insertIndirectBranch not implemented but SizeRelaxationPass uses it (LLVM-55)

[bjoernQ]

I'm using a Rust port (https://github.com/MabezDev/rust-xtensa) that uses this LLVM backend. It works on my OSX system but not on my Windows system - but it used to work on Windows using some quite early commits of this LLVM backend. For me actually it crashes during compilation on Windows.

I investigated a bit and was able to track it down to insertIndirectBranch isn't implemented

Then I realized that insertIndirectBranch is called via BranchRelaxation which is enabled by https://github.com/espressif/llvm-xtensa/blob/757e18f722dbdcd98b8479e25041b1eee1128ce9/lib/Target/Xtensa/XtensaTargetMachine.cpp#L137

Probably it used to work for me before since previously XtensaTargetMachine configured the passes like this

  addPass(createXtensaBranchSelectionPass());
  addPass(createXtensaSizeReductionPass());

So it wasn't using BranchRelaxation before.

I don't understand why it's working on OSX and apparently Linux and it's only me who runs into this problem (and only on Windows) - however given the code it shouldn't be platform dependent.

I guess implementing insertIndirectBranch would be a good solution.

Locally I just commented out addPass(&BranchRelaxationPassID); from XtensaPassConfig::addPreEmitPass() and I'm now able to compile and run code.
Unfortunately implementing a proper solution (i.e. insertIndirectBranch) myself is way over my head.

[aykevl]

Have you tried compiling LLVM with assertions enabled? I'm working on a compiler frontend and found many bugs that way - including things that looked like optimizer/backend bugs but were really frontend bugs.

The fact that it's only an issue on Windows tells me it might be a memory corruption or similar in LLVM, which you can often catch with assertions.

Also maybe check differences in LLVM build config. I've seen bugs only happening on Windows because I had disabled threading there but was still using it.

[bjoernQ]

Thanks for the hints.

Actually with assertions enabled I get an assertion at https://github.com/espressif/llvm-xtensa/blob/757e18f722dbdcd98b8479e25041b1eee1128ce9/lib/CodeGen/MachineBasicBlock.cpp#L1360

I assumed it's normal for this work in progress backend currently but I'll compare the behavior on OSX.

I tried to build LLVM more or less like the usual Rust build does: https://github.com/rust-lang/rust/blob/41501a6b03a8f10d8c29dfcb37dbd5ff84b33f34/src/bootstrap/native.rs#L179

But I'm still puzzled why it generates apparently correct code as soon as I comment out BranchRelaxation and why insertIndirectBranch is apparently not called on OSX/Linux since the call is there: https://github.com/espressif/llvm-xtensa/blob/757e18f722dbdcd98b8479e25041b1eee1128ce9/lib/CodeGen/BranchRelaxation.cpp#L420

But to be honest I'm far from really understanding what is really happening there.

But it's a good idea to compare whats happening on Windows vs other platforms.

[aykevl]

That might just be a bug in the Xtensa backend - apparently someone somewhere is not updating liveness information?

[andreisfr]

Hi @bjoernQ ! Thank you for information!. Can you attach the code which causes an error to reproduce situation? For example like there https://github.com/espressif/llvm-xtensa/issues/4#issuecomment-481116928

[bjoernQ]

Hello @andreisfr !

Here are some LLVM BC files which should cause the error lib.zip

Just trying to use llc to generate the Xtensa assembly should show the problem on Windows. That might be the easiest way to quickly reproduce the problem.

I built LLVM with cmake configured like this
cmake ../llvm-xtensa -DLLVM_TARGETS_TO_BUILD="Xtensa;X86" -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_CRT_DEBUG=MT -DLLVM_USE_CRT_RELEASE=MT -DLLVM_USE_CRT_RELWITHDEBINFO=MT -G "Ninja"

and it then gets built using Visual Studio 2017 tools on my system

In my case with BranchRelaxation it crashes - when I comment out the BranchRelaxation pass it works fine for me.

[andreisfr]

Thank you, @bjoernQ ! I will investigate the problem

[andreisfr]

Hi @bjoernQ ! Please, check https://github.com/espressif/llvm-project/tree/xtensa_release_9.0.1 version, I suppose this version fixes problem.

[bjoernQ]

I will give it a try and soon as possible. Thanks for the update

[bjoernQ]

Works fine for me! Thanks!