Bitwise

BAND, BOR, BXOR and BNOT operate on the raw two’s-complement bit pattern of an i64; SHL and SHR shift it. Byte values, operand layouts and stack effects are in the Bitwise section of the opcode reference. None of these instructions touch a register, and none but the shifts can fail: BAND/BOR/BXOR/BNOT are defined for every i64 bit pattern, with no invalid input. SHL and SHR fault on a shift amount outside \([0, 64)\), and SHL also faults when the shift would lose a significant bit.

For the boolean-algebra equivalents that treat 0/non-zero as false/true instead of operating bit by bit, see Logical.

Shift Behaviour

SHL performs a left shift, filling the vacated low bits with zero. It does not discard bits that leave the high end: a shift that loses a significant bit takes the value outside the i64 range, so it raises ArithmeticOverflow like any other overflowing operation. PUSH 4611686018427387904; PUSH 1; SHL fails rather than yielding i64::MIN. Shifting the result back recovers the operand exactly whenever nothing was lost, which is the test the VM applies.

SHR performs an arithmetic (sign-preserving) right shift, not a logical (zero-filling) one: the sign bit is replicated into the vacated high bits, so a negative operand stays negative. -8 >> 1 yields -4, and i64::MIN >> 1 yields -4611686018427387904, half the magnitude of i64::MIN and still negative, rather than a small positive number a logical shift would produce. This matches Rust’s i64 >> b operator and Python’s >> on integers.

Both instructions require the shift amount to satisfy \(0 \le b < 64\) and error InvalidShift otherwise; a shift by 64 or more is not defined as “shift everything out to zero” the way it might be on some hardware, it is rejected outright: PUSH 8; PUSH 64; SHR fails with invalid shift amount 64.

Where a logical right shift is actually needed, mask the sign-extended bits off after SHR with BAND. For a shift of exactly 1, BAND with 0x7FFFFFFFFFFFFFFF (i64::MAX) clears only the replicated sign bit, which is everything an arithmetic shift by 1 could have filled from the sign: SHR -8 1 followed by masking with i64::MAX gives 9223372036854775804, the same result a zero-filling right shift by 1 would give. A shift by more than 1 replicates the sign into more than one bit, so the mask has to widen to match: BAND with a fixed i64::MAX mask only emulates a logical shift for b = 1.