[llvm-dev] fixup_aarch64_movw support for COFF AArch64

Tom Tan via llvm-dev llvm-dev at lists.llvm.org
Wed Oct 2 23:16:23 PDT 2019

Hi Adam,

SwiftShader calls LLVM to do JIT for ARM64. LLVM set CodeModel::Large for ARM64 in JIT mode [1] because it assumes a pointer could reach any place in 64-bit address space. The causes LLVM generating 4 MOV instructions to load 64-bit address/constant with each MOV loads 16-bit to the target. Windows ARM64 limited branch range (probably +/- 4GB which is reachable by ADRP/ADD instruction pair) so it doesn't support relocating the previous long branch (4 MOVs), both at compile/link time and load time. There are vary places in LLVM which checks CodeModel::Large and generate code accordingly [2].

Introduce new relocation types to COFF will cause interop issue with MSVC. You probably could change CodeModel for JIT and see how far SwiftShader can go.

[1]: https://github.com/llvm-mirror/llvm/blob/6e45beba86221def2571af2b4e3f00ea8b8f5643/lib/Target/AArch64/AArch64TargetMachine.cpp#L251 
[2]: https://github.com/llvm-mirror/llvm/blob/6e45beba86221def2571af2b4e3f00ea8b8f5643/lib/Target/AArch64/AArch64InstrInfo.cpp#L1525 


-----Original Message-----
From: Adam Kallai <kadam at inf.u-szeged.hu> 
Sent: Wednesday, October 2, 2019 7:35 AM
To: Martin Storsjö <martin at martin.st>
Cc: llvm-dev at lists.llvm.org; Tom Tan <Tom.Tan at microsoft.com>; Kristof Beyls <Kristof.Beyls at arm.com>; Peter Smith <Peter.Smith at arm.com>; Dave Rodgman <dave.rodgman at arm.com>
Subject: Re: fixup_aarch64_movw support for COFF AArch64


Thanks for your suggestion.

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


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

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