When a memory's min and max are equal, it will never move

This allows us to set the `readonly` and `can_move` flags on the load of the
memory's heap base, which allows the optimizer to perform GVN- and LICM-style
deduplication and code motion on the heap base load, which ultimately results in
better code.

Author: fitzgen

crates/environ/src/types.rs

@@ -2019,6 +2019,12 @@ impl Memory {
             return false;
         }
 
+        // If its minimum and maximum are the same, then the memory will never
+        // be resized, and therefore will never move.
+        if self.limits.max.is_some_and(|max| self.limits.min == max) {
+            return false;
+        }
+
         // If the maximum size of this memory is above the threshold of the
         // initial memory reservation then the memory may move.
         let max = self.maximum_byte_size().unwrap_or(u64::MAX);

tests/disas/memory-min-max-same.wat

@@ -0,0 +1,71 @@
+;;! target = "x86_64"
+;;! test = "optimize"
+;;! flags = "-O memory-may-move=y -O memory-reservation=0"
+
+(module
+  (import "" "" (func $imp))
+
+  ;; Minimum and maximum sizes are equal. Therefore, this memory should never
+  ;; move, regardless of any other memory-related settings.
+  (memory 1 1)
+
+  ;; And therefore, the heap base should be marked read-only and can-move and
+  ;; should get code-motioned up out of the loop below, even though we call some
+  ;; foreign function in the loop body.
+  (func $f (param $base i32)
+    (local $i i32)
+
+    (local.set $i (i32.const 0))
+
+    (loop
+      ;; Call a foreign function. This would usually otherwise defeat
+      ;; code-motioning the heap base out of the loop.
+      (call $imp)
+
+      ;; Do a memory operation, which should use the heap base, but should be
+      ;; code-motioned above the loop, because its load should be marked both
+      ;; read-only and can-move.
+      (i32.store (i32.add (local.get $base) (local.get $i)) (i32.const 0))
+
+      ;; Increment `i` and continue the loop.
+      (local.set $i (i32.add (local.get $i) (i32.const 1)))
+      (br 0)
+    )
+  )
+)
+;; function u0:1(i64 vmctx, i64, i32) tail {
+;;     gv0 = vmctx
+;;     gv1 = load.i64 notrap aligned readonly gv0+8
+;;     gv2 = load.i64 notrap aligned gv1+16
+;;     gv3 = vmctx
+;;     gv4 = load.i64 notrap aligned gv3+72
+;;     gv5 = load.i64 notrap aligned readonly can_move checked gv3+64
+;;     sig0 = (i64 vmctx, i64) tail
+;;     fn0 = u0:0 sig0
+;;     stack_limit = gv2
+;;
+;;                                 block0(v0: i64, v1: i64, v2: i32):
+;; @0028                               v3 = iconst.i32 0
+;; @0030                               v6 = load.i64 notrap aligned readonly can_move v0+80
+;; @0030                               v7 = load.i64 notrap aligned readonly can_move v0+96
+;; @0039                               v13 = iconst.i64 0x0001_0000
+;; @0039                               v17 = iconst.i64 0
+;; @0039                               v15 = load.i64 notrap aligned readonly can_move checked v0+64
+;; @003e                               v19 = iconst.i32 1
+;; @002e                               jump block2(v3)  ; v3 = 0
+;;
+;;                                 block2(v9: i32):
+;; @0030                               call_indirect.i64 sig0, v6(v7, v0)
+;;                                     v22 = iconst.i32 0
+;; @0036                               v10 = iadd.i32 v2, v9
+;; @0039                               v12 = uextend.i64 v10
+;;                                     v23 = iconst.i64 0x0001_0000
+;;                                     v24 = icmp ugt v12, v23  ; v23 = 0x0001_0000
+;; @0039                               v16 = iadd.i64 v15, v12
+;;                                     v25 = iconst.i64 0
+;;                                     v26 = select_spectre_guard v24, v25, v16  ; v25 = 0
+;; @0039                               store little heap v22, v26  ; v22 = 0
+;;                                     v27 = iconst.i32 1
+;;                                     v28 = iadd v9, v27  ; v27 = 1
+;; @0043                               jump block2(v28)
+;; }