Initial reorg.

This is largely the same as #305, but updated for the current tree.

crates/environ/src/tunables.rs

@@ -0,0 +1,44 @@
+/// Tunable parameters for WebAssembly compilation.
+#[derive(Clone)]
+pub struct Tunables {
+    /// For static heaps, the size of the heap protected by bounds checking.
+    pub static_memory_bound: u32,
+
+    /// The size of the offset guard for static heaps.
+    pub static_memory_offset_guard_size: u64,
+
+    /// The size of the offset guard for dynamic heaps.
+    pub dynamic_memory_offset_guard_size: u64,
+}
+
+impl Default for Tunables {
+    fn default() -> Self {
+        Self {
+            #[cfg(target_pointer_width = "32")]
+            /// Size in wasm pages of the bound for static memories.
+            static_memory_bound: 0x4000,
+            #[cfg(target_pointer_width = "64")]
+            /// Size in wasm pages of the bound for static memories.
+            ///
+            /// When we allocate 4 GiB of address space, we can avoid the
+            /// need for explicit bounds checks.
+            static_memory_bound: 0x1_0000,
+
+            #[cfg(target_pointer_width = "32")]
+            /// Size in bytes of the offset guard for static memories.
+            static_memory_offset_guard_size: 0x1_0000,
+            #[cfg(target_pointer_width = "64")]
+            /// Size in bytes of the offset guard for static memories.
+            ///
+            /// Allocating 2 GiB of address space lets us translate wasm
+            /// offsets into x86 offsets as aggressively as we can.
+            static_memory_offset_guard_size: 0x8000_0000,
+
+            /// Size in bytes of the offset guard for dynamic memories.
+            ///
+            /// Allocate a small guard to optimize common cases but without
+            /// wasting too much memor.
+            dynamic_memory_offset_guard_size: 0x1_0000,
+        }
+    }
+}