[analyzer] Left shifting a negative value is undefined

The analyzer did not return an UndefVal in case a negative value was left
shifted. I also altered the UndefResultChecker to emit a clear warning in this
case.

Differential Revision: https://reviews.llvm.org/D39423

llvm-svn: 316924

2 files changed, 6 insertions, 0 deletions

diff --git a/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp
index 21e8b107c9e..172ce346f1b 100644
--- a/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp
@@ -137,6 +137,10 @@ void UndefResultChecker::checkPostStmt(const BinaryOperator *B,
 
         OS << " greater or equal to the width of type '"
            << B->getLHS()->getType().getAsString() << "'.";
+      } else if (B->getOpcode() == BinaryOperatorKind::BO_Shl &&
+                 C.isNegative(B->getLHS())) {
+        OS << "The result of the left shift is undefined because the left "
+              "operand is negative";
       } else {
         OS << "The result of the '"
            << BinaryOperator::getOpcodeStr(B->getOpcode())
diff --git a/clang/lib/StaticAnalyzer/Core/BasicValueFactory.cpp b/clang/lib/StaticAnalyzer/Core/BasicValueFactory.cpp
index ebbace4e33b..ec7a7e9e4b1 100644
--- a/clang/lib/StaticAnalyzer/Core/BasicValueFactory.cpp
+++ b/clang/lib/StaticAnalyzer/Core/BasicValueFactory.cpp
@@ -225,6 +225,8 @@ BasicValueFactory::evalAPSInt(BinaryOperator::Opcode Op,
       // test these conditions symbolically.
 
       // FIXME: Expand these checks to include all undefined behavior.
+      if (V1.isSigned() && V1.isNegative())
+        return nullptr;
 
       if (V2.isSigned() && V2.isNegative())
         return nullptr;