restrict is a c99 keyword. Yes, Rpbert S. Barnes, I know that most compilers support __restrict__. You will note that anything with double underscores is, by definition, implementation specific and thus NOT C++, but a compiler specific version of it. – KitsuneYMG Jan 6, 2010 at 9:31

In general, data structures are made branchless by implicitly or explicitly padding them so that their operations take a constant number of iterations. Refer to the article for more complex examples.

rings. Oversimplifying things, an std::string is comprised of a pointer to a null-terminated char array (also known as a “C-string”) allocated somewhere on the heap and one integer containing the string size.

A common value for a string is the empty string — which is also its default value. You also need to handle them somehow, and the idiomatic approach is to assign nullptr as the pointer and 0 as the string size, and then check if the pointer is null or if the size is zero at the beginning of every procedure involving strings.

However, this requires a separate branch, which is costly (unless the majority of strings are either empty or non-empty). To remove the check and thus also the branch, we can allocate a “zero C-string,” which is just a zero byte allocated somewhere, and then simply point all empty strings there. Now all string operations with empty strings have to read this useless zero byte, but this is still much cheaper than a branch misprediction.

In an attempt to convert data stored in a buffer (maybe read over a network connection) into a high-level structure, a pointer to ‘struct measurements_t’ is aliased with a pointer to a ‘uint8_t’. Since both types are incompatible (pointer to struct vs. pointer to ‘uint8_t’) this code is a violation of the strict aliasing rule. Experienced C developers most likely recognized immediately that this code yields undefined behavior, but they would have probably attributed it to struct padding and alignment issues. The real reason, as we know by now, is a violation of the strict aliasing rule.

This bug (and some linked discussions) indicate that compiler developers don’t necessarily consider int8_t and uint8_t to be character types for aliasing purposes.

A lot of C code is broken under strict aliasing. Separate compilation is probably what protects us from broader compiler exploitation of the brokenness, but it is a very poor kind of protection.

Before reporting that GCC compiles your code incorrectly, compile it with gcc -Wall -Wextra and see whether this shows anything wrong with your code. Similarly, if compiling with -fno-strict-aliasing -fwrapv makes a difference, your code probably is not correct.