https://git.ruby-lang.org/ruby.git/commit/?id=9043ad3d74

From 9043ad3d74413823e62b11923652d272e41dc230 Mon Sep 17 00:00:00 2001
From: Aaron Patterson <tenderlove@r...>
Date: Thu, 10 Jun 2021 09:40:30 -0700
Subject: Implement topn instruction

This commit implements the topn instruction

Co-Authored-By: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@s...>
Co-Authored-By: Noah Gibbs <noah.gibbs@s...>
---
 bootstraptest/test_yjit.rb | 18 ++++++++++++++++++
 yjit_codegen.c             | 18 ++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/bootstraptest/test_yjit.rb b/bootstraptest/test_yjit.rb
index 51c82b55b5..fcf027d728 100644
--- a/bootstraptest/test_yjit.rb
+++ b/bootstraptest/test_yjit.rb
@@ -1,3 +1,21 @@ https://github.com/ruby/ruby/blob/trunk/bootstraptest/test_yjit.rb#L1
+# Test for topn
+assert_equal 'array', %q{
+  def threequals(a)
+    case a
+    when Array
+      "array"
+    when Hash
+      "hash"
+    else
+      "unknown"
+    end
+  end
+
+  threequals([])
+  threequals([])
+  threequals([])
+}
+
 # Test for opt_mod
 assert_equal '2', %q{
   def mod(a, b)
diff --git a/yjit_codegen.c b/yjit_codegen.c
index dc148c1951..487c1cee98 100644
--- a/yjit_codegen.c
+++ b/yjit_codegen.c
@@ -583,6 +583,23 @@ gen_setn(jitstate_t* jit, ctx_t* ctx) https://github.com/ruby/ruby/blob/trunk/yjit_codegen.c#L583
     return YJIT_KEEP_COMPILING;
 }
 
+// get nth stack value, then push it
+static codegen_status_t
+gen_topn(jitstate_t* jit, ctx_t* ctx)
+{
+    int32_t n = (int32_t)jit_get_arg(jit, 0);
+
+    // Get top n type / operand
+    val_type_t top_n_type = ctx_get_opnd_type(ctx, OPND_STACK(n));
+    x86opnd_t top_n_val = ctx_stack_opnd(ctx, n);
+
+    x86opnd_t loc0 = ctx_stack_push(ctx, top_n_type);
+    mov(cb, REG0, top_n_val);
+    mov(cb, loc0, REG0);
+
+    return YJIT_KEEP_COMPILING;
+}
+
 static codegen_status_t
 gen_pop(jitstate_t* jit, ctx_t* ctx)
 {
@@ -3191,6 +3208,7 @@ yjit_init_codegen(void) https://github.com/ruby/ruby/blob/trunk/yjit_codegen.c#L3208
     yjit_reg_op(BIN(dup), gen_dup);
     yjit_reg_op(BIN(dupn), gen_dupn);
     yjit_reg_op(BIN(setn), gen_setn);
+    yjit_reg_op(BIN(topn), gen_topn);
     yjit_reg_op(BIN(pop), gen_pop);
     yjit_reg_op(BIN(adjuststack), gen_adjuststack);
     yjit_reg_op(BIN(newarray), gen_newarray);
-- 
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