[Analyzer] Clarify error messages for undefined result

Differential Revision: https://reviews.llvm.org/D30295

llvm-svn: 315462

1 files changed, 40 insertions, 7 deletions

diff --git a/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp
index f3c2ffc5866..21e8b107c9e 100644
--- a/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/UndefResultChecker.cpp
@@ -59,6 +59,11 @@ static bool isArrayIndexOutOfBounds(CheckerContext &C, const Expr *Ex) {
   return StOutBound && !StInBound;
 }
 
+static bool isShiftOverflow(const BinaryOperator *B, CheckerContext &C) {
+  return C.isGreaterOrEqual(
+      B->getRHS(), C.getASTContext().getIntWidth(B->getLHS()->getType()));
+}
+
 void UndefResultChecker::checkPostStmt(const BinaryOperator *B,
                                        CheckerContext &C) const {
   ProgramStateRef state = C.getState();
@@ -97,18 +102,46 @@ void UndefResultChecker::checkPostStmt(const BinaryOperator *B,
     }
 
     if (Ex) {
-      OS << "The " << (isLeft ? "left" : "right")
-         << " operand of '"
+      OS << "The " << (isLeft ? "left" : "right") << " operand of '"
          << BinaryOperator::getOpcodeStr(B->getOpcode())
          << "' is a garbage value";
       if (isArrayIndexOutOfBounds(C, Ex))
         OS << " due to array index out of bounds";
-    }
-    else {
+    } else {
       // Neither operand was undefined, but the result is undefined.
-      OS << "The result of the '"
-         << BinaryOperator::getOpcodeStr(B->getOpcode())
-         << "' expression is undefined";
+      if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
+           B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
+          C.isNegative(B->getRHS())) {
+        OS << "The result of the "
+           << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
+                                                              : "right")
+           << " shift is undefined because the right operand is negative";
+      } else if ((B->getOpcode() == BinaryOperatorKind::BO_Shl ||
+                  B->getOpcode() == BinaryOperatorKind::BO_Shr) &&
+                 isShiftOverflow(B, C)) {
+
+        OS << "The result of the "
+           << ((B->getOpcode() == BinaryOperatorKind::BO_Shl) ? "left"
+                                                              : "right")
+           << " shift is undefined due to shifting by ";
+
+        SValBuilder &SB = C.getSValBuilder();
+        const llvm::APSInt *I =
+            SB.getKnownValue(C.getState(), C.getSVal(B->getRHS()));
+        if (!I)
+          OS << "a value that is";
+        else if (I->isUnsigned())
+          OS << '\'' << I->getZExtValue() << "\', which is";
+        else
+          OS << '\'' << I->getSExtValue() << "\', which is";
+
+        OS << " greater or equal to the width of type '"
+           << B->getLHS()->getType().getAsString() << "'.";
+      } else {
+        OS << "The result of the '"
+           << BinaryOperator::getOpcodeStr(B->getOpcode())
+           << "' expression is undefined";
+      }
     }
     auto report = llvm::make_unique<BugReport>(*BT, OS.str(), N);
     if (Ex) {