1-1 on page C1-185, the sequence // is used as a comment leader and A64 assemblers are encouraged to accept this syntax.
My tests with aarch64-linux-gnuabihf-as confirm what the documentation says.
OK:
// mycomment
# mycomment
nop // mycomment
Fail:
nop # mycomment
Personal recommendation
If you can choose, just always compile your assembly with gcc or use the C preprocessor cpp explicitly, and use C preprocessor comments:
/* mycomment */
because:
- C is standardized, and it will work for all archs
- you will need the C preprocessor in any case, because GNU GAS macros are not powerful enough
#is bad as it could conflict with the#preprocessor directives
Tested on Ubuntu 16.04, Binutils 2.26.1.
Yes, keep using # and you'll get used to it.
There may be ways of getting / to work but then your code isn't just processor-specific but literally computer-specific. You're better-off getting used to small things than completely destroying the portability of your code to fancy a whim.
In Gnu assembler the comment start character is target specific. For i386 and x86_64 it is #. For ARMv7 it is @.
Some other comment conventions work under some conditions. I'm not sure about the details. // comment starter and /* */ multi line comments are examples I have seen.