https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95859
--- Comment #11 from Tobias Schlüter <tobi at gcc dot gnu.org> --- Works on trunk now but not 10.2. Compiler explorer link: https://godbolt.org/z/1zbh4YM4W On the trunk we get the following. I'm guessing that one could enhance the read pattern by using more registers, but without benchmarking I don't believe that this can be beat: func34(m34): pxor xmm0, xmm0 mov rax, rdi cvtss2sd xmm0, DWORD PTR [rsp+8] movsd QWORD PTR [rdi], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+24] movsd QWORD PTR [rdi+8], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+40] movsd QWORD PTR [rdi+16], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+12] movsd QWORD PTR [rdi+24], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+28] movsd QWORD PTR [rdi+32], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+44] movsd QWORD PTR [rdi+40], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+16] movsd QWORD PTR [rdi+48], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+32] movsd QWORD PTR [rdi+56], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+48] movsd QWORD PTR [rdi+64], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+20] movsd QWORD PTR [rdi+72], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+36] movsd QWORD PTR [rdi+80], xmm0 pxor xmm0, xmm0 cvtss2sd xmm0, DWORD PTR [rsp+52] movsd QWORD PTR [rdi+88], xmm0 ret Thanks to whoever did that. I see that a release candidate for 10.2.1 has been cut. I would assume that it's not fixed in 10.2.1 because there would be a bugfix mentioned here. My experience is clearly not representative and I can appreciate that there was no deluge of performance regression PRs, but I would think that Eigen is an important enough library that one should consider whether breaking it like this is really something that should survive several (sub-)releases.
