Expand documentation for ctl_data_ternops()

evm/src/cpu/cpu_stark.rs

@@ -59,9 +59,18 @@ fn ctl_data_binops<F: Field>(ops: &[usize]) -> Vec<Column<F>> {
 }
 
 /// Create the vector of Columns corresponding to the three inputs and
-/// one output of a ternary operation.
-/// If `is_shift` is `true`, we offset the memory_channels indices used for the inputs
-/// by 1. It will only be valid if the associated filter is one of the two shift flags.
+/// one output of a ternary operation. By default, ternary operations use
+/// the first three memory channels, and the last one for the result (binary
+/// operations do not use the third inputs).
+///
+/// Shift operations are different, as they are simulated with `MUL` or `DIV`
+/// on the arithmetic side. As we first convert the shift into the multiplicand
+/// (in case of `SHL`) or the divisor (in case of `SHR`), making the first memory
+/// channel not directly usable. We overcome this by adding an offset of 1 in
+/// case of shift operation, which will skip the first memory channel and use the
+/// next three as ternary inputs. Because both `MUL` and `DIV` are binary operations,
+/// the last memory channel used for inputs will be safely ignored. The exact mapping
+/// is described in [cpu/shift.rs](super::shift::eval_packed).
 fn ctl_data_ternops<F: Field>(ops: &[usize], is_shift: bool) -> Vec<Column<F>> {
     let offset = is_shift as usize;
     let mut res = Column::singles(ops).collect_vec();