[cfe-dev] Question on LiveVariables analysis

Anna Zaks ganna at apple.com
Sun Oct 9 16:44:45 PDT 2011


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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