i#1901: switch to r10 instead of rcx for the first syscall param

Switches to using r10 which is where the first syscall param is for
OP_syscall.  This allows tools to clobber rcx at the syscall.  This fixes
some weird behavior with tools on Windows 10.

Fixes #1901

Review-URL: https://codereview.appspot.com/310720043

core/win32/os_private.h

@@ -259,7 +259,11 @@ sys_param_addr(dcontext_t *dcontext, reg_t *param_base, int num)
     /* we force-inline get_mcontext() and so don't take it as a param */
     priv_mcontext_t *mc = get_mcontext(dcontext);
     switch (num) {
-    case 0: return &mc->xcx;
+    /* The first arg was in rcx, but that's clobbered by OP_sysycall, so the wrapper
+     * copies it to r10.  We need to use r10 as our own instru sometimes takes
+     * advantage of the dead rcx and clobbers it inside the wrapper (i#1901).
+     */
+    case 0: return &mc->r10;
     case 1: return &mc->xdx;
     case 2: return &mc->r8;
     case 3: return &mc->r9;

core/win32/syscall.c

@@ -1750,7 +1750,10 @@ presys_TerminateProcess(dcontext_t *dcontext, reg_t *param_base)
         /* FIXME: what if syscall returns w/ STATUS_PROCESS_IS_TERMINATING? */
         os_terminate_wow64_write_args(true/*process*/, process_handle, exit_status);
         cleanup_and_terminate(dcontext, syscalls[SYS_TerminateProcess],
-                              IF_X64_ELSE(mc->xcx, mc->xdx),
+                              /* r10, which will go to rcx in cleanup_and_terminate
+                               * and back to r10 in global_do_syscall_syscall (i#1901).
+                               */
+                              IF_X64_ELSE(mc->r10, mc->xdx),
                               mc->xdx, true /* entire process */, 0, 0);
     }
     return true;
@@ -1829,7 +1832,10 @@ presys_TerminateThread(dcontext_t *dcontext, reg_t *param_base)
         KSTOP(num_exits_dir_syscall);
         os_terminate_wow64_write_args(false/*thread*/, thread_handle, exit_status);
         cleanup_and_terminate(dcontext, syscalls[SYS_TerminateThread],
-                              IF_X64_ELSE(mc->xcx, mc->xdx),
+                              /* r10, which will go to rcx in cleanup_and_terminate
+                               * and back to r10 in global_do_syscall_syscall (i#1901).
+                               */
+                              IF_X64_ELSE(mc->r10, mc->xdx),
                               mc->xdx, exitproc, 0, 0);
     }
 }
@@ -4474,8 +4480,7 @@ dr_syscall_invoke_another(void *drcontext)
     }
 # ifdef X64
     else if (get_syscall_method() == SYSCALL_METHOD_SYSCALL) {
-        /* we could instead have sys_param_addr() use r10, like we do on linux */
-        mc->r10 = mc->xcx;
+        /* sys_param_addr() is already using r10 */
     }
 # endif
 }