Add graphviz output

This adds a method to blocks to get outgoing ids, then uses the outgoing
ids to generate a graphviz graph.  Two methods were added to the Block
object.  One method returns an id for the block, which is just the
address of the underlying block.  The other method returns a list of
outgoing block ids.  We can use Block#id in conjunction with
Block#outgoing_ids to construct a graph of blocks

yjit.rb

@@ -55,6 +55,43 @@ module YJIT
     def self.comments_for(start_address, end_address)
       Primitive.comments_for(start_address, end_address)
     end
+
+    def self.graphviz_for(iseq)
+      iseq = RubyVM::InstructionSequence.of(iseq)
+      buff = ''
+      blocks = blocks_for(iseq)
+      buff << "digraph g {"
+      buff << "  node [shape=record];"
+      blocks.each do |block|
+        buff << "b#{block.id} [label=\"#{disasm_block(block).gsub(/\n/, "\\l\n")}\"];"
+        block.outgoing_ids.each do |id|
+          buff << "b#{block.id} -> b#{id};"
+        end
+      end
+      buff << "}"
+      buff
+    end
+
+    def self.disasm_block(block)
+      cs = YJIT::Disasm.new
+      comments = comments_for(block.address, block.address + block.code.length)
+      comment_idx = 0
+      str = ''
+      cs.disasm(block.code, block.address).each do |i|
+        while (comment = comments[comment_idx]) && comment.address <= i.address
+          str << "  ; #{comment.comment}\n"
+          comment_idx += 1
+        end
+
+        str << sprintf(
+          "  %<address>08x:  %<instruction>s\t%<details>s\n",
+          address: i.address,
+          instruction: i.mnemonic,
+          details: i.op_str
+        )
+      end
+      str
+    end
   end
 
   # Return a hash for statistics generated for the --yjit-stats command line option.

yjit_iface.c

@@ -1025,6 +1025,55 @@ rb_yjit_call_threshold(void)
     return rb_yjit_opts.call_threshold;
 }
 
+# define PTR2NUM(x)   (LONG2NUM((long)(x)))
+
+/**
+ *  call-seq: block.id -> unique_id
+ *
+ *  Returns a unique integer ID for the block.  For example:
+ *
+ *      blocks = blocks_for(iseq)
+ *      blocks.group_by(&:id)
+ */
+static VALUE
+block_id(VALUE self)
+{
+    block_t * block;
+    TypedData_Get_Struct(self, block_t, &yjit_block_type, block);
+    return PTR2NUM(block);
+}
+
+/**
+ *  call-seq: block.outgoing_ids -> list
+ *
+ *  Returns a list of outgoing ids for the current block.  This list can be used
+ *  in conjunction with Block#id to construct a graph of block objects.
+ */
+static VALUE
+outgoing_ids(VALUE self)
+{
+    block_t * block;
+    TypedData_Get_Struct(self, block_t, &yjit_block_type, block);
+
+    VALUE ids = rb_ary_new();
+
+    rb_darray_for(block->outgoing, branch_idx) {
+        branch_t* out_branch = rb_darray_get(block->outgoing, branch_idx);
+
+        for (size_t succ_idx = 0; succ_idx < 2; succ_idx++) {
+            block_t* succ = out_branch->blocks[succ_idx];
+
+            if (succ == NULL)
+                continue;
+
+            rb_ary_push(ids, PTR2NUM(succ));
+        }
+
+    }
+
+    return ids;
+}
+
 // Can raise RuntimeError
 void
 rb_yjit_init(struct rb_yjit_options *options)
@@ -1069,9 +1118,11 @@ rb_yjit_init(struct rb_yjit_options *options)
     // YJIT::Block (block version, code block)
     cYjitBlock = rb_define_class_under(mYjit, "Block", rb_cObject);
     rb_define_method(cYjitBlock, "address", block_address, 0);
+    rb_define_method(cYjitBlock, "id", block_id, 0);
     rb_define_method(cYjitBlock, "code", block_code, 0);
     rb_define_method(cYjitBlock, "iseq_start_index", iseq_start_index, 0);
     rb_define_method(cYjitBlock, "iseq_end_index", iseq_end_index, 0);
+    rb_define_method(cYjitBlock, "outgoing_ids", outgoing_ids, 0);
 
     // YJIT disassembler interface
 #if HAVE_LIBCAPSTONE