Remove is_keccak_sponge (#1410)

* Remove is_keccak_sponge

* Apply comment

evm/spec/tables/cpu.tex

@@ -31,7 +31,6 @@ but change the code context, which is where the instructions are read from.
     \item \texttt{is\_kernel\_mode}: Boolean indicating whether we are in kernel (i.e. privileged) mode. This means we are executing kernel code, and we have access to
 privileged instructions.
     \item \texttt{gas}: The current amount of gas used in the current context. It is eventually checked to be below the current gas limit. Must fit in 32 bits.
-    \item \texttt{is\_keccak\_sponge}: Boolean indicating whether we are executing a Keccak hash. Only used as a filter for CTLs.
     \item \texttt{clock}: Monotonic counter which starts at 0 and is incremented by 1 at each row. Used to enforce correct ordering of memory accesses. 
     \item \texttt{opcode\_bits}: 8 boolean columns, which are the bit decomposition of the opcode being read at the current PC.
 \end{itemize}

evm/src/cpu/columns/mod.rs

@@ -78,9 +78,6 @@ pub(crate) struct CpuColumnsView<T: Copy> {
     /// If CPU cycle: the opcode, broken up into bits in little-endian order.
     pub opcode_bits: [T; 8],
 
-    /// Filter. 1 iff a Keccak sponge lookup is performed on this row.
-    pub is_keccak_sponge: T,
-
     /// Columns shared by various operations.
     pub(crate) general: CpuGeneralColumnsView<T>,
 

evm/src/cpu/cpu_stark.rs

@@ -48,8 +48,15 @@ pub(crate) fn ctl_data_keccak_sponge<F: Field>() -> Vec<Column<F>> {
 }
 
 /// CTL filter for a call to the Keccak sponge.
+// KECCAK_GENERAL is differentiated from JUMPDEST by its second bit set to 0.
 pub(crate) fn ctl_filter_keccak_sponge<F: Field>() -> Filter<F> {
-    Filter::new_simple(Column::single(COL_MAP.is_keccak_sponge))
+    Filter::new(
+        vec![(
+            Column::single(COL_MAP.op.jumpdest_keccak_general),
+            Column::linear_combination_with_constant([(COL_MAP.opcode_bits[1], -F::ONE)], F::ONE),
+        )],
+        vec![],
+    )
 }
 
 /// Creates the vector of `Columns` corresponding to the two inputs and

evm/src/witness/operation.rs

@@ -129,7 +129,6 @@ pub(crate) fn generate_keccak_general<F: Field>(
     state: &mut GenerationState<F>,
     mut row: CpuColumnsView<F>,
 ) -> Result<(), ProgramError> {
-    row.is_keccak_sponge = F::ONE;
     let [(context, _), (segment, log_in1), (base_virt, log_in2), (len, log_in3)] =
         stack_pop_with_log_and_fill::<4, _>(state, &mut row)?;
     let len = u256_to_usize(len)?;