[PATCH] D13126: New static analyzer checker for loss of sign/precision

[Daniel Marjamäki via cfe-commits]

danielmarjamaki added inline comments.

================
Comment at: lib/StaticAnalyzer/Checkers/ConversionChecker.cpp:84
@@ +83,3 @@
+// Can E value be greater or equal than Val?
+static bool canBeGreaterEqual(CheckerContext &C, const Expr *E,
+                              unsigned long long Val) {
----------------
danielmarjamaki wrote:
> zaks.anna wrote:
> > danielmarjamaki wrote:
> > > zaks.anna wrote:
> > > > This function returns true if the value "is" greater or equal, not "can be" greater or equal. The latter would be "return StGE".
> > > > 
> > > > Also, it's slightly better to return the StGE state and use it to report the bug. This way, our assumption is explicitly recorded in the error state.
> > > NoQ made the same comment. I disagree.
> > > 
> > >     int A = 0;
> > >     if (X) {
> > >          A = 1000;
> > >     }
> > >     U8 = A;  // <- Imho; A _can_ be 1000
> > > 
> > > Imho it's better to say that A _can_ be 1000 unless A is 1000 for all possible execution paths through the code.
> > > 
> > > Do you still think "is" is better than "can be"?
> > The Clang Static Analyzer performs path sensitive analysis of the program. (It does not merge the paths at the "U8 = A" statement!!!) You will only be changing the state along a single execution path of this program. Along that path, A will always be 1000. 
> > 
> > When analyzing your example, the analyzer is going to separately analyze 2 paths:
> > 1st path: A=0; X != 0; A =1000; U8 = A; // Here U8 is definitely 1000.
> > 2d  path: A=0; X == 0; U8 = A;               // Here U8 is definitely 0.
> > 
> > This video contains an intuitive explanation of symbolic execution technique we use: http://llvm.org/devmtg/2012-11/videos/Zaks-Rose-Checker24Hours.mp4
> I understand that and I still think that value of A "can be" 1000. Yes in that path the value "is" 1000.
> 
> But as far as I see, you and others disagree with me. And therefore I will change to "is".
For your information in Cppcheck I say that a value is "possible" if some path(s) generates that value. And "always" when all paths generate that value.

Code example:

    int f(int x) {
      int a = 1000;
      int b = 0;
      if (x == 500)
        a = 3;
      return a + b - x;
    }

Debug output (cppcheck --debug-normal file.c):

    ##Value flow
    Line 3
      1000 always 1000
    Line 4
      0 always 0
    Line 5
      x possible 500
      == possible 1
      500 always 500
    Line 6
      3 always 3
    Line 7
      a possible {1000,3}
      + possible {1000,3}
      b always 0
      x possible 500

For me personally it is confusing to say that A "is" 1000. That is different to how I normally think of it in Cppcheck.


http://reviews.llvm.org/D13126