Remove the MemoryStyle enum

crates/cranelift/src/translate/heap.rs

@@ -35,29 +35,6 @@ entity_impl!(Heap, "heap");
 ///    to always cause a trap when accessed. It is used to optimize bounds
 ///    checking for heap accesses with a shared base pointer. They are
 ///    addressable but not accessible.
-///
-/// The *heap bound* is the total size of the mapped and unmapped pages. This is
-/// the bound that `heap_addr` checks against. Memory accesses inside the heap
-/// bounds can trap if they hit an unmapped page (which is not accessible).
-///
-/// Two styles of heaps are supported, *static* and *dynamic*. They behave
-/// differently when resized.
-///
-/// #### Static heaps
-///
-/// A *static heap* starts out with all the address space it will ever need, so
-/// it never moves to a different address. At the base address is a number of
-/// mapped pages corresponding to the heap's current size. Then follows a number
-/// of unmapped pages where the heap can grow up to its maximum size. After the
-/// unmapped pages follow the offset-guard pages which are also guaranteed to
-/// generate a trap when accessed.
-///
-/// #### Dynamic heaps
-///
-/// A *dynamic heap* can be relocated to a different base address when it is
-/// resized, and its bound can move dynamically. The offset-guard pages move
-/// when the heap is resized. The bound of a dynamic heap is stored in a global
-/// value.
 #[derive(Clone, PartialEq, Hash)]
 pub struct HeapData {
     /// The address of the start of the heap's storage.

crates/environ/src/module.rs

@@ -7,66 +7,6 @@ use core::ops::Range;
 use cranelift_entity::{packed_option::ReservedValue, EntityRef};
 use serde_derive::{Deserialize, Serialize};
 
-/// Implementation styles for WebAssembly linear memory.
-#[derive(Debug, Clone, Hash, Serialize, Deserialize)]
-pub enum MemoryStyle {
-    /// The actual memory can be resized and moved.
-    Dynamic {
-        /// Extra space to reserve when a memory must be moved due to growth.
-        reserve: u64,
-    },
-    /// Address space is allocated up front.
-    Static {
-        /// The number of bytes which are reserved for this linear memory. Only
-        /// the lower bytes which represent the actual linear memory need be
-        /// mapped, but other bytes must be guaranteed to be unmapped.
-        byte_reservation: u64,
-    },
-}
-
-impl MemoryStyle {
-    /// Decide on an implementation style for the given `Memory`.
-    pub fn for_memory(memory: Memory, tunables: &Tunables) -> Self {
-        let is_static =
-            // Ideally we would compare against (an upper bound on) the target's
-            // page size, but unfortunately that is a little hard to plumb
-            // through here.
-            memory.page_size_log2 >= Memory::DEFAULT_PAGE_SIZE_LOG2
-            && tunables.signals_based_traps
-            && match memory.maximum_byte_size() {
-                Ok(mut maximum) => {
-                    if !tunables.memory_may_move {
-                        maximum = maximum.min(tunables.memory_reservation);
-                    }
-
-                    // Ensure the minimum is less than the maximum; the minimum
-                    // might exceed the maximum when the memory is artificially
-                    // bounded via `memory_may_move` above
-                    memory.minimum_byte_size().unwrap() <= maximum
-                        && maximum <= tunables.memory_reservation
-                }
-
-                // If the maximum size of this memory is not representable with
-                // `u64` then use the `memory_may_move` to indicate whether
-                // it's a static memory or not. It should be ok to discard the
-                // linear memory's maximum size here as growth to the maximum
-                // is always fallible and never guaranteed.
-                Err(_) => !tunables.memory_may_move,
-            };
-
-        if is_static {
-            return Self::Static {
-                byte_reservation: tunables.memory_reservation,
-            };
-        }
-
-        // Otherwise, make it dynamic.
-        Self::Dynamic {
-            reserve: tunables.memory_reservation_for_growth,
-        }
-    }
-}
-
 /// A WebAssembly linear memory initializer.
 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct MemoryInitializer {

crates/wasmtime/src/runtime/memory.rs

@@ -1048,16 +1048,9 @@ mod tests {
         let ty = MemoryType::new(1, None);
         let mem = Memory::new(&mut store, ty).unwrap();
         let store = store.as_context();
-        let style = wasmtime_environ::MemoryStyle::for_memory(
-            store[mem.0].memory,
-            store.engine().tunables(),
-        );
-
-        assert_eq!(store.engine().tunables().memory_guard_size, 0);
-        match style {
-            wasmtime_environ::MemoryStyle::Dynamic { .. } => {}
-            other => panic!("unexpected style {other:?}"),
-        }
+        let tunables = store.engine().tunables();
+        assert_eq!(tunables.memory_guard_size, 0);
+        assert!(!store[mem.0].memory.can_elide_bounds_check(tunables, 12));
     }
 
