reinterpret_cast UB или нет?

~ g++ -O3 -std=c++17 LvRK05uQ.cpp && ./a.out
(5, 3, 10): 0.327668sec, 6.103744GB/s, 1.638tpc
(1, 3, 10): 0.435120sec, 4.596429GB/s, 1.234tpc
(1, 2, 10): 0.459338sec, 4.354094GB/s, 1.169tpc
(3, 6, 10): 0.780734sec, 2.561693GB/s, 0.688tpc
(3, 5, 20): 0.294056sec, 6.801418GB/s, 1.826tpc
(4, 4, 10): 0.287162sec, 6.964710GB/s, 1.870tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(4, 4, 10): 0.289929sec, 6.898228GB/s, 1.852tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(4, 4, 10): 0.289550sec, 6.907258GB/s, 1.854tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(5, 3, 10): 0.298832sec, 6.692722GB/s, 1.797tpc
(3, 5, 10): 0.966036sec, 2.070312GB/s, 0.556tpc
(1, 8, 10): 0.385803sec, 5.183987GB/s, 1.392tpc
(1, 4, 10): 0.404743sec, 4.941400GB/s, 1.326tpc
(1, 2, 10): 0.457901sec, 4.367756GB/s, 1.172tpc
(1, 2, 10): 0.457947sec, 4.367315GB/s, 1.172tpc
(1, 2, 10): 1.712740sec, 1.167719GB/s, 0.313tpc

~ gcc --version
gcc (GCC) 9.2.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

~ lscpu
Architecture:                    x86_64
CPU op-mode(s):                  32-bit, 64-bit
Byte Order:                      Little Endian
Address sizes:                   39 bits physical, 48 bits virtual
CPU(s):                          8
On-line CPU(s) list:             0-7
Thread(s) per core:              2
Core(s) per socket:              4
Socket(s):                       1
NUMA node(s):                    1
Vendor ID:                       GenuineIntel
CPU family:                      6
Model:                           142
Model name:                      Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
Stepping:                        10
CPU MHz:                         804.002
CPU max MHz:                     4000.0000
CPU min MHz:                     400.0000
BogoMIPS:                        3984.00
Virtualization:                  VT-x
L1d cache:                       128 KiB
L1i cache:                       128 KiB
L2 cache:                        1 MiB
L3 cache:                        8 MiB
NUMA node0 CPU(s):               0-7
Vulnerability L1tf:              Mitigation; PTE Inversion; VMX conditional cache flushes, SMT vulnerable
Vulnerability Mds:               Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Meltdown:          Mitigation; PTI
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl and seccomp
Vulnerability Spectre v1:        Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:        Mitigation; Full generic retpoline, IBPB conditional, IBRS_FW, STIBP conditional, RSB filling
Flags:                           fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe
                                 syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf
                                  tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic
                                 movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb invpcid_single pti s
                                 sbd ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mp
                                 x rdseed adx smap clflushopt intel_pt xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts hwp hwp_notify hwp_act_wind
                                 ow hwp_epp md_clear flush_l1d

size_t cpuinfo_max_freq = 2260000000; // std::strtoul(read_file("/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq").c_str(), nullptr, 10) * 1000;

/*__always_inline*/inline static void process(const char * in, char * out) {

auto dataset = (char *)mmap(nullptr, n + 4096, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|/*MAP_POPULATE|*/MAP_PRIVATE, 0, 0);

g++-9 -O3 -fomit-frame-pointer -march=native  -std=gnu++2a -lstdc++fs last.cpp -o last

./last
(5, 3, 10): 1.285655sec, 1.555627GB/s, 0.739tpc
(1, 3, 10): 1.392527sec, 1.436237GB/s, 0.682tpc
(1, 2, 10): 1.353350sec, 1.477814GB/s, 0.702tpc
(3, 6, 10): 1.206201sec, 1.658098GB/s, 0.788tpc
(3, 5, 20): 1.243445sec, 1.608432GB/s, 0.764tpc
(4, 4, 10): 1.165896sec, 1.715418GB/s, 0.815tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(4, 4, 10): 1.159045sec, 1.725558GB/s, 0.820tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(4, 4, 10): 1.161732sec, 1.721567GB/s, 0.818tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(5, 3, 10): 1.290603sec, 1.549663GB/s, 0.736tpc
(3, 5, 10): 1.306882sec, 1.530358GB/s, 0.727tpc
(1, 8, 10): 1.231989sec, 1.623391GB/s, 0.771tpc
(1, 4, 10): 1.238509sec, 1.614844GB/s, 0.767tpc
(1, 2, 10): 1.351298sec, 1.480058GB/s, 0.703tpc
(1, 2, 10): 1.348495sec, 1.483134GB/s, 0.705tpc
(1, 2, 10): 4.031501sec, 0.496093GB/s, 0.236tpc

g++-9 -Ofast -march=native -fwhole-program -funroll-all-loops  -std=gnu++2a -lstdc++fs last.cpp -o last
./last
(5, 3, 10): 1.236477sec, 1.617498GB/s, 0.768tpc
(1, 3, 10): 1.260593sec, 1.586554GB/s, 0.754tpc
(1, 2, 10): 1.177917sec, 1.697912GB/s, 0.807tpc
(3, 6, 10): 1.150058sec, 1.739042GB/s, 0.826tpc
(3, 5, 20): 1.149489sec, 1.739901GB/s, 0.827tpc
(4, 4, 10): 1.119731sec, 1.786143GB/s, 0.849tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(4, 4, 10): 1.117426sec, 1.789827GB/s, 0.850tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(4, 4, 10): 1.124264sec, 1.778941GB/s, 0.845tpc
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC^TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
GCGTGAGCCACCGCGCCCGGCCTTTTTGAGAC_GGAGTCTCGCTCTGTCGCCCAGGCTGGAGTGCAGTGGCGCGATCTCGGCTCACTGCAACC_TCCGCCTCCCGGGTTCAAGCGATTCTCCTGCCTCAG
(5, 3, 10): 1.240749sec, 1.611929GB/s, 0.766tpc
(3, 5, 10): 1.204658sec, 1.660220GB/s, 0.789tpc
(1, 8, 10): 1.142005sec, 1.751305GB/s, 0.832tpc
(1, 4, 10): 1.107829sec, 1.805332GB/s, 0.858tpc
(1, 2, 10): 1.186179sec, 1.686086GB/s, 0.801tpc
(1, 2, 10): 1.174458sec, 1.702913GB/s, 0.809tpc
(1, 2, 10): 3.060147sec, 0.653563GB/s, 0.311tpc