[analyzer] Teach constraint managers about unsigned comparisons.

In C, comparisons between signed and unsigned numbers are always done in
unsigned-space. Thus, we should know that "i >= 0U" is always true, even
if 'i' is signed. Similarly, "u >= 0" is also always true, even though '0'
is signed.

Part of <rdar://problem/13239003> (false positives related to std::vector)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177806 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 21 insertions, 10 deletions

diff --git a/lib/StaticAnalyzer/Core/APSIntType.cpp b/lib/StaticAnalyzer/Core/APSIntType.cpp
index 884b0faa9e..c7e9526821 100644
--- a/lib/StaticAnalyzer/Core/APSIntType.cpp
+++ b/lib/StaticAnalyzer/Core/APSIntType.cpp
@@ -13,20 +13,31 @@ using namespace clang;
 using namespace ento;
 
 APSIntType::RangeTestResultKind
-APSIntType::testInRange(const llvm::APSInt &Value) const {
+APSIntType::testInRange(const llvm::APSInt &Value,
+                        bool AllowSignConversions) const {
+
   // Negative numbers cannot be losslessly converted to unsigned type.
-  if (IsUnsigned && Value.isSigned() && Value.isNegative())
+  if (IsUnsigned && !AllowSignConversions &&
+      Value.isSigned() && Value.isNegative())
     return RTR_Below;
 
-  // Signed integers can be converted to signed integers of the same width
-  // or (if positive) unsigned integers with one fewer bit.
-  // Unsigned integers can be converted to unsigned integers of the same width
-  // or signed integers with one more bit.
   unsigned MinBits;
-  if (Value.isSigned())
-    MinBits = Value.getMinSignedBits() - IsUnsigned;
-  else
-    MinBits = Value.getActiveBits() + !IsUnsigned;
+  if (AllowSignConversions) {
+    if (Value.isSigned() && !IsUnsigned)
+      MinBits = Value.getMinSignedBits();
+    else
+      MinBits = Value.getActiveBits();
+
+  } else {
+    // Signed integers can be converted to signed integers of the same width
+    // or (if positive) unsigned integers with one fewer bit.
+    // Unsigned integers can be converted to unsigned integers of the same width
+    // or signed integers with one more bit.
+    if (Value.isSigned())
+      MinBits = Value.getMinSignedBits() - IsUnsigned;
+    else
+      MinBits = Value.getActiveBits() + !IsUnsigned;
+  }
 
   if (MinBits <= BitWidth)
     return RTR_Within;