     #[test]

crates/wasmtime/src/runtime/trampoline/memory.rs

@@ -11,8 +11,7 @@ use crate::store::{InstanceId, StoreOpaque};
 use crate::MemoryType;
 use alloc::sync::Arc;
 use wasmtime_environ::{
-    DefinedMemoryIndex, DefinedTableIndex, EntityIndex, HostPtr, MemoryStyle, Module, Tunables,
-    VMOffsets,
+    DefinedMemoryIndex, DefinedTableIndex, EntityIndex, HostPtr, Module, Tunables, VMOffsets,
 };
 
 #[cfg(feature = "component-model")]
@@ -106,13 +105,7 @@ impl RuntimeMemoryCreator for MemoryCreatorProxy {
         minimum: usize,
         maximum: Option<usize>,
     ) -> Result<Box<dyn RuntimeLinearMemory>> {
-        let style = MemoryStyle::for_memory(*ty, tunables);
-        let reserved_size_in_bytes = match style {
-            MemoryStyle::Static { byte_reservation } => {
-                Some(usize::try_from(byte_reservation).unwrap())
-            }
-            MemoryStyle::Dynamic { .. } => None,
-        };
+        let reserved_size_in_bytes = Some(tunables.memory_reservation.try_into().unwrap());
         self.0
             .new_memory(
                 MemoryType::from_wasmtime_memory(ty),

crates/wasmtime/src/runtime/vm/cow.rs

@@ -9,15 +9,14 @@ use super::sys::DecommitBehavior;
 use crate::prelude::*;
 use crate::runtime::vm::sys::vm::{self, MemoryImageSource};
 use crate::runtime::vm::{
-    round_usize_up_to_host_pages, usize_is_multiple_of_host_page_size, MmapVec, SendSyncPtr,
+    host_page_size, round_usize_up_to_host_pages, usize_is_multiple_of_host_page_size, MmapVec,
+    SendSyncPtr,
 };
 use alloc::sync::Arc;
 use core::ffi::c_void;
 use core::ops::Range;
 use core::ptr::{self, NonNull};
-use wasmtime_environ::{
-    DefinedMemoryIndex, MemoryInitialization, MemoryStyle, Module, PrimaryMap, Tunables,
-};
+use wasmtime_environ::{DefinedMemoryIndex, MemoryInitialization, Module, PrimaryMap, Tunables};
 
 /// Backing images for memories in a module.
 ///
@@ -430,13 +429,14 @@ impl MemoryImageSlot {
         }
 
         // If (1) the accessible region is not in its initial state, and (2) the
-        // memory relies on virtual memory at all (i.e. has offset guard pages
-        // and/or is static), then we need to reset memory protections. Put
-        // another way, the only time it is safe to not reset protections is
-        // when we are using dynamic memory without any guard pages.
-        let style = MemoryStyle::for_memory(*ty, tunables);
+        // memory relies on virtual memory at all (i.e. has offset guard
+        // pages), then we need to reset memory protections. Put another way,
+        // the only time it is safe to not reset protections is when we are
+        // using dynamic memory without any guard pages.
+        let host_page_size_log2 = u8::try_from(host_page_size().ilog2()).unwrap();
         if initial_size_bytes_page_aligned < self.accessible
-            && (tunables.memory_guard_size > 0 || matches!(style, MemoryStyle::Static { .. }))
+            && (tunables.memory_guard_size > 0
+                || ty.can_elide_bounds_check(tunables, host_page_size_log2))
         {
             self.set_protection(initial_size_bytes_page_aligned..self.accessible, false)?;
             self.accessible = initial_size_bytes_page_aligned;

crates/wasmtime/src/runtime/vm/instance/allocator/pooling/memory_pool.rs

@@ -547,11 +547,11 @@ impl SlabConstraints {
         // static memory slot (see `Config::memory_reservation`); even
         // if the memory never grows to this size (e.g., it has a lower memory
         // maximum), codegen will assume that this unused memory is mapped
-        // `PROT_NONE`. Typically `static_memory_bound` is 4GiB which helps
+        // `PROT_NONE`. Typically `memory_reservation` is 4GiB which helps
         // elide most bounds checks. `MemoryPool` must respect this bound,
         // though not explicitly: if we can achieve the same effect via
         // MPK-protected stripes, the slot size can be lower than the
-        // `static_memory_bound`.
+        // `memory_reservation`.
         let expected_slot_bytes: usize = tunables
             .memory_reservation
             .try_into()

tests/all/memory_creator.rs

@@ -103,7 +103,7 @@ mod not_for_windows {
             guard_size: usize,
         ) -> Result<Box<dyn LinearMemory>, String> {
             assert_eq!(guard_size, 0);
-            assert!(reserved_size.is_none());
+            assert_eq!(reserved_size, Some(0));
             assert!(!ty.is_64());
             unsafe {
                 let mem = Box::new(CustomMemory::new(