About writing an alias analysis pass for LLVM 3.1

Project Infrastructure LLVM Dev List Archives
1

Hi,

I am now trying to write an alias analysis pass for LLVM 3.1. The pass is compiled into a .so library. When I loaded it into opt to perform evaluation with command:

opt -load my-so-lib -aa-eval foo.bc

the following errors occurred:

opt: raw_ostream.cpp:261: void llvm::raw_ostream::flush_nonempty(): Assertion OutBufCur > OutBufStart && “Invalid call to flush_nonempty.”’ failed.`

As stated in the documentation I found on LLVM official website, an alias analysis pass should subclass both Pass and AliasAnalysis class. I used GDB for some debugging, and found why the error emerged.

By MyAA note the pass class written by me, and by obj note the instance of MyAA. In the crash scene, When LLVM was trying to call obj.alias, a virtual function of class llvm::AliasAnalysis, it actually called obj.print, a virtual function of llvm::Pass, because of the confusion of two virtual function tables. It seems that LLVM assesses obj via a AliasAnalysis* pointer without performing static_cast. However, I have no idea why this happens. I guess it is because I registered the Pass in a wrong way, but still don’t know to fix it.

Anyone can help me? I am not a C++ or LLVM expert, so I got really stranded.

Thank you in advance.

Pei Wang

2

Hi,

Does your pass use multiple inheritance? Sounds like your problem is
that you need to define getAdjustedAnalysisPointer, see:

http://llvm.org/docs/doxygen/html/classllvm_1_1Pass.html#a03d3a81b1c46aff7c38ef3a6750ba225

An example implementation (very likely exactly what you want) is in
LibCallAliasAnalysis:

http://llvm.org/docs/doxygen/html/LibCallAliasAnalysis_8h_source.html#l00060

Hope this helps!

~Will

3

Hi Will,

That's exactly the solution. Thanks!

Pei