rt: Use an out pointer for rust_new_stack

upcall_call_shim_on_c_stack does not handle return values

src/rt/arch/i386/morestack.S

@@ -75,11 +75,12 @@ MORESTACK:
 
 	// The arguments to rust_new_stack2
 	movl 40(%esp),%eax  // Size of stack arguments
-	movl %eax,16(%esp)
+	movl %eax,20(%esp)
 	leal 48(%esp),%eax // Address of stack arguments
-	movl %eax,12(%esp)
+	movl %eax,16(%esp)
 	movl 36(%esp),%eax // The amount of stack needed
-	movl %eax,8(%esp)
+	movl %eax,12(%esp)
+	movl $0, 8(%esp)   // Out pointer
 
 #ifdef __APPLE__
 	call 1f
@@ -97,7 +98,7 @@ MORESTACK:
 	movl 32(%esp),%edx          // Grab the return pointer.
 	inc  %edx    // Skip past the ret instruction in the parent fn
 
-	movl %eax,%esp          // Switch stacks.
+	movl 8(%esp),%esp          // Switch stacks.
 	call *%edx             // Re-enter the function that called us.
 
 	// Now the function that called us has returned, so we need to delete the

src/rt/arch/x86_64/morestack.S

@@ -81,9 +81,12 @@ MORESTACK:
 	movq %rbp, %rcx
 	addq $24, %rcx  // Base pointer, return address x2
 
+	pushq $0 // Alignment
+
 	pushq %r11 // Size of stack arguments
 	pushq %rcx // Address of stack arguments
 	pushq %r10 // The amount of stack needed
+	pushq $0   // Out pointer
 
 	movq UPCALL_NEW_STACK@GOTPCREL(%rip), %rsi
 	movq %rsp, %rdi
@@ -95,7 +98,8 @@ MORESTACK:
 #endif
 
 	// Pop the new_stack_args struct
-	addq $24, %rsp
+	popq %rax
+	addq $32, %rsp
 
 	// Pop the saved arguments
 	popq %r9

src/rt/rust_upcall.cpp

@@ -217,28 +217,40 @@ extern "C" void record_sp(void *limit);
  */
 extern "C" CDECL void
 upcall_call_shim_on_c_stack(void *args, void *fn_ptr) {
+    rust_task *task = rust_scheduler::get_task();
+
     // FIXME (1226) - The shim functions generated by rustc contain the
     // morestack prologue, so we need to let them know they have enough
     // stack.
-    record_sp(0);
-    rust_task *task = rust_scheduler::get_task();
+    //record_sp(0);
+
     rust_scheduler *sched = task->sched;
-    sched->c_context.call_shim_on_c_stack(args, fn_ptr);
-    task->record_stack_limit();
+    try {
+        sched->c_context.call_shim_on_c_stack(args, fn_ptr);
+    } catch (...) {
+        //task = rust_scheduler::get_task();
+        //task->record_stack_limit();
+        throw;
+    }
+    //task = rust_scheduler::get_task();
+    //task->record_stack_limit();
 }
 
 struct rust_new_stack2_args {
-  size_t stk_sz;
-  void *args_addr;
-  size_t args_sz;
+    void *new_stack;
+    size_t stk_sz;
+    void *args_addr;
+    size_t args_sz;
 };
 
 // A new stack function suitable for calling through
 // upcall_call_shim_on_c_stack
-extern "C" CDECL void *
+extern "C" CDECL void
 upcall_new_stack(struct rust_new_stack2_args *args) {
     rust_task *task = rust_scheduler::get_task();
-    return task->new_stack(args->stk_sz, args->args_addr, args->args_sz);
+    args->new_stack = task->new_stack(args->stk_sz,
+                                      args->args_addr,
+                                      args->args_sz);
 }
 
 extern "C" CDECL void