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[CPP-434] Additional test case; improve QHelp by including themes from the BadAdditionOverflowCheck QHelp.
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cpp/ql/src/Likely Bugs/Arithmetic/SignedOverflowCheck-bad.cpp renamed to cpp/ql/src/Likely Bugs/Arithmetic/SignedOverflowCheck-bad1.cpp

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‎cpp/ql/src/Likely Bugs/Arithmetic/SignedOverflowCheck-good1.cpp‎

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bool baf(unsigned short n1, unsigned short delta) {
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return n1 + (unsigned)delta < n1; // GOOD
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}

cpp/ql/src/Likely Bugs/Arithmetic/SignedOverflowCheck-good.cpp renamed to cpp/ql/src/Likely Bugs/Arithmetic/SignedOverflowCheck-good2.cpp

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‎cpp/ql/src/Likely Bugs/Arithmetic/SignedOverflowCheck.qhelp‎

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@@ -24,25 +24,36 @@ type is promoted to the larger type used in the addition and comparison,
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namely a <code>signed int</code>. As a result, the entire expression is
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evaluated using <code>signed</code> values and its value is therefore undefined.
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</p>
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<sample src="SignedOverflowCheck-bad.cpp" />
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<sample src="SignedOverflowCheck-bad1.cpp" />
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<p>
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In the following example, even though both <code>n</code> and <code>delta</code>
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have been declared <code>unsigned short</code>, C/C++ type promotion rules
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require that both parameters be promoted to the next bigger <code>signed</code>
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integer type (in this case <code>signed int</code>) before being added together
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so as to avoid overflows or underflows. As a result, the entire expression is
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evaluated using <code>signed</code> values and its value is therefore undefined.
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integer type (in this case <code>signed int</code>) before being added together.
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As a result, the entire expression is evaluated using <code>signed</code> values
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and its value is therefore undefined. (Note, however, that the addition cannot
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overflow since we are adding two "small" <code>unsigned short</code> values.)
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</p>
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<sample src="SignedOverflowCheck-bad2.cpp" />
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<p>
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The following example builds upon the previous one. Again, we have two
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<code>unsigned short</code> values getting promoted to a wider type. However,
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since <code>delta</code> is explicitly cast to an <code>unsigned</code> type,
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<code>n1</code> (on both sides of the comparison) is promoted to
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<code>unsigned</code> as well. Since we are now operating on
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<code>unsigned</code> values, the overflow check is defined and supported by
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standard C/C++.
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</p>
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<sample src="SignedOverflowCheck-good1.cpp" />
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<p>
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In the next example, a value of type <code>signed int</code> is
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added to a value of type <code>unsigned int</code>. Because
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the types are of the same size, C/C++ promotion rules dictate that
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<code>unsigned int</code> is chosen as the overall type of the addition
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operation. The entire expression is evaluated using <code>unsigned</code>
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values, which is allowed and defined behavior per the C/C++ standard.
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</p>
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<sample src="SignedOverflowCheck-good.cpp" />
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<sample src="SignedOverflowCheck-good2.cpp" />
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</example>
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<references>
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<li><a href="http://c-faq.com/expr/preservingrules.html">comp.lang.c FAQ list · Question 3.19 (Preserving rules)</a></li>

‎cpp/ql/test/query-tests/Likely Bugs/Arithmetic/BadAdditionOverflowCheck/SignedOverflowCheck.cpp‎

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@@ -112,3 +112,7 @@ bool multipleCasts2(char x) {
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// msvc 19.22 /O2: not deleted
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return (int)(unsigned short)(x + '1') < (int)(unsigned short)x; // GOOD [FALSE POSITIVE]
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}
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int does_it_overflow(int n1, unsigned short delta) {
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return n1 + (unsigned)delta < n1; // GOOD
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}

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