Poor codegen for enumerations where variants have power-of-two values.

Background

Consider the code in the link below:
https://rust.godbolt.org/z/z6WrPEM68

It defines an enumeration where all variants have a value that are equal to a power-of-two.

Creation of an enumeration value

Two of the functions create an enummeration value out of an unsigned integer. The first function wraps the result in an Option with None being used for illegal inputs, the second function uses undefined behaviour instead. In both cases, legal input values and enumeration values are identical.

Expectation

Since legal input values and enumeration values are identical, the functions compile to little more than mov instructions and possibly checks.

Reality

Giant jump tables.

Matching on an enumeration value

Since the enumeration values have only one bit set, it is possible to make various tests much cheaper. In this instance, there are several functions testing wether an enumeration value is a member of a set.

Expectation

The functions compile to a bitwise AND or similar, using branchless instructions in all cases.

Reality

The functions are compiled into several instructions including branches.

Start with the Rust Compiler Explorer reproducer at https://rust.godbolt.org/z/z6WrPEM68 and inspect the generated assembly for the enumeration-construction and set-membership functions. Compare the output with the expected checks and bitwise operations described in the issue. Done means the relevant cases no longer generate giant jump tables or unnecessary branches while preserving correct behavior.