[CI only] Use pessimistic long branches on ARM

[filipnavara]

@MichalStrehovsky can you run NativeAOT outer loop with this, please?

Ref: #99079 (comment)

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Issue Details

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@MichalStrehovsky can you run NativeAOT outer loop with this, please?

Ref: #99079 (comment)

Author:	filipnavara
Assignees:	-
Labels:	NO-MERGE, NO-REVIEW, area-NativeAOT-coreclr
Milestone:	-

[jkotas]

/azp run runtime-nativeaot-outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[filipnavara]

Thanks, this uncovered some new failure mode. Unfortunately I left my Raspberry Pi at office so I won't be able to reproduce it immediately.

[filipnavara]

@am11 Do you remember if there's a reason not to use -fPIC for shared libraries in src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Unix.targets? We already have PositionIndependentExecutable and default to true. Do you think it would make sense to do something like that for shared libraries too, or even enable -fPIC unconditionally?

[jkotas]

Should -fPIC be on by default for shared libraries? If it is not, it is certainly fine to enable it.

[filipnavara]

I assumed it is but the CI suggests otherwise (for ARM32, at least). I'll check it on Raspberry Pi tomorrow morning.

[am11]

-fPIC -fPIE are for compilation and -pie is for linking.

shared libraries and ELF are both executable by default, we set entrypoint to 0x0; something we picked up from how llvm toolchain build on Ubuntu:

runtime/src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Unix.targets

Line 202 in 8ec2b50

<LinkerArg Include="-Wl,-e0x0" Condition="'$(NativeLib)' == 'Shared' and '$(_IsApplePlatform)' != 'true'" />

so the idea that PIE is for executable and PIC is for shared lib is not definitive.

[filipnavara]

-fPIC -fPIE are for compilation and -pie is for linking.

Well, the linker in the PR failed with

ld.lld(0,0): error : relocation R_ARM_THM_MOVW_PREL_NC cannot be used against symbol 'memmove'; recompile with -fPIC [/__w/1/s/src/tests/nativeaot/SmokeTests/SharedLibrary/SharedLibrary.csproj]
ld.lld(0,0): error : relocation R_ARM_THM_MOVT_PREL cannot be used against symbol 'memmove'; recompile with -fPIC [/__w/1/s/src/tests/nativeaot/SmokeTests/SharedLibrary/SharedLibrary.csproj]

I was caught a bit off guard by that because I assumed that (on Linux) shared libraries are always position independent. Both R_ARM_THM_MOVT_PREL and R_ARM_THM_MOVW_PREL_NC are very much position independent.

I guess I am just misinterpreting the error, and undefined symbols that result in dynamic relocations cannot express this.