YJIT: Generate Block::entry_exit with block entry PC

Previously, when Block::entry_exit is requested from any instruction
that is not the first one in the block, we generated the exit with an
incorrect PC. We should always be using the PC for the entry of the
block for Block::entry_exit.

It was a simple typo. The bug was [introduced][1] while we were
refactoring to use the current backend. Later, we had a chance to spot
this issue while [preparing][2] to enable unused variable warnings, but
didn't spot the issue.

Fixes [Bug #19463]

[1]: 27fcab9
[2]: 31461c7

test/ruby/test_yjit.rb

@@ -1155,6 +1155,47 @@ def test(str)
     RUBY
   end
 
+  def test_return_to_invalidated_block
+    # [Bug #19463]
+    assert_compiles(<<~RUBY, result: [1, 1, :ugokanai])
+      klass = Class.new do
+        def self.lookup(hash, key) = hash[key]
+
+        def self.foo(a, b) = []
+
+        def self.test(hash, key)
+          [lookup(hash, key), key, "".freeze]
+          # 05 opt_send_without_block :lookup
+          # 07 getlocal_WC_0          :hash
+          # 09 opt_str_freeze         ""
+          # 12 newarray               3
+          # 14 leave
+          #
+          # YJIT will put instructions (07..14) into a block.
+          # When String#freeze is redefined from within lookup(),
+          # the return address to the block is still on-stack. We rely
+          # on invalidation patching the code at the return address
+          # to service this situation correctly.
+        end
+      end
+
+      # get YJIT to compile test()
+      hash = { 1 => [] }
+      31.times { klass.test(hash, 1) }
+
+      # inject invalidation into lookup()
+      evil_hash = Hash.new do |_, key|
+        class String
+          undef :freeze
+          def freeze = :ugokanai
+        end
+
+        key
+      end
+      klass.test(evil_hash, 1)
+    RUBY
+  end
+
   private
 
   def code_gc_helpers

yjit/src/codegen.rs

@@ -489,8 +489,8 @@ pub fn jit_ensure_block_entry_exit(jit: &mut JITState, ocb: &mut OutlinedCb) {
         // Generate the exit with the cache in jitstate.
         block.entry_exit = Some(get_side_exit(jit, ocb, &block_ctx).unwrap_code_ptr());
     } else {
-        let _pc = unsafe { rb_iseq_pc_at_idx(blockid.iseq, blockid.idx) };
-        block.entry_exit = Some(gen_outlined_exit(jit.pc, &block_ctx, ocb));
+        let block_entry_pc = unsafe { rb_iseq_pc_at_idx(blockid.iseq, blockid.idx) };
+        block.entry_exit = Some(gen_outlined_exit(block_entry_pc, &block_ctx, ocb));
     }
 }