adding NEON optimizations

Author: lemire

popcnt_generic.go

@@ -1,5 +1,5 @@
-//go:build !amd64 || appengine
-// +build !amd64 appengine
+//go:build (!amd64 && !arm64) || appengine
+// +build !amd64,!arm64 appengine
 
 package roaring
 

popcnt_neon_arm64.go

@@ -0,0 +1,65 @@
+//go:build arm64 && !appengine
+// +build arm64,!appengine
+
+package roaring
+
+// The functions below are implemented in popcnt_neon_arm64.s using NEON
+// (Advanced SIMD). NEON is mandatory in the ARMv8-A baseline that every arm64
+// CPU implements, so — unlike the amd64 AVX2 path, which is gated on a runtime
+// _hasAVX2 check — these routines are always used on arm64. The pure-Go
+// fallbacks in popcnt_slices.go remain in use on other architectures and on
+// appengine builds, which compile popcnt_generic.go instead.
+
+//go:noescape
+func _popcntSliceNEON(s []uint64) uint64
+
+//go:noescape
+func _popcntMaskSliceNEON(s, m []uint64) uint64
+
+//go:noescape
+func _popcntAndSliceNEON(s, m []uint64) uint64
+
+//go:noescape
+func _popcntOrSliceNEON(s, m []uint64) uint64
+
+//go:noescape
+func _popcntXorSliceNEON(s, m []uint64) uint64
+
+// useNEON is always true on arm64; it exists so tests can force the scalar
+// fallback path and to mirror the amd64 dispatch structure.
+var useNEON = true
+
+func popcntSlice(s []uint64) uint64 {
+	if useNEON {
+		return _popcntSliceNEON(s)
+	}
+	return popcntSliceGo(s)
+}
+
+func popcntMaskSlice(s, m []uint64) uint64 {
+	if useNEON {
+		return _popcntMaskSliceNEON(s, m)
+	}
+	return popcntMaskSliceGo(s, m)
+}
+
+func popcntAndSlice(s, m []uint64) uint64 {
+	if useNEON {
+		return _popcntAndSliceNEON(s, m)
+	}
+	return popcntAndSliceGo(s, m)
+}
+
+func popcntOrSlice(s, m []uint64) uint64 {
+	if useNEON {
+		return _popcntOrSliceNEON(s, m)
+	}
+	return popcntOrSliceGo(s, m)
+}
+
+func popcntXorSlice(s, m []uint64) uint64 {
+	if useNEON {
+		return _popcntXorSliceNEON(s, m)
+	}
+	return popcntXorSliceGo(s, m)
+}