[LLVMdev] PHINode containing itself causes segfault when compiling Blender OpenCL kernel with R600 backend

[Tom Stellard]

On Sun, Sep 21, 2014 at 01:54:40AM +0400, Vitaliy Filippov wrote:
> Hi!
> 
> I'm trying to run Blender using Mesa OpenCL implementation on a
> radeonsi card. First the kernel didn't want to compile, but that was
> caused by a bug in it (they were using . instead of -> in 1 place),
> and after fixing this bug I've got the kernel to compile...
> 
> ...But after that, LLVM started to crash during translation of IR
> into shader code with R600 backend.
> 

Can you file a bug for this at bugs.freedesktop.org and post the output
of blender with the environment variable R600_DEUBG=cs,compute

-Tom

> I've done some investigation and figured out that the crash is
> caused by a PHINode containing itself.
> SIAnnotateControlFlow::handleLoopCondition() can't handle such
> situation - it recurses into itself, calls Phi->eraseFromParent()
> inside the inner execution, returns into outer one, gets zeroed out
> object and crashes when trying to do something with its members...
> for example when trying to erase it again.
> 
> I have no real background in LLVM or GCC, so the concept of PHINode
> itself was a real discovery for me :) and PHINode containing itself
> does look even more strange... I've tried to understand the
> semantics of such PHINodes from reading the code and got a suspicion
> that the rest of LLVM code just ignores PHINodes equal to their
> parent... So I've tried to fix the bug by making
> handleLoopCondition() skip IncomingValues equal to the Phi itself,
> but the bug didn't go away! Surprisingly, PHINode may not just
> contain itself directly, but it also may contain itself inside
> another PHINode, i.e. Phi->getIncomingValue(0)->getIncomingValue(0)
> == Phi, which results in the same problem with
> SIAnnotateControlFlow...
> 
> Besides "how to make a correct fix" :), my question also is: what
> are the real semantics of a PHINode containing itself directly or
> indirectly? I've done some tracing and saw such PHINodes added by
> the optimizer, in llvm::InlineFunction()... but what do they mean
> and how to deal with them correctly?
> 
> -- 
> With best regards,
>   Vitaliy Filippov
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