fix index out of bounds error of x86_64 ABI

src/librustc/middle/trans/cabi_x86_64.rs

@@ -256,10 +256,10 @@ fn fixup(ty: TypeRef, cls: &mut [x86_64_reg_class]) {
                         cls[i] = sse_int_class;
                     } else if is_sse(cls[i]) {
                         i += 1;
-                        while cls[i] == sseup_class { i += 1u; }
+                        while i != e && cls[i] == sseup_class { i += 1u; }
                     } else if cls[i] == x87_class {
                         i += 1;
-                        while cls[i] == x87up_class { i += 1u; }
+                        while i != e && cls[i] == x87up_class { i += 1u; }
                     } else {
                         i += 1;
                     }

src/test/run-pass/issue-5754.rs

@@ -0,0 +1,19 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct S {
+    r: float
+}
+
+extern "C" {
+    fn rust_dbg_extern_identity_double(arg1: S) -> float;
+}
+
+pub fn main() {}