[AArch64] Assembly syntax for relocation specifier in data directives

[MaskRay]

https://reviews.llvm.org/D81446 and #72584 , introduced the assembly syntax .word sym@plt-. and .word sym@gotpcrel for R_AARCH64_PLT32 and R_AARCH64_GOTPCREL32 relocations.
This syntax was chosen due to a quirk in LLVM's AsmParser, where @plt was universally accepted until my recent update (commit a067175) made it an opt-in parsing feature.

( https://reviews.llvm.org/D156505 introduced @AUTH(ib, 1234, addr))

FYI: For RISC-V, I am proposing to remove @plt @gotpcrel as there is no backward compatibility concern on -fexperimental-c++-abi-vtables generated assembly #132569

The use of @plt as a specifier applied to every subexpression is considered awkward.
The reliance on MCSymbolRefExpr::VariantKind for representing these relocations introduces complexity and inconsistencies within LLVM's internal representation.
As detailed in https://maskray.me/blog/2025-03-16-relocation-generation-in-assemblers#relocation-specifier-flavors , this approach highlights inelegance in how LLVM handles relocation specifiers.

As a more concrete example, https://reviews.llvm.org/D156505 , which introduced @AUTH(ib, 1234, addr), had to reject all kinds of invalid syntax
after parsing time (e.g. .quad _g9@AUTH(ia,42) - _g8@AUTH(ia,42)), which is not ideal.

A more standard syntax like .word :specifier: expr would be more desirable (another specifier cannot appear in a subexpression), but there is a parsing ambiguity: .word :plt:fun would be parsed as labels, and not as a relocation specifier.
(RISC-V could use .word %specifier(expr), which has no ambiguity. I have proposed #132569 )

This issue aims to record the current undesirable and inconsistent state. @smithp35

(AArch32's expr(specifier) is probably not a good choice either, as (specifier) could also appear at any subexpression, e.g. .long f(got)+3)

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Fangrui Song (MaskRay)

https://reviews.llvm.org/D81446 and https://github.com//pull/72584 , introduced the assembly syntax `.word sym@plt-.` and `.word sym@gotpcrel` for R_AARCH64_PLT32 and R_AARCH64_GOTPCREL32 relocations. This syntax was chosen due to an existing quirk in LLVM's AsmParser, where `@plt` was universally accepted until my recent update (commit a067175) made it an opt-in parsing feature.

( https://reviews.llvm.org/D156505 introduced @<!-- -->AUTH(ib, 1234, addr)

The use of @<!-- -->plt as a specifier applied to every subexpression is considered awkward.
The reliance on `MCSymbolRefExpr::VariantKind for representing these relocations introduces complexity and inconsistencies within LLVM's internal representation.
As detailed in https://maskray.me/blog/2025-03-16-relocation-generation-in-assemblers#relocation-specifier-flavors , this approach highlights inelegance in how LLVM handles relocation specifiers.

As a more concrete example, https://reviews.llvm.org/D156505 , which introduced @<!-- -->AUTH(ib, 1234, addr), had to reject all kinds of invalid syntax
after parsing time (e.g. .quad _g9@<!-- -->AUTH(ia,42) - _g8@<!-- -->AUTH(ia,42)), which is not ideal.

A more standard syntax like .word :specifier: expr would be more desirable (another specifier cannot appear in a subexpression), but there is a parsing ambiguity: .word :plt:fun would be parsed as labels, and not as a relocation specifier.
(RISC-V could use .word %specifier(expr), which has no ambiguity. I have proposed #132569 )

This issue aims to record the current undesirable and inconsistent state. @smithp35

[smithp35]

Apologies for the delay in responding. Just to confirm I've understood it, the Sysvabi64 documents the modifiers for instructions https://github.com/ARM-software/abi-aa/blob/main/sysvabi64/sysvabi64.rst#64assembler-language-addressing-mode-conventions which all use :modifier: syntax. AAIU these all prefix the symbol, for example adrp x0, :got: sym

What is wanted is some kind of syntax for data directives that isn't @? With something like :modifier(expr) a suggestion.

It is something we can look at for new modifiers that are proposed. I guess we're stuck with @ for the existing use cases for backwards compatibility.

[MaskRay]

Before https://reviews.llvm.org/D81446 in 2020, which added data directive sym@plt for the clang -fexperimental-relative-c++-abi-vtables feature, the sym@specifier syntax was either rejected outright or triggered assertion failures in AArch64ELFObjectWriter::getRelocType. In non-assertion builds, it might be simply accepted and ignored (for MCSymbolRefExpr::VariantKind constants that I am eliminating).

Indeed, we aim for proper assembly syntax in data directives, rather than having @ inadvertently accepted by the LLVM AsmParser (fixed by my a067175).

Because of the label ambiguity in .word: specifier: expr (accepted by existing assemblers, even unaware of the new syntax), the :specifier: expr syntax for code might not be repurposed.

.word %gotpcrel(sym), %gotpcrel(sym) and .word %auth(...)(expr) (AUTH needs extra arguments) are a choice.

The only allowed ELF @specifier are @got and @gotpcrel, dedicated to the clang -fexperimental-relative-c++-abi-vtables feature.
GNU assembler doesn't support @specifier. I believe there is no backward compatibility concern.

[smithp35]

I've made a suggestion to use :operator(expr) for data directives as part of ARM-software/abi-aa#330 we'll see what the feedback is from the GNU team.

Another possibility that avoids the parsing ambiguity is to add the operator postfix `.xword expr :: but I'm not sure that helps.