Change startup ABI for untrusted code to be C-compatible

src/trusted/service_runtime/arch/x86_64/nacl_switch_64.S

 /*
- * Copyright 2009 The Native Client Authors.  All rights reserved.
- * Use of this source code is governed by a BSD-style license that can
- * be found in the LICENSE file.
+ * Copyright (c) 2011 The Native Client Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
  */
 
 /*
  * This code gets executed when switching from the 64-bit service
  * runtime to a 32-bit nacl module. NaClSwitch has one parameter only,
  * which is a struct passed by reference.
  */
 
 #include "native_client/src/trusted/service_runtime/nacl_config.h"
 
         /*
          * This function does not return.  Thus, we need to preserve
          * any callee-saved registers.
          */
 
         .text
 .globl  IDENTIFIER(NaClSwitch)
         HIDDEN(NaClSwitch)
 IDENTIFIER(NaClSwitch):
 #if NACL_WINDOWS
         /* if Windows, 1st param is already in %rcx, not %rdi */
-        mov     %rcx, %rdi
 #elif NACL_LINUX || NACL_OSX
         /* elif Linux/OSX, 1st param is already in %rdi. */
+        mov     %rdi, %rcx
 #else
 # error "What OS/compiler is the service runtime being compiled with?"
 #endif
 
-        movq    0x8(%rdi), %rbx
-        movq    0x20(%rdi), %rbp
-        movq    0x60(%rdi), %r12
-        movq    0x68(%rdi), %r13
-        movq    0x70(%rdi), %r14
-        movq    0x78(%rdi), %r15
+        movq    0x8(%rcx), %rbx
+        movq    0x20(%rcx), %rbp
+        movq    0x60(%rcx), %r12
+        movq    0x68(%rcx), %r13
+        movq    0x70(%rcx), %r14
+        movq    0x78(%rcx), %r15
 
         /* there is no springboard for x86_64 */
-        movq    0x38(%rdi), %rsp  /* rsp -- switch stack */
-        movq    0x90(%rdi), %rax  /* syscall return */
-        jmp     *0x88(%rdi)
+        movq    0x38(%rcx), %rsp  /* rsp -- switch stack */
+        movq    0x90(%rcx), %rax  /* syscall return */
+
         /*
-         * This leaves %rdi pointing to the thread context, but that
+         * %rdi is the first argument in the user calling convention.
+         * When starting the initial thread, we are passing the address
+         * of the parameter block here.  The initial stack pointer has
+         * been adjusted to one word below there, to insert a dummy
+         * return address for the user entry point function.
+         */
+        leal    8(%rsp), %edi
+
+        /*
+         * Zero all unused registers.  The 32-bit instructions
+         * are a byte shorter than their 64-bit counterparts
+         * when the target register is one of the first eight,
+         * and they implicitly zero the high halves.
+         */
+        xorl    %edx, %edx
+        movl    %edx, %esi
+        movq    %rdx, %r8
+        movq    %rdx, %r9
+        movq    %rdx, %r10
+        movq    %rdx, %r11
+
+        jmp     *0x88(%rcx)
+        /*
+         * This leaves %rcx pointing to the thread context, but that
          * should be memory that's inaccessible by NaCl module code.
          */