(optimization): felt252_div constant propogation support

[eytan-starkware]

What changed?

• Added constant folding for the felt252_div libfunc with three optimization cases:
  1. Division by 1 returns the original value
  2. Division of 0 by any non-zero value returns 0
  3. Division of two constants is computed at compile time
• Improved error handling in the test runner to provide better diagnostics when lowering fails
• Added test cases for all three optimization scenarios

[reviewable-StarkWare]

This change is 

[eytan-starkware]

• (optimization): felt252_div constant propogation support #8604 👈 (View in Graphite)
• Adding support to felt add mul and sub in constant propogation #8594
• main

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[TomerStarkware]

@TomerStarkware reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ilyalesokhin-starkware and @orizi)

[orizi]

Reviewable status: all files reviewed, 6 unresolved discussions (waiting on @ilyalesokhin-starkware)

---

crates/cairo-lang-lowering/src/optimizations/const_folding.rs line 556 at r1 (raw file):

            // There is no need to check if divisor is 0 and throw error, because casting to NonZero
            // will add a panic that will always be thrown. TODO(eytan-starkware): Make
            // sure that NonZero on a const zero leads to a compile error.

It does.
shorten comment.

Suggestion:

            // Note that divisor is never 0, due to NonZero type always being the divisor.

---

crates/cairo-lang-lowering/src/optimizations/const_folding.rs line 560 at r1 (raw file):

                && rhs.is_one()
            {
                // Check if dividing by 1 (returns the original value)

Suggestion:

                // Returns the original value when dividing by 1.

---

crates/cairo-lang-lowering/src/optimizations/const_folding.rs line 566 at r1 (raw file):

                && lhs.is_zero()
            {
                // Check if 0 is being divided (returns 0)

Suggestion:

// If the value is 0, result is 0 regardless of the divisor.

---

crates/cairo-lang-lowering/src/optimizations/const_folding.rs line 569 at r1 (raw file):

                Some(self.propagate_zero_and_get_statement(stmt.outputs[0]))
            } else if let (Some(lhs), Some(rhs)) =
                (self.as_int(stmt.inputs[0].var_id), self.as_int(stmt.inputs[1].var_id))

maybe better now when let-chains works - no need for an extra tuple structuring and destructuring.

Suggestion:

            } else if let Some(lhs) = self.as_int(stmt.inputs[0].var_id)
              && let Some(rhs)) = self.as_int(stmt.inputs[1].var_id)

---

crates/cairo-lang-lowering/src/optimizations/const_folding.rs line 570 at r1 (raw file):

            } else if let (Some(lhs), Some(rhs)) =
                (self.as_int(stmt.inputs[0].var_id), self.as_int(stmt.inputs[1].var_id))
                && !rhs.is_zero()

Suggestion:

            } else if let (Some(lhs), Some(rhs)) =
                (self.as_int(stmt.inputs[0].var_id), self.as_int(stmt.inputs[1].var_id))

---

crates/cairo-lang-lowering/src/optimizations/const_folding.rs line 578 at r1 (raw file):

                let rhs_felt = Felt252::from(rhs);
                // For non-zero divisor, use field_div; the libfunc should handle zero checks
                let rhs_nonzero = rhs_felt.try_into().expect("Non-zero divisor");

if you want to do that - you can do it as part of the initial let-chain.
&& let Some(rhs_nonzero) = Felt252::from(rhs).try_into() {

Code quote:

let rhs_nonzero = rhs_felt.try_into().expect("Non-zero divisor");

[orizi]

Reviewable status: all files reviewed, 8 unresolved discussions (waiting on @eytan-starkware and @ilyalesokhin-starkware)

---

crates/cairo-lang-lowering/src/optimizations/test_data/const_folding line 6514 at r1 (raw file):

//! > module_code
use core::felt252;

remove all around - already in prelude.

Code quote:

use core::felt252;

---

crates/cairo-lang-lowering/src/optimizations/test_data/const_folding line 6578 at r1 (raw file):

Statements:
  (v1: core::zeroable::NonZero::<core::felt252>) <- NonZero(1)
  (v2: core::felt252) <- core::felt252_div(v0, v1)

nothing happened.

Code quote:

  (v2: core::felt252) <- core::felt252_div(v0, v1)