utilize @branchHint more

lib/std/hash/xxhash.zig

@@ -593,7 +593,7 @@ pub const XxHash3 = struct {
     }
 
     fn hash3(seed: u64, input: anytype, noalias secret: *const [192]u8) u64 {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         std.debug.assert(input.len > 0 and input.len < 4);
 
         const flip: [2]u32 = @bitCast(secret[0..8].*);
@@ -625,7 +625,7 @@ pub const XxHash3 = struct {
     }
 
     fn hash16(seed: u64, input: anytype, noalias secret: *const [192]u8) u64 {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         std.debug.assert(input.len > 8 and input.len <= 16);
 
         const flip: [4]u64 = @bitCast(secret[24..56].*);
@@ -641,7 +641,7 @@ pub const XxHash3 = struct {
     }
 
     fn hash128(seed: u64, input: anytype, noalias secret: *const [192]u8) u64 {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         std.debug.assert(input.len > 16 and input.len <= 128);
 
         var acc = XxHash64.prime_1 *% @as(u64, input.len);
@@ -657,7 +657,7 @@ pub const XxHash3 = struct {
     }
 
     fn hash240(seed: u64, input: anytype, noalias secret: *const [192]u8) u64 {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         std.debug.assert(input.len > 128 and input.len <= 240);
 
         var acc = XxHash64.prime_1 *% @as(u64, input.len);
@@ -676,7 +676,7 @@ pub const XxHash3 = struct {
     }
 
     noinline fn hashLong(seed: u64, input: []const u8) u64 {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         std.debug.assert(input.len >= 240);
 
         const block_count = ((input.len - 1) / @sizeOf(Block)) * @sizeOf(Block);

lib/std/hash_map.zig

@@ -1187,17 +1187,13 @@ pub fn HashMapUnmanaged(
         }
 
         /// Find the index containing the data for the given key.
-        /// Whether this function returns null is almost always
-        /// branched on after this function returns, and this function
-        /// returns null/not null from separate code paths.  We
-        /// want the optimizer to remove that branch and instead directly
-        /// fuse the basic blocks after the branch to the basic blocks
-        /// from this function.  To encourage that, this function is
-        /// marked as inline.
-        inline fn getIndex(self: Self, key: anytype, ctx: anytype) ?usize {
+        fn getIndex(self: Self, key: anytype, ctx: anytype) ?usize {
             comptime verifyContext(@TypeOf(ctx), @TypeOf(key), K, Hash, false);
 
             if (self.size == 0) {
+                // We use cold instead of unlikely to force a jump to this case,
+                // no matter the weight of the opposing side.
+                @branchHint(.cold);
                 return null;
             }