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

Anna Zaks via cfe-commits cfe-commits at lists.llvm.org
Tue Mar 22 11:05:32 PDT 2016

zaks.anna 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:
> > 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


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