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

[Jan Beulich]

On 29.08.2025 08:51, Jan Beulich wrote:
> 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.

I've meanwhile realized though that [1] wouldn't cover this issue. Some
alternative syntax would still be needed, and the suggested form looks
quite reasonable to me.

Jan

> 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