Codegen: Align representation of stackmap with SpiderMonkey

This commit aligns the representation of stackmaps to be the same
as Spidermonkey's by:
 * Reversing the order of the bitmap from low addresses to high addresses
 * Including incoming stack arguments
 * Excluding outgoing stack arguments

Additionally, some accessor functions were added to allow Spidermonkey
to access the internals of the bitmap.

cranelift/codegen/src/ir/stackslot.rs

@@ -162,6 +162,23 @@ impl fmt::Display for StackSlotData {
     }
 }
 
+/// Stack frame layout information.
+///
+/// This is computed by the `layout_stack()` method.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
+pub struct StackLayoutInfo {
+    /// The total size of the stack frame.
+    ///
+    /// This is the distance from the stack pointer in the current function to the stack pointer in
+    /// the calling function, so it includes a pushed return address as well as space for outgoing
+    /// call arguments.
+    pub frame_size: StackSize,
+
+    /// The total size of the stack frame for inbound arguments pushed by the caller.
+    pub inbound_args_size: StackSize,
+}
+
 /// Stack frame manager.
 ///
 /// Keep track of all the stack slots used by a function.
@@ -177,14 +194,8 @@ pub struct StackSlots {
     /// All the emergency slots.
     emergency: Vec<StackSlot>,
 
-    /// The total size of the stack frame.
-    ///
-    /// This is the distance from the stack pointer in the current function to the stack pointer in
-    /// the calling function, so it includes a pushed return address as well as space for outgoing
-    /// call arguments.
-    ///
-    /// This is computed by the `layout()` method.
-    pub frame_size: Option<StackSize>,
+    /// Layout information computed from `layout_stack`.
+    pub layout_info: Option<StackLayoutInfo>,
 }
 
 /// Stack slot manager functions that behave mostly like an entity map.
@@ -195,7 +206,7 @@ impl StackSlots {
             slots: PrimaryMap::new(),
             outgoing: Vec::new(),
             emergency: Vec::new(),
-            frame_size: None,
+            layout_info: None,
         }
     }
 
@@ -204,7 +215,7 @@ impl StackSlots {
         self.slots.clear();
         self.outgoing.clear();
         self.emergency.clear();
-        self.frame_size = None;
+        self.layout_info = None;
     }
 
     /// Allocate a new stack slot.