[LLVMdev] Issues with DependenceAnalysis

[Cristianno Martins]

Hi there,  

I've been playing with some benchmarks for a while, but I'm stuck with a very specific issue, that, I guess, must be some kind of bug.
Well, I wrote a pass that simply counts how many combinations of memory accesses (only stores and loads, on a given loop) has any dependence between them. Curiously, I didn't have any problems until I tried it out on one of the loops in the make_dragons function (SPEC int 2006, gobmk benchmark, engine/dragon.c), when I got the following error message:
(PS: the "Verifying ..." line was printed by my pass, only to show which instructions would be checked right after it)

> $ opt -basicaa -load Hello.dylib -hello engine/dragon.bc -debug-only=da > /dev/null
> […] Verifying: "%1525 = load i32* %1524, align 4, !dbg !1371, !tbaa !693" AND  "store i32 1, i32* %1532, align 4, !dbg !1381, !tbaa !693"
>     common nesting levels = 1
>     maximum nesting levels = 1
>     SrcPtrSCEV = @worm
>     DstPtrSCEV = @worm
>     using GEPs
> Assertion failed: (isSCEVable(V->getType()) && "Value is not SCEVable!"), function getSCEV, file /Developer/llvm/lib/Analysis/ScalarEvolution.cpp, line 2721.
> 0  libLLVM-3.3svn.dylib 0x0000000105b01cee llvm::sys::PrintStackTrace(__sFILE*) + 46
> 1  libLLVM-3.3svn.dylib 0x0000000105b01ffb PrintStackTraceSignalHandler(void*) + 27
> 2  libLLVM-3.3svn.dylib 0x0000000105b02309 SignalHandler(int) + 297
> 3  libsystem_c.dylib      0x00007fff8bae694a _sigtramp + 26
> 4  libsystem_c.dylib      0x00007fff5b3d0d88 _sigtramp + 3482231896
> 5  libLLVM-3.3svn.dylib 0x0000000105b0202b raise + 27
> 6  libLLVM-3.3svn.dylib 0x0000000105b020e2 abort + 18
> 7  libLLVM-3.3svn.dylib 0x0000000105b020c1 __assert_rtn + 129
> 8  libLLVM-3.3svn.dylib 0x0000000104d570e1 llvm::ScalarEvolution::getSCEV(llvm::Value*) + 145
> 9  libLLVM-3.3svn.dylib 0x0000000104c3c7ac llvm::DependenceAnalysis::depends(llvm::Instruction*, llvm::Instruction*, bool) + 2076
> 10 Hello.dylib                0x000000010871221a llvm::Hello::countMemoryDependences() + 858
> 11 Hello.dylib                0x000000010872b073 llvm::Hello::initialize(llvm::Loop*) + 691
> 12 Hello.dylib                0x00000001087142e5 llvm::Hello::runOnLoop(llvm::Loop*, llvm::LPPassManager&) + 389
> 13 libLLVM-3.3svn.dylib 0x0000000104cab04d llvm::LPPassManager::runOnFunction(llvm::Function&) + 1149
> 14 libLLVM-3.3svn.dylib 0x000000010528c07f llvm::FPPassManager::runOnFunction(llvm::Function&) + 431
> 15 libLLVM-3.3svn.dylib 0x000000010528c368 llvm::FPPassManager::runOnModule(llvm::Module&) + 104
> 16 libLLVM-3.3svn.dylib 0x000000010528c76a llvm::MPPassManager::runOnModule(llvm::Module&) + 634
> 17 libLLVM-3.3svn.dylib 0x000000010528cfae llvm::PassManagerImpl::run(llvm::Module&) + 302
> 18 libLLVM-3.3svn.dylib 0x000000010528d251 llvm::PassManager::run(llvm::Module&) + 33
> 19 opt                  0x000000010483d351 main + 6593
> 20 libdyld.dylib        0x00007fff8e8b67e1 start + 0
> Stack dump:
> 0. Program arguments: opt -basicaa -load Hello.dylib -hello engine/dragon.bc -debug-only=da  
> 1. Running pass 'Function Pass Manager' on module 'engine/dragon.bc'.
> 2. Running pass 'Loop Pass Manager' on function '@make_dragons'
> 3. Running pass 'Hello Pass' on basic block '%1516'
>  
>  


So, I have two questions:  
1. What does it mean when the *PtrSCEV of a instruction has the value "@worm" in it?
2. Is this the correct behavior, given the two instructions been handled by the DependenceAnalysis::depends function? If so, how can I avoid having this assertion failed? (or how can I identify precisely when this could happen and avoid by myself calling depends?)

Thanks in advance,

--  
Cristianno Martins
PhD Candidate in Computer Science
University of Campinas
cmartins at ic.unicamp.br

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