Edit: It seems that this forums's markdown render have problem on rendering this post. You can review the origin post on StackOverflow - https://stackoverflow.com/questions/63837100. I ran into a strange situation that alignof(m512) is not equal to std::alignment_of<m512>::value compiled by Apple's clang. After some testing I found that when alignof(T) is evaluated inside a template with T=m512, the result is different with direct alignof(m512). I also ran several test compiled by g++ and non-Apple's clang on ubuntu(WSL) and got correct(I thought) behavior. Is this a bug of Apple's clang? #include <immintrin.h> //avx headers #include <cstdio> #include <typeinfo> #include <type_traits> void test_directly() { &#9;printf("directly: typeid %s alignof %zu\n", typeid(m512).name(), alignof(m512)); } template<typename T> void test_as_template_argument() { &#9;static_assert(std::is_same<T, __m512>::value, "assert"); &#9;printf("template: typeid %s alignof %zu\n", typeid(T).name(), alignof(T)); } int main() { &#9;test_directly(); &#9;test_as_template_argument<__m512>(); &#9;return 0; } output(compiled with clang -std=c17 -march=native): directly: typeid Dv16_f alignof 64 template: typeid Dv16_f alignof 32 clang's version: Apple clang version 11.0.3 (clang-1103.0.32.62) Target: x86_64-apple-darwin19.4.0 Thread model: posix InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin macOS's version: macOS Catalina 10.15.4 (19E2269) Darwin 19.4.0