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chore[docs]: document the copy maxbound heuristic reasoning #4593

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chore[docs]: document the copy maxbound heuristic reasoning #4593

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7 changes: 6 additions & 1 deletion vyper/codegen/core.py
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Original file line number Diff line number Diff line change
Expand Up @@ -205,9 +205,12 @@ def _prefer_copy_maxbound_heuristic(dst, src, item_size):
# a heuristic - it's cheaper to just copy the extra buffer bytes
# than calculate the number of bytes
# copy(dst, src, 32 + itemsize*load(src))
# DUP<src> MLOAD PUSH1 ITEMSIZE MUL PUSH1 32 ADD (3 * 4 + 8 = 20 gas | 8 bytes)
# DUP<src> MLOAD PUSH1 32 ADD (3 * 4 = 12 gas | 5 bytes) if ITEM_SIZE == 1
# =>
# copy(dst, src, bound)
# (32 + itemsize*(load(src))) costs 4*3 + 8 - 3 gas over just `bound`
# PUSH1 BOUND (3 gas | 2 bytes)
# (32 + itemsize*(load(src))) costs 3 * 4 [+ 8] - 3 gas over just `bound`
length_calc_cost = 4 * 3 - 3
length_calc_cost += 8 * (item_size != 1) # PUSH MUL

Expand Down Expand Up @@ -237,6 +240,8 @@ def _prefer_copy_maxbound_heuristic(dst, src, item_size):
# or not.
# (dload(src) expands to `codecopy(0, add(CODE_END, src), 32); mload(0)`,
# and we have already accounted for an `mload(ptr)`).
# PUSH1 32 DUP2 PUSH CODE_END ADD PUSH0 CODECOPY (3 * 4 + 2 + 6 = 20 gas)
# PUSH1 32 PUSH OFFSET PUSH0 CODECOPY (3 * 2 + 2 + 6 = 14 gas) if src is a literal
# for simplicity, skip the 14 case.
if src.location == DATA and copy_cost <= (20 + length_calc_cost):
return True
Expand Down