[cfe-dev] Question on LiveVariables analysis
Anna Zaks
ganna at apple.com
Mon Oct 10 20:56:54 PDT 2011
Hi Guoping,
I thought that you saw an assertion failure by running $ clang --analyze. I did not realize that you are running custom code to build up the UninitializedVariables. I am not familiar enough with that code to know what's wrong without debugging it.
Most likely there is some issue with the way you build the analysis in your code; you should probably look at other uses of UninitializedVariables to see what you are missing. Also, try to find out at which step the first symptoms of the problem occur - for example, do you get a CFG different from what you see when dumping the CFG with a debug checker? (See the command from Ted's reply below.)
I noticed that you do not pass CFGBuildOptions when constructing the AnalysisContext. Here is some code from AnalysisBasedWarnings.cpp, which implies that you might need to request linearized CFG.
// Unreachable code analysis requires a linearized CFG.
AC.getCFGBuildOptions().setAllAlwaysAdd();
Also, what are you trying to do? Maybe just writing a checker would be a simpler solution.
Hope this helps,
Anna.
On Oct 9, 2011, at 5:32 PM, Guoping Long wrote:
> Hi, Anna
>
> Here are steps on how I got this error:
> 1 Get the latest code:
> A. svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
> B. svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
> 2. Configure & Make
> A. make a separate directory for build: makedir llvm-build
> B. In llvm-build/, using cmake to generate the Makefile:
> cmake ../llvm -DCMAKE_BUILD_TYPE=Debug
> C. make & make install
> 3. Add a test:
> A. create a directory: tools/clang/examples/Debug
> B. write a test file, as attached.
> C. Modify tools/clang/CMakeLists.txt to build the examples/ directory.
> D. Add CMakeLists.txt in tools/clang/examples/Debug to build the newly added test program.
> E. when the program is built, run it with the previous test program to get this error.
>
> I am using MacOS 10.6.8, x86-64 Intel platform.
> It seems my source tree is not in the right configuration? Please let me know what's wrong in my environment. I will be deeply appreciated. Thanks.
>
> -------
> Guoping
>
> 2011/10/9 Anna Zaks <ganna at apple.com>
> Hi Guoping,
>
> I cannot reproduce the assertion failure.
>
> Is it still reproducible with the latest revision of Clang/LLVM? If yes, can you send the command line you used.
>
> Thanks,
> Anna.
>
> On Oct 9, 2011, at 10:23 AM, Guoping Long wrote:
>
>> Also, it seems there is an assert error in UninitializedValues.cpp. Giving this test program:
>> 1 int main(int argc, char *argv[])
>> 2 {
>> 3 int x=3, y = 5, z = 6;
>> 4
>> 5 if(x<0) {
>> 6 while(y>0){
>> 7 y--;
>> 8 x = z+1;
>> 9 }
>> 10 }else x = y;
>> 11 return 0;
>> 12 }
>> Here is the assert error:
>> Assertion failed: (res.getDeclRefExpr() == lastDR), function VisitUnaryOperator, file /Users/gplong/llvm/tools/clang/lib/Analysis/UninitializedValues.cpp, line 529.
>> This message is triggered in call VisitUnaryOperator(). The line number may not be correct because I added some print statements.
>>
>> -----
>> Guoping
>>
>> 2011/10/8 Guoping Long <longguoping at gmail.com>
>> On my testing environment, the UninitializedValues.cpp has the same problem (do not have any output). Both analyses are based on the StmtVisitor interface. I think there may be a version mismatch, that is these two analyses are based on an older version of StmtVisitor class to work? I just downloaded a new version from website and did all tests again and still have this problem.
>>
>> ----
>> Guoping
>>
>>
>> 2011/10/8 Guoping Long <longguoping at gmail.com>
>> Hi, Ted
>>
>> I will be very curious why results in your environment are correct. Attached is my test code. I stepped into the program using GDB, it turned out that no live variables can be found. Take the last return statement in my previous example program as an example: return b * d + c. The transfer function eventually called the default StmtVisit function which did not recognize variables b, d, c which are actually used.The only function in LiveVariables.cpp that can generate live variables is VisitDeclRefExpr(), but when running the program this function is never called.
>>
>> I am using the newest version of Clang/LLVM. Is it because my environment setup is wrong?
