fix: do "Brillig Array Gets" optimization during brillig gen

compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs

@@ -803,9 +803,24 @@ impl<'block, Registers: RegisterAllocator> BrilligBlock<'block, Registers> {
 
                 let array_variable = self.convert_ssa_value(*array, dfg);
 
-                let index_variable = self.convert_ssa_single_addr_value(*index, dfg);
+                // If the index is a constant we can directly fetch the value from the brillig array or vector
+                // by skipping the `[RC, ...` (for arrays) or `[RC, Size, Capacity, ...` (for vectors) elements.
+                let index_constant = dfg.get_numeric_constant(*index);
+                let index_variable = if let Some(index_constant) = index_constant {
+                    let offset = if matches!(dfg.type_of_value(*array), Type::Array(..)) {
+                        // Brillig arrays are [RC, ...items]
+                        1u128
+                    } else {
+                        // Brillig vectors are [RC, Size, Capacity, ...items]
+                        3u128
+                    };
+                    let shifted_index = index_constant + offset.into();
+                    self.brillig_context.make_usize_constant_instruction(shifted_index)
+                } else {
+                    self.convert_ssa_single_addr_value(*index, dfg)
+                };
 
-                if dfg.is_constant(*index) {
+                if index_constant.is_some() {
                     self.brillig_context.codegen_load_with_offset(
                         array_variable.extract_register(),
                         index_variable,

compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_globals.rs

@@ -441,8 +441,6 @@ mod tests {
         ";
 
         let ssa = Ssa::from_str(src).unwrap();
-        // Need to run SSA pass that sets up Brillig array gets
-        let ssa = ssa.brillig_array_gets();
         // Need to run DIE to generate the used globals map, which is necessary for Brillig globals generation.
         let mut ssa = ssa.dead_instruction_elimination();
 

compiler/noirc_evaluator/src/ssa.rs

@@ -204,13 +204,6 @@ pub fn primary_passes(options: &SsaEvaluatorOptions) -> Vec<SsaPass> {
         // Remove any potentially unnecessary duplication from the Brillig entry point analysis.
         SsaPass::new(Ssa::remove_unreachable_functions, "Removing Unreachable Functions"),
         SsaPass::new(Ssa::remove_truncate_after_range_check, "Removing Truncate after RangeCheck"),
-        // This pass makes transformations specific to Brillig generation.
-        // It must be the last pass to either alter or add new instructions before Brillig generation,
-        // as other semantics in the compiler can potentially break (e.g. inserting instructions).
-        // We can safely place the pass before DIE as that pass only removes instructions.
-        // We also need DIE's tracking of used globals in case the array get transformations
-        // end up using an existing constant from the globals space.
-        SsaPass::new(Ssa::brillig_array_gets, "Brillig Array Get Optimizations"),
         SsaPass::new(Ssa::dead_instruction_elimination, "Dead Instruction Elimination"),
         // A function can be potentially unreachable post-DIE if all calls to that function were removed.
         SsaPass::new(Ssa::remove_unreachable_functions, "Removing Unreachable Functions"),
@@ -247,8 +240,6 @@ pub fn minimal_passes() -> Vec<SsaPass<'static>> {
         SsaPass::new(Ssa::remove_unreachable_functions, "Removing Unreachable Functions"),
         // We need a DIE pass to populate `used_globals`, otherwise it will panic later.
         SsaPass::new(Ssa::dead_instruction_elimination, "Dead Instruction Elimination"),
-        // We need to add an offset to constant array indices in Brillig.
-        SsaPass::new(Ssa::brillig_array_gets, "Brillig Array Get Optimizations"),
     ]
 }
 

compiler/noirc_evaluator/src/ssa/opt/constant_folding.rs

@@ -1575,8 +1575,6 @@ mod test {
             }
             ";
         let ssa = Ssa::from_str(src).unwrap();
-        // Need to run SSA pass that sets up Brillig array gets
-        let ssa = ssa.brillig_array_gets();
         let brillig = ssa.to_brillig(&BrilligOptions::default());
 
         let ssa = ssa.fold_constants_with_brillig(&brillig);

compiler/noirc_evaluator/src/ssa/opt/mod.rs

@@ -8,7 +8,6 @@ mod array_set;
 mod as_slice_length;
 mod assert_constant;
 mod basic_conditional;
-mod brillig_array_gets;
 pub(crate) mod brillig_entry_points;
 mod check_u128_mul_overflow;
 mod checked_to_unchecked;