Integer Overflow: Don't warn about left-shift expressions with negative result like '1<<31'. That is common practice.

lib/checktype.cpp

@@ -113,7 +113,7 @@ void CheckType::tooBigBitwiseShiftError(const Token *tok, int lhsbits, const Val
 void CheckType::checkIntegerOverflow()
 {
     // unknown sizeof(int) => can't run this checker
-    if (_settings->platformType == Settings::Unspecified)
+    if (_settings->platformType == Settings::Unspecified || _settings->int_bit >= 64)
         return;
 
     // max int value according to platform settings.
@@ -136,7 +136,13 @@ void CheckType::checkIntegerOverflow()
             const ValueFlow::Value *value = tok->getValueGE(maxint + 1, _settings);
             if (!value)
                 value = tok->getValueLE(-maxint - 2, _settings);
-            if (value)
+            if (!value)
+                continue;
+
+            // For left shift, it's common practice to shift into the sign bit
+            if (tok->str() == "<<" && value->intvalue > 0 && value->intvalue < (1LL << _settings->int_bit))
+                continue;
+
             integerOverflowError(tok, *value);
         }
     }

test/testtype.cpp

@@ -42,7 +42,7 @@ private:
         TEST_CASE(checkFloatToIntegerOverflow);
     }
 
-    void check(const char code[], Settings* settings = 0) {
+    void check(const char code[], Settings* settings = 0, const char filename[] = "test.cpp") {
         // Clear the error buffer..
         errout.str("");
 
@@ -55,7 +55,7 @@ private:
         // Tokenize..
         Tokenizer tokenizer(settings, this);
         std::istringstream istr(code);
-        tokenizer.tokenize(istr, "test.cpp");
+        tokenizer.tokenize(istr, filename);
 
         // Check..
         CheckType checkType(&tokenizer, settings, this);
@@ -137,6 +137,11 @@ private:
               "   return 123456U * x;\n"
               "}",&settings);
         ASSERT_EQUALS("", errout.str());
+
+        check("int foo() {\n"
+              "  x = 1 << 31;\n" // this is technically integer overflow but it's common code
+              "}", &settings, "test.c");
+        ASSERT_EQUALS("", errout.str());
     }
 
     void signConversion() {