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perf: remove the three most significant bytes of a operand in LT table. #1678

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Oct 20, 2024
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perf: remove the three most significant bytes of a operand in LT table. #1678

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14 changes: 5 additions & 9 deletions crates/core/machine/src/alu/lt/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -45,7 +45,7 @@ pub struct LtCols<T> {
pub is_sltu: T,

/// The output operand.
pub a: Word<T>,
pub a: T,

/// The first input operand.
pub b: Word<T>,
Expand Down Expand Up @@ -183,7 +183,7 @@ impl LtChip {
let c = event.c.to_le_bytes();

cols.shard = F::from_canonical_u32(event.shard);
cols.a = Word(a.map(F::from_canonical_u8));
cols.a = F::from_canonical_u8(a[0]);
cols.b = Word(b.map(F::from_canonical_u8));
cols.c = Word(c.map(F::from_canonical_u8));

Expand Down Expand Up @@ -249,7 +249,7 @@ impl LtChip {
cols.bit_b = cols.msb_b * cols.is_slt;
cols.bit_c = cols.msb_c * cols.is_slt;

assert_eq!(cols.a[0], cols.bit_b * (F::one() - cols.bit_c) + cols.is_sign_eq * cols.sltu);
assert_eq!(cols.a, cols.bit_b * (F::one() - cols.bit_c) + cols.is_sign_eq * cols.sltu);

blu.add_byte_lookup_event(ByteLookupEvent {
shard: event.shard,
Expand Down Expand Up @@ -351,13 +351,9 @@ where

// Check that `a[0]` is set correctly.
builder.assert_eq(
local.a[0],
local.a,
local.bit_b * (AB::Expr::one() - local.bit_c) + local.is_sign_eq * local.sltu,
);
// Check the 3 most significant bytes of 'a' are zero.
builder.assert_zero(local.a[1]);
builder.assert_zero(local.a[2]);
builder.assert_zero(local.a[3]);

// Verify that the byte equality flags are set correctly, i.e. all are boolean and only
// at most a single byte flag is set.
Expand Down Expand Up @@ -451,7 +447,7 @@ where
builder.receive_alu(
local.is_slt * AB::F::from_canonical_u32(Opcode::SLT as u32)
+ local.is_sltu * AB::F::from_canonical_u32(Opcode::SLTU as u32),
local.a,
Word([local.a.into(), AB::Expr::zero(), AB::Expr::zero(), AB::Expr::zero()]),
local.b,
local.c,
local.shard,
Expand Down
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