[llvm-dev] fixup_aarch64_movw support for COFF AArch64

[Adam Kallai via llvm-dev]

Martin,

Thanks for your suggestion.

I look at these tests, try to make them work for COFF.

Adam

On 2019. 10. 02. 12:23, Martin Storsjö wrote:
> On Wed, 2 Oct 2019, Adam Kallai wrote:
>
>> I'm working Chromium targeting Windows on ARM64 platform. As a part 
>> of this work I ran into an issue related to llvm in Swiftshader.
>>
>> Currently fixup_aarch64_movw relocation type is not supported for 
>> COFF ARM64 (AArch64WinCOFFObjectWriter). As far as I see, Microsoft 
>> hasn't defined indicator for this relocation type. I haven't seen 
>> documented anywhere.
>>
>> For AArch32 mova/movt indicators were implemented, I'm not sure but 
>> maybe we need to have something similar for AArch64?
>
> The AArch32 movw/movt relocation was for a true relocation, where the 
> target of the relocation is a symbol that is unknown at the assembly 
> stage. But for AArch64 there is no such relocation defined for COFF.
>
>> Could someone give me some pointers how I could handle/fix this?
>
> I'm not entirely sure, but it seems like this fixup type is only used 
> for absolute values that are resolved before the object file is 
> written - from AArch64AsmBackend.cpp, adjustFixupValue:
>
>   case AArch64::fixup_aarch64_movw:
>     [...]
>     if (!IsResolved) {
>       // FIXME: Figure out when this can actually happen, and verify our
>       // behavior.
>       Ctx.reportError(Fixup.getLoc(), "unresolved movw fixup not yet "
>                                       "implemented");
>       return Value;
>     }
>
> Despite this, it seems like AArch64ELFObjectWriter::getRelocType does 
> return some ELF/AArch64 specific relocation types for this (but which 
> never end up emitted to object files).
>
> I tried adding an error in AArch64AsmBackend.cpp for this fixup type 
> and running the tests in llvm/test/MC (I didn't check other parts of 
> the testsuite), and it broke two tests, MC/AArch64/fixup-absolute.s 
> and MC/AArch64/fixup-absolute-signed.s. And these two produce ELF 
> object files without relocations.
>
> So I would aim at making these two testcases work for COFF files, 
> which shouldn't require making up any new COFF relocation types, at 
> least not ones that would end up visible outside of the 
> lib/Target/AArch64 internals.
>
> // Martin
>