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feat: skip codegen of zero iteration loops #7030

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Jan 13, 2025
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feat: skip codegen of zero iteration loops #7030

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22 changes: 16 additions & 6 deletions compiler/noirc_evaluator/src/ssa/ssa_gen/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -519,7 +519,7 @@
/// br loop_entry(v0)
/// loop_entry(i: Field):
/// v2 = lt i v1
/// brif v2, then: loop_body, else: loop_end

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Unknown word (brif)
/// loop_body():
/// v3 = ... codegen body ...
/// v4 = add 1, i
Expand All @@ -528,6 +528,22 @@
/// ... This is the current insert point after codegen_for finishes ...
/// ```
fn codegen_for(&mut self, for_expr: &ast::For) -> Result<Values, RuntimeError> {
self.builder.set_location(for_expr.start_range_location);
let start_index = self.codegen_non_tuple_expression(&for_expr.start_range)?;

self.builder.set_location(for_expr.end_range_location);
let end_index = self.codegen_non_tuple_expression(&for_expr.end_range)?;

if let (Some(start_constant), Some(end_constant)) = (
self.builder.current_function.dfg.get_numeric_constant(start_index),
self.builder.current_function.dfg.get_numeric_constant(end_index),
) {
// If we can determine that the loop contains zero iterations then there's no need to codegen the loop.
if start_constant >= end_constant {
return Ok(Self::unit_value());
}
}

let loop_entry = self.builder.insert_block();
let loop_body = self.builder.insert_block();
let loop_end = self.builder.insert_block();
Expand All @@ -540,12 +556,6 @@
// within the loop which need to jump to them.
self.enter_loop(loop_entry, loop_index, loop_end);

self.builder.set_location(for_expr.start_range_location);
let start_index = self.codegen_non_tuple_expression(&for_expr.start_range)?;

self.builder.set_location(for_expr.end_range_location);
let end_index = self.codegen_non_tuple_expression(&for_expr.end_range)?;

// Set the location of the initial jmp instruction to the start range. This is the location
// used to issue an error if the start range cannot be determined at compile-time.
self.builder.set_location(for_expr.start_range_location);
Expand Down Expand Up @@ -580,7 +590,7 @@
///
/// ```text
/// v0 = ... codegen cond ...
/// brif v0, then: then_block, else: else_block

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/// then_block():
/// v1 = ... codegen a ...
/// br end_if(v1)
Expand All @@ -595,7 +605,7 @@
///
/// ```text
/// v0 = ... codegen cond ...
/// brif v0, then: then_block, else: end_if

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Unknown word (brif)
/// then_block:
/// v1 = ... codegen a ...
/// br end_if()
Expand Down
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