Remove nacl_user[] array lookup from syscall code path on x86-64 and MIPS

src/trusted/service_runtime/arch/mips/nacl_syscall.S

 /*
  * Copyright (c) 2012 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.
  */
 
 #include "native_client/src/trusted/service_runtime/arch/mips/sel_ldr_mips.h"
 #include "native_client/src/trusted/service_runtime/arch/mips/sel_rt.h"
 #include "native_client/src/trusted/service_runtime/nacl_config.h"
 
@@ skipping 70 matching lines @@
   sw    $a3, 12($sp)
   sw    $a2,  8($sp)
   sw    $a1,  4($sp)
   sw    $a0,  0($sp)
   /* Save return address for returning to untrusted code. */
   sw    $ra, -4($sp)
   /* Save return address that indicates which trampoline was called. */
   sw    $t5, -8($sp)
 
   /* Load the __thread variable's offset into a3. */
-  lui   $a3, %tprel_hi(gNaClThreadIdx)
-  addiu $a3, $a3, %tprel_lo(gNaClThreadIdx)
+  lui   $a3, %tprel_hi(nacl_current_thread)
+  addiu $a3, $a3, %tprel_lo(nacl_current_thread)
 
-  /* Fetch the thread-local variable holding the thread index. */
+  /* Fetch the thread-local variable: set a0 = nacl_current_thread */
   rdhwr $v1, $29
   addu  $a3, $v1, $a3
-  lw    $a1, 0($a3)
-
-  /*
-   * Load the nacl_user entry of this thread.
-   * a0 = nacl_user[a1*4]
-   */
-  lw    $a0,%got(nacl_user)($28)
-  sll   $a1, $a1, 2
-  addu  $a0, $a0, $a1
-  lw    $a0, 0($a0)
+  lw    $a0, 0($a3)
 
 DEFINE_GLOBAL_HIDDEN_IDENTIFIER(NaClSyscallThreadCaptureFault):
   sw $s0, NACL_THREAD_CONTEXT_OFFSET_S0($a0)
   sw $s1, NACL_THREAD_CONTEXT_OFFSET_S1($a0)
   sw $s2, NACL_THREAD_CONTEXT_OFFSET_S2($a0)
   sw $s3, NACL_THREAD_CONTEXT_OFFSET_S3($a0)
   sw $s4, NACL_THREAD_CONTEXT_OFFSET_S4($a0)
   sw $s5, NACL_THREAD_CONTEXT_OFFSET_S5($a0)
   sw $s6, NACL_THREAD_CONTEXT_OFFSET_S6($a0)
   sw $s7, NACL_THREAD_CONTEXT_OFFSET_S7($a0)
@@ skipping 10 matching lines @@
 
   /* NOTREACHED */
 
   /*
    * If the thread returns, which must not happen, it will be halted
    * by the following instruction.
    */
 
   .word NACL_HALT_WORD