[llvm-dev] Backward references in assembly absolute expressions

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Tue May 17 18:23:43 PDT 2016

Looping in people with at least some interest in this side of things...

On Tue, May 17, 2016 at 2:59 PM Petr Hosek via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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?
> http://reviews.llvm.org/D20337
> 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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