[llvm-dev] Backward references in assembly absolute expressions

Petr Hosek via llvm-dev llvm-dev at lists.llvm.org
Tue May 17 14:59:35 PDT 2016

I've created a patch which adds support for symbolic expressions in
absolute expressions which matches the behavior of GNU assembler, would
someone be willing to review it?


On Tue, Apr 19, 2016 at 2:19 PM Petr Hosek <phosek at chromium.org> wrote:

> While trying to compile an existing codebase which uses handwritten
> assembly with LLVM, I ran into an issue around using backward references in
> assembly absolute expressions. A simple example can be the following
> snippet:
> _foo:
>   .fill 0x100
> _bar:
>   .fill _bar - _foo
> _baz:
>   .fill 0x100
> While gas compiles this snippet without any errors, the integrated
> assembler throws an error: expected absolute expression for _bar - _foo.
> I haven't found any definition of absolute expression in gas manual, and
> it's arguable whether this case should be considered an absolute
> expression, but at the point of evaluating the directive, the addresses of
> backward labels should be already known so this expression could be treated
> as absolute. A quick search also revealed several bug reports related to
> this issue so it seems like this use case is fairly common.
> I'd be happy to try and implement the support for backward references in
> MC (unless someone else is already working on this), but before I do invest
> more time on this issue, I'd like to ask if this is desirable or if the
> lack of support for backward references is intentional?
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