compile.c: statically resolve swap instructions when possible

If the two preceeding instructions are known not to be dependent
on the state of the stack, then we can eliminate the `swap` and inverse
the two previous instructions.

Co-Authored-By: John Hawthorn <john@hawthorn.email>

Author: byroot

compile.c

@@ -359,6 +359,26 @@ static void iseq_add_setlocal(rb_iseq_t *iseq, LINK_ANCHOR *const seq, const NOD
 #define IS_NEXT_INSN_ID(link, insn) \
     ((link)->next && IS_INSN((link)->next) && IS_INSN_ID((link)->next, insn))
 
+static inline bool
+IS_INDEPENDENT_INSN(LINK_ELEMENT *link)
+{
+    if (!IS_INSN(link)) {
+        return false;
+    }
+
+    enum ruby_vminsn_type type = INSN_OF(link);
+
+    return (
+        type == BIN(putobject) ||
+        type == BIN(putspecialobject) ||
+        type == BIN(putnil) ||
+        type == BIN(putself) ||
+        type == BIN(duphash) ||
+        type == BIN(getinstancevariable) ||
+        type == BIN(getlocal)
+    );
+}
+
 /* error */
 #if CPDEBUG > 0
 RBIMPL_ATTR_NORETURN()
@@ -1225,6 +1245,25 @@ ELEM_REMOVE(LINK_ELEMENT *elem)
     }
 }
 
+/*
+ * elem1, elem2 => elem2, elem1
+ */
+static void
+ELEM_SWAP(LINK_ELEMENT *first, LINK_ELEMENT *second)
+{
+    RUBY_ASSERT(first->next == second);
+    RUBY_ASSERT(first == second->prev);
+
+    first->prev->next = second;
+    second->next->prev = first;
+
+    first->next = second->next;
+    second->next = first;
+
+    second->prev = first->prev;
+    first->prev = second;
+}
+
 static LINK_ELEMENT *
 FIRST_ELEMENT(const LINK_ANCHOR *const anchor)
 {
@@ -4253,6 +4292,24 @@ iseq_peephole_optimize(rb_iseq_t *iseq, LINK_ELEMENT *list, const int do_tailcal
         }
     }
 
+    if (iobj->link.next && iobj->link.next->next && IS_NEXT_INSN_ID(iobj->link.next, swap)) {
+        LINK_ELEMENT *first = &iobj->link;
+        LINK_ELEMENT *second = first->next;
+        LINK_ELEMENT *swap = second->next;
+        /*
+         *  putself / (or any other independent instruction)
+         *  putnil  / (or any other independent instruction)
+         *  swap
+         * =>
+         *  putnil  / (or any other independent instruction)
+         *  putself / (or any other independent instruction)
+         */
+        if (IS_INSN_ID(swap, swap) && IS_INDEPENDENT_INSN(first) && IS_INDEPENDENT_INSN(second)) {
+            ELEM_REMOVE(swap);
+             ELEM_SWAP(first, second);
+        }
+    }
+
     return COMPILE_OK;
 }