VM: possible fix for #1319.

Fix for reset_context() so that the top two stack items are never
removed from the stack so if the parent->init_context(ctx) call triggers
a GC, those items are never collected.

basis/bootstrap/assembler/x86.32.factor

@@ -353,14 +353,22 @@ IN: bootstrap.x86
 
 : jit-start-context-and-delete ( -- )
     jit-load-vm
-    jit-load-context
+
+    ! Updates the context to match the values in the data and retain
+    ! stack registers. reset_context can GC.
+    jit-save-context
+
+    ! Resets the context. The top two ds item are preserved.
     vm-reg "reset_context" jit-call-1arg
 
-    jit-save-quot-and-param
+    ! Switches to the same context I think, uses ctx-reg
     ctx-reg jit-switch-context
-    jit-push-param
 
-    EAX EDX [] MOV
+    ! Pops the quotation from the stack and puts it in EAX.
+    EAX ds-reg [] MOV
+    ds-reg 4 SUB
+
+    ! Jump to the quotation in EAX.
     jit-jump-quot ;
 
 [

basis/bootstrap/assembler/x86.64.factor

@@ -78,7 +78,8 @@ IN: bootstrap.x86
     jit-restore-context
 ] jit-primitive jit-define
 
-: jit-jump-quot ( -- ) arg1 quot-entry-point-offset [+] JMP ;
+: jit-jump-quot ( -- )
+    arg1 quot-entry-point-offset [+] JMP ;
 
 : jit-call-quot ( -- ) arg1 quot-entry-point-offset [+] CALL ;
 
@@ -306,13 +307,24 @@ IN: bootstrap.x86
     jit-set-context
 ] \ (set-context-and-delete) define-sub-primitive
 
+! Resets the active context and instead the passed in quotation
+! becomes the new code that it executes.
 : jit-start-context-and-delete ( -- )
+    ! Updates the context to match the values in the data and retain
+    ! stack registers. reset_context can GC.
+    jit-save-context
 
-    jit-load-context
+    ! Resets the context. The top two ds items are preserved.
     vm-reg "reset_context" jit-call-1arg
-    jit-pop-quot-and-param
+
+    ! Switches to the same context I think.
     ctx-reg jit-switch-context
-    jit-push-param
+
+    ! Pops the quotation from the stack and puts it in arg1.
+    arg1 ds-reg [] MOV
+    ds-reg 8 SUB
+
+    ! Jump to quotation arg1
     jit-jump-quot ;
 
 [

vm/contexts.cpp

@@ -123,8 +123,18 @@ VM_C_API void delete_context(factor_vm* parent) {
 
 /* Allocates memory (init_context()) */
 VM_C_API void reset_context(factor_vm* parent) {
-  parent->ctx->reset();
-  parent->init_context(parent->ctx);
+
+  // The function is used by (start-context-and-delete) which expects
+  // the top two datastack items to be preserved after the context has
+  // been resetted.
+
+  context* ctx = parent->ctx;
+  cell arg1 = ctx->pop();
+  cell arg2 = ctx->pop();
+  ctx->reset();
+  ctx->push(arg2);
+  ctx->push(arg1);
+  parent->init_context(ctx);
 }
 
 /* Allocates memory */