[cmaloney]

Long ASCII runs now OR eight aligned words and test the high bits once, one branch per 64 bytes (32 on 32-bit) instead of one per word. When a block contains a non-ASCII byte, the existing one-word loop finds it. It is plain C: GCC and Clang compile it to SSE2/NEON loads, other compilers to ORed word loads with the same single branch.

Multiple compiler and architecture codegen sample: https://godbolt.org/z/8bM83vx3a

pyperf, PGO+LTO, 2 interleaved rounds:

Benchmark	main	this
(b'a' * 4096).decode()	165 ns	108 ns: 1.53x faster
(b'a' * 200).decode()	28.9 ns	26.5 ns: 1.09x faster
b'a' * 200 + 'é' + ..., .decode()	60.9 ns	56.0 ns: 1.09x faster
b'a' * 136 + 'é' + ..., .decode()	53.8 ns	51.3 ns: 1.05x faster
8-100 byte decodes, 'é' within 100		within 1.2 ns

import pyperf

runner = pyperf.Runner()
tail = 'é'.encode() + b'a' * 50
for n in (8, 100, 200, 4096):
    runner.timeit(f"(b'a' * {n}).decode()", "s.decode()",
                  globals={"s": b'a' * n})
for n in (16, 100, 136, 200):
    runner.timeit(f"(b'a' * {n} + 'é'.encode() + b'a' * 50).decode()",
                  "s.decode()", globals={"s": b'a' * n + tail})

• Issue: Speed up UTF-8 validation in tokenizer and ASCII scanning in UTF-8 decoding #161145