How to make the GNU Assembler use a slash / for comments?

Question

This is indeed a stupid idiosyncrasy of mine, but I can't stand the way GNU AS uses to insert a comment. I am too accustomed to the Sun way (the same used in most UNIX assemblers), that uses a simple slash "/" to comment out the code till the end of the line. Do you know of a way to accomplish my little whim?

Accepted Answer

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.

How to make the GNU Assembler use a slash / for comments?

Solution

Problem