Split LHS of CMP if it's an immediate (Shopify/zjit#94)

Author: k0kubun

test/ruby/test_zjit.rb

@@ -147,6 +147,14 @@ def test(a, b) = a < b
     }, call_threshold: 2
   end
 
+  def test_opt_lt_with_literal_lhs
+    assert_compiles '[false, false, true]', %q{
+      def test(n) = 2 < n
+      test(2) # profile opt_lt
+      [test(1), test(2), test(3)]
+    }, call_threshold: 2
+  end
+
   def test_opt_le
     assert_compiles '[true, true, false]', %q{
       def test(a, b) = a <= b

zjit/src/backend/x86_64/mod.rs

@@ -202,9 +202,17 @@ impl Assembler
                             asm.push_insn(Insn::Test { left: *left, right: *left });
                         }
                         _ => {
-                            if let (Opnd::Mem(_), Opnd::Mem(_)) = (&left, &right) {
-                                let loaded = asm.load(*right);
-                                *right = loaded;
+                            // Split the instruction if `cmp` can't be encoded with given operands
+                            match (&left, &right) {
+                                // One of the operands should not be a memory operand
+                                (Opnd::Mem(_), Opnd::Mem(_)) => {
+                                    *right = asm.load(*right);
+                                }
+                                // The left operand needs to be either a register or a memory operand
+                                (Opnd::UImm(_) | Opnd::Imm(_), _) => {
+                                    *left = asm.load(*left);
+                                }
+                                _ => {},
                             }
                             asm.push_insn(insn);
                         }