Add tunables.rs.

lib/environ/src/tunables.rs

@@ -0,0 +1,27 @@
+/// 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.
+    pub offset_guard_size: u64,
+}
+
+impl Default for Tunables {
+    fn default() -> Self {
+        Self {
+            /// 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,
+
+            /// Size in bytes of the offset guard.
+            ///
+            /// Allocating 2 GiB of address space lets us translate wasm
+            /// offsets into x86 offsets as aggressively as we can.
+            offset_guard_size: 0x8000_0000,
+        }
+    }
+}