[LLVMdev] Pass Incompatibility

[John Criswell]

  On 10/20/10 6:05 AM, Luke Dalessandro wrote:
> I have a transformation where I'd like to use both DominatorTree (for ExtractCodeRegion), and DemoteRegisterToMemory (i.e., reg2mem). The transformation is phased, so all occurrences of getAnalysis<DominatorTree>(Function) happen before any occurrence of getAnalysisID<FunctionPass>(&DemoteRegisterToMemoryID, Function).

I believe that reg2mem is a transform pass, correct?  If so, then you 
should not be making it a prerequisite for your pass.  Instead, whatever 
is scheduling the set of transform passes (e.g, the user on the opt 
command line or the source code that schedules your pass to be run with 
a PassManager) should run reg2mem right before your pass is run.  Your 
pass should assert that the LLVM bitcode has the property that reg2mem 
creates.

In general, passes cannot depend upon transform passes.  There are 
certain situations in which PassManager cannot resolve dependencies when 
a transform pass is part of a dependency chain (e.g., Transform Pass A 
requires Transform Pass B and Analysis Pass C, but B invalidates C), and 
so you get cryptic error messages.  That is probably what you are seeing 
here.

-- John T.


> If I register these two passes with DominatorTree first, I get the following assert.
>
> PassAnalysisSupport.h:239: AnalysisType&  llvm::Pass::getAnalysisID(const void*, llvm::Function&) [with AnalysisType = llvm::DominatorTree]: Assertion `ResultPass&&  "Unable to find requested analysis info"' failed.
>
> If I register DemoteRegisterToMemoryID first then I get this assert during code extraction.
>
> CodeExtractor.cpp:681: llvm::Function*<unnamed>::CodeExtractor::ExtractCodeRegion(const std::vector<llvm::BasicBlock*, std::allocator<llvm::BasicBlock*>  >&): Assertion `BlocksToExtract.count(*PI)&&  "No blocks in this region may have entries from outside the region" " except for the first block!"' failed.
>
> If I split my pass in half, then the transformation works as expected, with region extraction and DemoteRegisterToMemory going on their merry way. The split pass is harder to understand though, and injects a pass into -help that doesn't actually work on its own.
>
> Is it simply the case the, in order to use these two passes, I must split my transformation into two passes, or is there another/a better way?
>
> Thanks,
> Luke
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