HWY_CMAKE_ARM7:BOOL=ON vs Dynamic Dispatch

hwy claims to have dynamic dispatch. However when using the cmake flag HWY_CMAKE_ARM7:BOOL=ON the complete codebase is compiled with neon instruction (`-mfpu=neon-vfpv4`).



It turns out the shared library is taking advantages of this compilation flag, and generate code with neon intructions:



```

% gdb ./obj-arm-linux-gnueabihf/hwy_list_targets

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Reading symbols from ./obj-arm-linux-gnueabihf/hwy_list_targets...

(gdb) r

Starting program: /home/malat/highway-1.0.3/obj-arm-linux-gnueabihf/hwy_list_targets

[Thread debugging using libthread_db enabled]

Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".



Program received signal SIGILL, Illegal instruction.

hwy::(anonymous namespace)::robust_statistics::CountingSort<unsigned long long> (values=0xbeffed08, num_values=256) at ./hwy/nanobenchmark.cc:214

214       std::vector<Unique> unique;

(gdb) bt full

#0  hwy::(anonymous namespace)::robust_statistics::CountingSort<unsigned long long> (values=0xbeffed08, num_values=256) at ./hwy/nanobenchmark.cc:214

        unique = std::vector of length 0, capacity 200277584

        p = <optimized out>

#1  0xb6ff4256 in hwy::(anonymous namespace)::robust_statistics::Mode<unsigned long long> (num_values=256, values=0xbeffed08) at ./hwy/nanobenchmark.cc:286

No locals.

#2  hwy::(anonymous namespace)::robust_statistics::Mode<unsigned long long, 256> (values=...) at ./hwy/nanobenchmark.cc:292

No locals.

#3  hwy::platform::TimerResolution () at ./hwy/nanobenchmark.cc:480

        samples = {1640, 680, 720, 720, 720, 720, 720, 680, 600, 680, 720, 720, 760, 720, 720, 680, 600, 680, 720, 760, 720, 720, 760, 640, 600, 640, 760, 720, 760, 720, 720, 640, 600, 680, 720, 720, 760,

          720, 720, 680, 600, 680, 720, 719, 720, 720, 720, 680, 600, 680, 720, 720, 720, 720, 720, 680, 600, 640, 760, 720, 720, 720, 720, 640, 600, 680, 720, 720, 760, 720, 720, 640, 600, 680, 720, 760,

          720, 720, 720, 640, 600, 640, 720, 720, 720, 720, 720, 640, 600, 680, 720, 720, 720, 720, 760, 640, 600, 680, 760, 720, 760, 720, 720, 680, 599, 680, 720, 720, 720, 720, 720, 680, 640, 680, 720,

          760, 720, 720, 720, 640, 600, 680, 720, 720, 720, 720, 720, 640, 600, 680, 720, 760, 720, 720, 720, 680, 600, 640, 720, 720, 720, 720, 720, 680, 640, 680, 720, 720, 720, 720, 720, 680, 600, 640,

          720, 720, 760, 720, 720, 640, 600, 680, 720, 720, 760, 720, 720, 680, 600, 640, 720, 720, 720, 720, 760, 680, 600, 680, 720, 720, 720, 720, 720, 640, 640, 680, 720, 720, 720, 720, 720, 680, 600,

          680, 720, 720, 720, 720, 720, 680...}

        i = <optimized out>

        t0 = <optimized out>

        t1 = <optimized out>

        rep = <optimized out>

        can_use_stop = true

        repetitions = {0 <repeats 32 times>, 13186041144842129408, 1, 17592186044416, 279787914, 142542138799204, 17592186048511, 0, 1, 142541374619648, 5770084, 263640, 520, 2164, 1680014846, 279787914,

          1680014810, 752179689, 1680014810, 764179556, 1673090989, 0, 0, 4294967550, 0, 13762973729367785472, 13186435773802365071, 13185760231415605028, 1108307720798211, 0, 0, 0, 0, 1108307720798208,

          1103205299907420, 65536, 1389782697508869, 1407804380614656, 281474977038444, 12885159936, 0, 204933669240, 13185811011904471040, 150323855360, 13186435770732249088, 13170777113315222376,

          16089343920, 13762973007813279746, 13185765720249593216, 0, 16089343952, 13762973007813279746, 13185765720249593216, 0, 17595256270208, 16089343880, 13186532544935362560, 0, 13762975155296927744,

          8309530817697147188, 25770127360, 281478044361982, 1407838740021249, 8589934595, 1407841806909548, 65025, 5770084, 65025, 5770084, 4295000484, 0, 0, 263640, 4096, 520, 1680014846, 279787914,

          1673090989, 0, 1680014810, 13170903170605382208, 13186528954342703104, 13170903167535153152, 13185775014558802296, 13762975106961762556, 13185776139974469871, 13186429108013211984, 1, 3204442356,

          3204442383, 0, 4294967306, 7365196392, 1, 13762988919442350416, 2199023255566, 282579962709375, 0, 4297588739, 223338299392, 360292368236413384, 11259024840327220, 4296605722, 0, 1103205299650560,

          21475093340, 4295032832, 1406206652710640, 1597728161520, 25769804156, 8590000128, 1406241012449016, 1133871693560, 25769804040, 17179869188, 1047972020468, 292057776372, 17179869252,

          7238662637146341380, 0, 0, 25769803776, 7238662641441308688, 1406206652710640, 1168231431920, 17179869456, 17179869185, 12884901908, 1295390749551316551, 10439841620871673989, 11396069235890618897,

          68719476740, 24011439870050305, 12884901888, 2, 73014445019, 55834575104, 71198746626085, 23932014281626402, 9944168979650592768, 4632040454706176000, 13873901602337195072, 153122390170800208,

          2341871814990364692, 4614043964063155330, 83334332495831810, 1010620087226897409, 4611687117939015697, 2322250218881419280, 5343807179059625984, 141287514736965, 1152925945713262688,

          4612398501962188802, 5261757287512868864, 2305881045444609312, 558346862592, 10378898593745469700, 738783853203685377, 13762980258003298314, 13172183419976679425, 13762980260922518864,

          13170903170605359480, 13185704267991739608, 0, 13185944611157180416, 3204443540, 13762980260788111365, 3204443540, 13762977632402533780, 13185944571411950376, 7365196300, 13762979917187579656,

          13186435773936692640, 13762979401791206054, 0 <repeats 20 times>, 37429944632, 13762979195636390016, 13185881157310349312, 37429945464, 13762979195636392312, 13185881157310349312,

          13762978852173245608...}

#4  0xb6ff2e0e in __static_initialization_and_destruction_0 (__priority=65535, __initialize_p=1) at ./hwy/nanobenchmark.cc:488

No locals.

#5  _GLOBAL__sub_I_nanobenchmark.cc(void) () at ./hwy/nanobenchmark.cc:763

No locals.

#6  0xb6fd244c in ?? () from /lib/ld-linux-armhf.so.3

No symbol table info available.

Backtrace stopped: previous frame identical to this frame (corrupt stack?)

(gdb) x/i $pc

=> 0xb6ff3fbe <hwy::(anonymous namespace)::robust_statistics::CountingSort<unsigned long long>(unsigned long long*, size_t)+14>:        vmov.i32        d16, #0 @ 0x00000000

```

[]