Optimized std.mem.eql with SIMD (#18389)

* optimized memeql
* add `sched_setaffinity` to `std.os.linux`

Co-authored-by: Protty <45520026+kprotty@users.noreply.github.com>
Co-authored-by: Ryan Liptak <squeek502@hotmail.com>

lib/std/mem.zig

@@ -634,14 +634,77 @@ test "lessThan" {
 
 /// Compares two slices and returns whether they are equal.
 pub fn eql(comptime T: type, a: []const T, b: []const T) bool {
+    if (@sizeOf(T) == 0) return true;
+    if (!@inComptime() and std.meta.hasUniqueRepresentation(T)) return eqlBytes(sliceAsBytes(a), sliceAsBytes(b));
+
     if (a.len != b.len) return false;
-    if (a.ptr == b.ptr) return true;
+    if (a.len == 0 or a.ptr == b.ptr) return true;
+
     for (a, b) |a_elem, b_elem| {
         if (a_elem != b_elem) return false;
     }
     return true;
 }
 
+/// std.mem.eql heavily optimized for slices of bytes.
+fn eqlBytes(a: []const u8, b: []const u8) bool {
+    if (a.len != b.len) return false;
+    if (a.len == 0 or a.ptr == b.ptr) return true;
+
+    if (a.len <= 16) {
+        if (a.len < 4) {
+            const x = (a[0] ^ b[0]) | (a[a.len - 1] ^ b[a.len - 1]) | (a[a.len / 2] ^ b[a.len / 2]);
+            return x == 0;
+        }
+        var x: u32 = 0;
+        for ([_]usize{ 0, a.len - 4, (a.len / 8) * 4, a.len - 4 - ((a.len / 8) * 4) }) |n| {
+            x |= @as(u32, @bitCast(a[n..][0..4].*)) ^ @as(u32, @bitCast(b[n..][0..4].*));
+        }
+        return x == 0;
+    }
+
+    // Figure out the fastest way to scan through the input in chunks.
+    // Uses vectors when supported and falls back to usize/words when not.
+    const Scan = if (std.simd.suggestVectorSize(u8)) |vec_size|
+        struct {
+            pub const size = vec_size;
+            pub const Chunk = @Vector(size, u8);
+            pub inline fn isNotEqual(chunk_a: Chunk, chunk_b: Chunk) bool {
+                return @reduce(.Or, chunk_a != chunk_b);
+            }
+        }
+    else
+        struct {
+            pub const size = @sizeOf(usize);
+            pub const Chunk = usize;
+            pub inline fn isNotEqual(chunk_a: Chunk, chunk_b: Chunk) bool {
+                return chunk_a != chunk_b;
+            }
+        };
+
+    inline for (1..6) |s| {
+        const n = 16 << s;
+        if (n <= Scan.size and a.len <= n) {
+            const V = @Vector(n / 2, u8);
+            var x = @as(V, a[0 .. n / 2].*) ^ @as(V, b[0 .. n / 2].*);
+            x |= @as(V, a[a.len - n / 2 ..][0 .. n / 2].*) ^ @as(V, b[a.len - n / 2 ..][0 .. n / 2].*);
+            const zero: V = @splat(0);
+            return !@reduce(.Or, x != zero);
+        }
+    }
+    // Compare inputs in chunks at a time (excluding the last chunk).
+    for (0..(a.len - 1) / Scan.size) |i| {
+        const a_chunk: Scan.Chunk = @bitCast(a[i * Scan.size ..][0..Scan.size].*);
+        const b_chunk: Scan.Chunk = @bitCast(b[i * Scan.size ..][0..Scan.size].*);
+        if (Scan.isNotEqual(a_chunk, b_chunk)) return false;
+    }
+
+    // Compare the last chunk using an overlapping read (similar to the previous size strategies).
+    const last_a_chunk: Scan.Chunk = @bitCast(a[a.len - Scan.size ..][0..Scan.size].*);
+    const last_b_chunk: Scan.Chunk = @bitCast(b[a.len - Scan.size ..][0..Scan.size].*);
+    return !Scan.isNotEqual(last_a_chunk, last_b_chunk);
+}
+
 /// Compares two slices and returns the index of the first inequality.
 /// Returns null if the slices are equal.
 pub fn indexOfDiff(comptime T: type, a: []const T, b: []const T) ?usize {

lib/std/os/linux.zig

@@ -1553,6 +1553,16 @@ pub fn sched_getaffinity(pid: pid_t, size: usize, set: *cpu_set_t) usize {
     return 0;
 }
 
+pub fn sched_setaffinity(pid: pid_t, set: *const cpu_set_t) !void {
+    const size = @sizeOf(cpu_set_t);
+    const rc = syscall3(.sched_setaffinity, @as(usize, @bitCast(@as(isize, pid))), size, @intFromPtr(set));
+
+    switch (std.os.errno(rc)) {
+        .SUCCESS => return,
+        else => |err| return std.os.unexpectedErrno(err),
+    }
+}
+
 pub fn epoll_create() usize {
     return epoll_create1(0);
 }