Re: add AVX2 support to simd.h

Nathan Bossart <nathandbossart@gmail.com> writes:
> I'm tempted to propose that we move forward with this patch as-is after
> adding a buildfarm machine that compiles with -mavx2 or -march=x86-64-v3.
> There is likely still follow-up work to make these improvements more
> accessible, but I'm not sure that is a strict prerequisite here.

The patch needs better comments (as in, more than "none whatsoever").
It doesn't need to be much though, perhaps like

+#if defined(__AVX2__)
+
+/*
+ * When compiled with -mavx2 or allied options, we prefer AVX2 instructions.
+ */
+#include <immintrin.h>
+#define USE_AVX2
+typedef __m256i Vector8;
+typedef __m256i Vector32;

Also, do you really want to structure the header so that USE_SSE2
doesn't get defined?  In that case you are committing to provide
an AVX2 replacement every single place that there's USE_SSE2, which
doesn't seem like a great thing to require.  OTOH, maybe there's
no choice given than we need a different definition for Vector8 and
Vector32?

            regards, tom lane