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Commit ID cff9a8672c1da7d36bc54d168d10ea2b1ce5c728
Author 31m059
Date 2019-11-04 22:50:14 UTC
Make Square and Bitboard operators commutative As Stockfish developers, we aim to make our code as legible and as close to simple English as possible. However, one of the more notable exceptions to this rule concerns operations between Squares and Bitboards. Prior to this pull request, AND, OR, and XOR were only defined when the Bitboard was the first operand, and the Square the second. For example, for a Bitboard b and Square s, "b & s" would be valid but "s & b" would not. This conflicts with natural reasoning about logical operators, both mathematically and intuitively, which says that logical operators should commute. More dangerously, however, both Square and Bitboard are defined as integers "under the hood." As a result, code like "s & b" would still compile and give reasonable bench values. This trap occasionally ensnares even experienced Stockfish developers, but it is especially dangerous for new developers not aware of this peculiarity. Because there is no compilation or runtime error, and a reasonable bench, only a close review by approvers can spot this error when a test has been submitted--and many times, these bugs have slipped past review. This is by far the most common logical error on Fishtest, and has wasted uncountable STC games over the years. However, it can be fixed by adding three non-functional lines of code. In this patch, we define the operators when the operands are provided in the opposite order, i.e., we make AND, OR, and XOR commutative for Bitboards and Squares. Because these are inline methods and implemented identically, the executable does not change at all. This patch has the small side-effect of requiring Squares to be explicitly cast to integers before AND, OR, or XOR with integers. This is only performed twice in Stockfish's source code, and again does not change the executable at all (since Square is an enum defined as an integer anyway). For demonstration purposes, this pull request also inverts the order of one AND and one OR, to show that neither the bench nor the executable change. (This change can be removed before merging, if preferred.) I hope that this pull request significantly lowers the barrier-of-entry for new developer to join the Stockfish project. I also hope that this change will improve our efficiency in using our generous CPU donors' machines, since it will remove one of the most common causes of buggy tests. Following helpful review and comments by Michael Stembera (@mstembera), we add a further clean-up by implementing OR for two Squares, to anticipate additional traps developers may encounter and handle them cleanly. Closes No functional change.
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