rt: Add a call stub that switches to the C stack, untested as of yet

mk/rt.mk

@@ -37,7 +37,8 @@ RUNTIME_CS := rt/sync/timer.cpp \
 
 RUNTIME_LL :=
 
-RUNTIME_S := rt/arch/i386/_context.S
+RUNTIME_S := rt/arch/i386/_context.S \
+             rt/arch/i386/ccall.S
 
 RUNTIME_HDR := rt/globals.h \
                rt/rust.h \

src/rt/arch/i386/ccall.S

@@ -0,0 +1,14 @@
+    .text
+
+// upcall_call_c_stack(void (*fn)(), void *new_esp)
+.globl _upcall_call_c_stack
+_upcall_call_c_stack:
+    movl %esp,%ecx          // grab esp
+    movl 8(%esp),%eax       // save fn
+    movl 12(%esp),%esp      // switch stack
+    pushl %ecx              // save esp on stack
+    calll *%eax
+    popl %esp               // restore esp
+    ret
+
+

src/rt/arch/i386/context.h

@@ -37,6 +37,7 @@ public:
 
   void swap(context &out);
   void call(void *f, void *arg, void *sp);
+  void call(void *f, void *sp);
 
   // Note that this doesn't actually adjust esp. Instead, we adjust esp when
   // we actually do the call. This is needed for exception safety -- if the

src/rt/rustrt.def.in

@@ -66,6 +66,7 @@ task_yield
 task_join
 unsupervise
 upcall_alloc_c_stack
+upcall_call_c_stack
 upcall_cmp_type
 upcall_dynastack_alloc
 upcall_dynastack_alloc_2