https://git.ruby-lang.org/ruby.git/commit/?id=fd34c831f6

From fd34c831f6f61691cb9f2d171c690c9769183437 Mon Sep 17 00:00:00 2001
From: John Hawthorn <john@h...>
Date: Sat, 4 Sep 2021 01:50:25 -0700
Subject: Allow special case of expandarray with nil

---
 test/ruby/test_yjit.rb | 13 +++++++++++++
 yjit_codegen.c         | 11 +++++++++++
 2 files changed, 24 insertions(+)

diff --git a/test/ruby/test_yjit.rb b/test/ruby/test_yjit.rb
index 94989dae9e..ccd0e6bae0 100644
--- a/test/ruby/test_yjit.rb
+++ b/test/ruby/test_yjit.rb
@@ -136,6 +136,19 @@ class TestYJIT < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_yjit.rb#L136
     RUBY
   end
 
+  def test_expandarray
+    assert_compiles(<<~'RUBY', insns: %i[expandarray], result: [1, 2])
+      a, b = [1, 2]
+    RUBY
+  end
+
+  def test_expandarray_nil
+    assert_compiles(<<~'RUBY', insns: %i[expandarray], result: [nil, nil])
+      a, b = nil
+      [a, b]
+    RUBY
+  end
+
   def test_compile_opt_getinlinecache
     assert_compiles(<<~RUBY, insns: %i[opt_getinlinecache], result: 123, min_calls: 2)
       def get_foo
diff --git a/yjit_codegen.c b/yjit_codegen.c
index f63166725a..9eff50b2e4 100644
--- a/yjit_codegen.c
+++ b/yjit_codegen.c
@@ -987,8 +987,19 @@ gen_expandarray(jitstate_t* jit, ctx_t* ctx) https://github.com/ruby/ruby/blob/trunk/yjit_codegen.c#L987
     // num is the number of requested values. If there aren't enough in the
     // array then we're going to push on nils.
     rb_num_t num = (rb_num_t) jit_get_arg(jit, 0);
+    val_type_t array_type = ctx_get_opnd_type(ctx, OPND_STACK(0));
     x86opnd_t array_opnd = ctx_stack_pop(ctx, 1);
 
+    if (array_type.type == ETYPE_NIL) {
+        // special case for a, b = nil pattern
+        // push N nils onto the stack
+        for (int i = 0; i < num; i++) {
+            x86opnd_t push = ctx_stack_push(ctx, TYPE_NIL);
+            mov(cb, push, imm_opnd(Qnil));
+        }
+        return YJIT_KEEP_COMPILING;
+    }
+
     // Move the array from the stack into REG0 and check that it's an array.
     mov(cb, REG0, array_opnd);
     guard_object_is_heap(cb, REG0, ctx, COUNTED_EXIT(side_exit, expandarray_not_array));
-- 
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