Add a readonly flag for loads (#562)

* Add readonly MemFlag

* Add readonly flag verifier check

* Make global loads readonly

* Fix gvn to consider readonly loads

cranelift/docs/ir.rst

@@ -502,12 +502,13 @@ Memory operation flags
 Loads and stores can have flags that loosen their semantics in order to enable
 optimizations.
 
-======= ===========================================
-Flag    Description
-======= ===========================================
-notrap  Memory is assumed to be :term:`accessible`.
-aligned Trapping allowed for misaligned accesses.
-======= ===========================================
+=======  ===========================================
+Flag     Description
+=======  ===========================================
+notrap   Memory is assumed to be :term:`accessible`.
+aligned  Trapping allowed for misaligned accesses.
+readonly The data at the specified address will not modified between when this function is called and exited.
+=======  ===========================================
 
 When the ``accessible`` flag is set, the behavior is undefined if the memory
 is not :term:`accessible`.

cranelift/filetests/simple_gvn/readonly.clif

@@ -0,0 +1,25 @@
+test simple-gvn
+
+target x86_64
+
+function %eliminate_redundant_global_loads(i32, i64 vmctx) {
+    gv0 = vmctx
+    gv1 = load.i64 notrap aligned readonly gv0
+    heap0 = static gv1, min 0x1_0000, bound 0x1_0000_0000, guard 0x8000_0000, index_type i32
+
+ebb0(v0: i32, v1: i64):
+    v2 = heap_addr.i64 heap0, v0, 1
+    v3 = heap_addr.i64 heap0, v0, 1
+
+    v4 = iconst.i32 0
+    store.i32 notrap aligned v4, v2
+    store.i32 notrap aligned v4, v3
+
+    return
+}
+; check: v2 = heap_addr.i64 heap0, v0, 1
+; check: v3 -> v2
+; check: v4 = iconst.i32 0
+; check: store notrap aligned v4, v2
+; check: store notrap aligned v4, v2
+; check: return