Revert "perf: move common_prefix_len out of loop"

[rkrasiuk]

Reverts #5036

common_prefix_len was inside the loop because it gets truncated at the end of the iteration. Rare case that was not hit in tests

reth/crates/primitives/src/trie/hash_builder/mod.rs

Lines 329 to 331 in 821612c

	current.truncate(preceding_len);
	tracing::trace!(target: "trie::hash_builder", ?current, "truncated nibbles to {} bytes", preceding_len);

[DaniPopes]

Is there a way to test this?

[codecov]

Codecov Report

Merging #5043 (a1339db) into main (931065d) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

Files	Coverage Δ
crates/primitives/src/trie/hash_builder/mod.rs	86.91% <100.00%> (ø)

... and 9 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	17.04% <100.00%> (-0.01%)	⬇️
unit-tests	62.26% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	30.85% <ø> (ø)
blockchain tree	80.82% <ø> (ø)
pipeline	88.41% <ø> (ø)
storage (db)	74.43% <ø> (ø)
trie	94.51% <ø> (+0.03%)	⬆️
txpool	54.78% <ø> (-0.46%)	⬇️
networking	78.19% <ø> (ø)
rpc	58.43% <ø> (+<0.01%)	⬆️
consensus	63.01% <ø> (ø)
revm	28.20% <ø> (ø)
payload builder	7.96% <ø> (ø)
primitives	86.55% <100.00%> (+<0.01%)	⬆️

[robinsdan]

Actually, I have noticed this truncate code, but it will not shrink to a length less than common_prefix_len within the loop as I commented in the original pr, because:

1. to execute this line, it need first pass this if line :

  if preceding_len <= common_prefix_len && !succeeding.is_empty() {
                tracing::trace!(target: "trie::hash_builder", "no common prefix to create branch nodes from, returning");
                return // return here
 }

so preceding_len > common_prefix_len || succeeding.is_empty()

3. if succeeding.is_empty() , then common_prefix_len == 0, and it will always be 0, move out of loop is safe.
4. if preceding_len > common_prefix_len, then after truncation, current = current[.. preceding_len], common_prefix_len will still not change.