>>
>> ----
>> Guoping
>>
>> 2011/10/7 Ted Kremenek <kremenek at apple.com>
>> This is what I am seeing:
>>
>> $ clang -cc1 -analyze -analyzer-checker=debug.DumpLiveVars t.c
>>
>> [ B0 (live variables at block exit) ]
>>
>> [ B1 (live variables at block exit) ]
>>
>> [ B2 (live variables at block exit) ]
>> b <t.c:2:10>
>> d <t.c:2:16>
>>
>> [ B3 (live variables at block exit) ]
>> a <t.c:2:7>
>> b <t.c:2:10>
>> d <t.c:2:16>
>>
>> [ B4 (live variables at block exit) ]
>>
>>
>> I also think the number of the blocks is not quite right in your example, or at least how they correspond to Clang's CFG:
>>
>> $ clang -cc1 -analyze -analyzer-checker=debug.DumpCFG t.c
>>
>> [ B4 (ENTRY) ]
>> Predecessors (0):
>> Successors (1): B3
>>
>> [ B1 ]
>> 1: 4
>> 2: c
>> 3: [B1.2] = [B1.1]
>> 4: b
>> 5: [B1.4]
>> 6: d
>> 7: [B1.6]
>> 8: [B1.5] * [B1.7]
>> 9: c
>> 10: [B1.9]
>> 11: [B1.8] + [B1.10]
>> 12: return [B1.11];
>> Predecessors (2): B2 B3
>> Successors (1): B0
>>
>> [ B2 ]
>> 1: a
>> 2: [B2.1]
>> 3: b
>> 4: [B2.3]
>> 5: [B2.2] + [B2.4]
>> 6: c
>> 7: [B2.6] = [B2.5]
>> 8: 2
>> 9: d
>> 10: [B2.9] = [B2.8]
>> Predecessors (1): B3
>> Successors (1): B1
>>
>> [ B3 ]
>> 1: int a;
>> 2: int b;
>> 3: int c;
>> 4: int d;
>> 5: int x;
>> 6: 3
>> 7: a
>> 8: [B3.7] = [B3.6]
>> 9: 5
>> 10: b
>> 11: [B3.10] = [B3.9]
>> 12: 4
>> 13: d
>> 14: [B3.13] = [B3.12]
>> 15: 100
>> 16: x
>> 17: [B3.16] = [B3.15]
>> 18: a
>> 19: [B3.18]
>> 20: b
>> 21: [B3.20]
>> 22: [B3.19] > [B3.21]
>> T: if [B3.22]
>> Predecessors (1): B4
>> Successors (2): B2 B1
>>
>> [ B0 (EXIT) ]
>> Predecessors (1): B1
>> Successors (0):
>>
>>
>> Block B3 is actually the entrance block, since the CFG is built bottom-up.
>>
>> Perhaps there is something not quite right with how you are running the checkers?
>>
>> On Oct 7, 2011, at 9:32 PM, Guoping Long wrote:
>>
>>> Hi,
>>>
>>> While LiveVariables analysis (tools/clang/lib/Analysis/LiveVariables.cpp) does not generate any output? Specifically, for this simple program:
>>> 1 int main(int argc, char *argv[])
>>> 2 {
>>> 3 int a,b,c,d,x;
>>> 4
>>> 5 a = 3; //Basic block B1
>>> 6 b = 5;
>>> 7 d = 4;
>>> 8 x = 100;
>>> 9
>>> 10 if(a>b){ //Basic block B2
>>> 11 c = a+b;
>>> 12 d = 2;
>>> 13 }
>>> 14
>>> 15 c = 4; //Basic block B3
>>> 16 return b * d + c;
>>> 17 }
>>> This simple example is taken from this wiki page illustrating live variables analysis (http://en.wikipedia.org/wiki/Live_variable_analysis).
>>> Both B2 and B3 should have live variables, but the output (using LiveVariables::dumpBlockLiveness(SourceManager &)) says no live variables exist. I am trying to use this analysis as an example on how to write a good analysis algorithm for Clang, but really did not expect this behavior for this analysis.
>>> Anybody who knows the answer please let me know. Many thanks!!!
>>>
>>> --------
>>> Guoping
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>
>>
>>
>>
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>
>
> <main.cpp>
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