Re: gas: Clarify whitespace between a label’s symbol and its colon

[Jan Beulich]

On 29.08.2025 06:50, Fangrui Song wrote:
> Filed https://sourceware.org/bugzilla/show_bug.cgi?id=33336 but also
> posted it here for visibility.
> 
> AArch32 and AArch64 adopt this colon-framed prefix notation for
> relocation specifiers
> 
> // AArch32
> movw    r0, :lower16:x
> 
> // AArch64
> add     x8, x8, :lo12:sym
> 
> While this syntax works fine for the second or third operand, applying
> it to the first operand creates parsing ambiguity.
> gas interprets the mnemonic or directive name followed by a colon as a
> label (e.g., insn :lo12:sym or .word :lo12:sym).

Does any insns exist though permitting the first operand to have a
relocation specifier? (For directives the issue is present, sure, and I
expect that right now this can be made work only ...

> What happens is that gas/app.c:do_scrub_chars transforms `.word
> :op:nop` into `.word:op:nop`   (#APP does not seem to suppress the
> transform), and the first word is interpreted as a label.

... by passing -f (placing more constraints on how the source file may
look like). Without -f (or #NO_APP at the start of the file) further
#APP in the file don't really have any effect.

> Would it be feasible to break backward compatibility by disallowing
> space before the colon?

There are far more issues in this area, i.e. dealing with just labels and
their colons won't work very well. I expect we'd end up with more hacks
rather than a tidier (and consistent) overall implementation. See [1],
which needed to be reverted for causing too much fallout, and which I
still mean to make a 2nd attempt on. Just need to find enough time (in
preferably large enough chunks) to actually get to it.

That said, app.c has some special treatment of colons already, so dealing
with just the special case may be possible in not overly intrusive a way.

> If not, we need an alternative notation for
> AArch32/AArch64 where the .word operand requires a relocation
> specifier.
> I recommend  .word %gotpcrel(sym) , identical to what RISC-V uses
> (https://maskray.me/blog/2025-03-16-relocation-generation-in-assemblers
> )
> 
> If we keep the implementation that interprets `.word :op:nop` as
> `.word: op: nop`, we should fix this sentence in the documentation
> https://sourceware.org/binutils/docs/as/Statements.html
> 
>> Whitespace before a label or after a colon is permitted, but you **may not have whitespace between a label’s symbol and its colon**. See Labels.

Documentation is (imo) correct; it's the scrubber which is flawed.

Jan

[1] https://sourceware.org/git?p=binutils-gdb.git;a=commitdiff;h=6ae8a30d44f016cafb46a75843b5109316eb1996