diff --git a/src/arch/x86_64/CodeGen.zig b/src/arch/x86_64/CodeGen.zig
index 3c64f15ef1aec0a8108813095d19ba512b9f6ba8..c7326ef3b069885546460ae2f3f7f5ec24acc9a7 100644
--- a/src/arch/x86_64/CodeGen.zig
+++ b/src/arch/x86_64/CodeGen.zig
@@ -7242,7 +7242,7 @@ fn airCmpxchg(self: *Self, inst: Air.Inst.Index) !void {
 
     try self.spillEflagsIfOccupied();
     _ = try self.addInst(.{ .tag = .cmpxchg, .ops = .lock_mr_sib, .data = .{ .rx = .{
-        .r = new_reg,
+        .r = registerAlias(new_reg, val_abi_size),
         .payload = try self.addExtra(Mir.MemorySib.encode(ptr_mem)),
     } } });
 
diff --git a/src/arch/x86_64/encodings.zig b/src/arch/x86_64/encodings.zig
index d0ded0136bb8766132b69abb18a850a2e30d5fc0..9683ef991af65fe2ab2f61c08809ed71c299bdd2 100644
--- a/src/arch/x86_64/encodings.zig
+++ b/src/arch/x86_64/encodings.zig
@@ -257,8 +257,8 @@ pub const table = &[_]Entry{
 
     .{ .cmpxchg, .mr, .rm8,  .r8,  .none, .none, &.{ 0x0f, 0xb0 }, 0, .none },
     .{ .cmpxchg, .mr, .rm8,  .r8,  .none, .none, &.{ 0x0f, 0xb0 }, 0, .rex  },
-    .{ .cmpxchg, .mr, .rm16, .r16, .none, .none, &.{ 0x0f, 0xb1 }, 0, .rex  },
-    .{ .cmpxchg, .mr, .rm32, .r32, .none, .none, &.{ 0x0f, 0xb1 }, 0, .rex  },
+    .{ .cmpxchg, .mr, .rm16, .r16, .none, .none, &.{ 0x0f, 0xb1 }, 0, .none },
+    .{ .cmpxchg, .mr, .rm32, .r32, .none, .none, &.{ 0x0f, 0xb1 }, 0, .none },
     .{ .cmpxchg, .mr, .rm64, .r64, .none, .none, &.{ 0x0f, 0xb1 }, 0, .long },
 
     .{ .cmpxchg8b , .m, .m64,  .none, .none, .none, &.{ 0x0f, 0xc7 }, 1, .none },
diff --git a/test/behavior/atomics.zig b/test/behavior/atomics.zig
index a1e3af6e9a3f2016b94f93cacef09649f3474f60..e6000cd8485e4d5bea24790951ef2ee4df1d7950 100644
--- a/test/behavior/atomics.zig
+++ b/test/behavior/atomics.zig
@@ -5,7 +5,6 @@ const expectEqual = std.testing.expectEqual;
 
 test "cmpxchg" {
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
@@ -96,7 +95,6 @@ test "cmpxchg with ptr" {
 
 test "cmpxchg with ignored result" {
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
@@ -143,7 +141,6 @@ var a_global_variable = @as(u32, 1234);
 
 test "cmpxchg on a global variable" {
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
@@ -384,7 +381,6 @@ fn testAtomicRmwInt128(comptime signedness: std.builtin.Signedness) !void {
 
 test "atomics with different types" {
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO