This is just the correctness fix from D21397.

SimplifyCFG is able to detect the pattern:

(i == 5334 || i == 5335)

to:

((i & -2) == 5334)

This transformation has some incorrect side conditions. Specifically, the transformation is only applied when the right-hand side constant (5334 in the example) is a power of two not equal and not equal to the negated mask. These side conditions were added in r258904 to fix PR26323. The correct side condition is that: ((Constant & Mask) == Constant)[(5334 & -2) == 5334].

It's a little bit hard to see why these transformations are correct and what the side conditions ought to be. Here is a CVC3 program to verify them for 64-bit values:

ONE : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63); x : BITVECTOR(64); y : BITVECTOR(64); z : BITVECTOR(64); mask : BITVECTOR(64) = BVSHL(ONE, z); QUERY( (y & ~mask = y) => ((x & ~mask = y) <=> (x = y OR x = (y | mask))) );

Please note that each pattern must be a dual implication (<--> or iff). One directional implication can create spurious matches. If the implication is only one-way, an unsatisfiable condition on the left side can imply a satisfiable condition on the right side. Dual implication ensures that satisfiable conditions are transformed to other satisfiable conditions and unsatisfiable conditions are transformed to other unsatisfiable conditions.

Here is a concrete example of a unsatisfiable condition on the left implying a satisfiable condition on the right:

mask = (1 << z)

(x & ~mask) == y --> (x == y || x == (y | mask))

Substituting y = 3, z = 0 yields:

(x & -2) == 3 --> (x == 3 || x == 2)

The version of this code before r258904 had no side-conditions and incorrectly justified itself in comments through one-directional implication.