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fix: avoid allocating new variable in exp_reverse_bits #57

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fix: avoid allocating new variable in exp_reverse_bits #57

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21 changes: 15 additions & 6 deletions vm/src/compiler/recursion/circuit/fri.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -284,12 +284,21 @@ where
let mut folded_eval: Ext<_, _> = builder.constant(CC::EF::ZERO);
let two_adic_generator: Felt<_> = builder.constant(CC::F::two_adic_generator(log_max_height));

// TODO: fix expreversebits address bug to avoid needing to allocate a new variable.
let mut x = CC::exp_reverse_bits(
builder,
two_adic_generator,
index_bits[..log_max_height].to_vec(),
);
// Initialize x with the two-adic generator and compute the reverse bits exponentiation
// directly to avoid allocating a new variable.
let mut x = two_adic_generator;
let power_bits = &index_bits[..log_max_height];
let bit_len = power_bits.len();

// Implement exp_reverse_bits logic directly to reuse the x variable
for i in 1..=bit_len {
let index = bit_len - i;
let bit = power_bits[index];
let x_squared = builder.eval(x * x);
// Use select_chain_f to choose between x_squared and x based on bit
let selected = CC::select_chain_f(builder, bit, [x_squared], [x]);
x = selected[0];
}

for (offset, log_folded_height, commit, step, beta) in izip!(
0..,
Expand Down