This compilation error was found by NodeJS when updating V8:
https://github.com/nodejs/node-v8/issues/240

MSVC reports an error with "too many initializer" for type uint32x4_t.

---

Under gcc/clang, this is a typedef to a builtin type.

For MSVC, it is a typedef to this union:
typedef union __n128
{
     unsigned __int64   n128_u64[2];
     unsigned __int32   n128_u32[4];
     ...
} __n128;

C++ mandates that only first member of union can be initialized at
declaration. Thus, it can only be initialized with {uint64_t, uint64_t}.

VS people proposed to use designated initializer instead:
var = {.n128_u32={1, 2, 3, 8}}
https://developercommunity.visualstudio.com/t/error-c2078-too-many-initializers-when-using-arm-n/402911
But, you need to use /std:c++20 for this, which is not the case in v8.

---

Thus, the only solution is to implement a hack specifically for MSVC,
where you build two uint64, from four uint32.

---------------------------------------

Once solved, another error is reported:
templated function extract_first_nonzero_index is specialized twice.

This is because, with MSVC, uint32x4_t and uint64x2_t are typedef to the
same __n128 union. The fix is to drop templates, and use explicit
function names instead.

Bug: v8:13312
Change-Id: I231d8cf01c05af01af319d56d5666c415f8b989b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3913035Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83404}