LibJS/JIT: Add fast path for decrement

Generate jitted code for the decrement opcode.
lanmonster · Nov 8, 2023 · 13726fd · 13726fd
1 parent 0e69f74
commit 13726fd
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diff --git a/Userland/Libraries/LibJIT/X86_64/Assembler.h b/Userland/Libraries/LibJIT/X86_64/Assembler.h
@@ -757,6 +757,21 @@ struct X86_64Assembler {
  }
  }
 
+ void dec32(Operand op, Optional<Label&> overflow_label)
+ {
+ if (op.is_register_or_memory()) {
+ emit_rex_for_slash(op, REX_W::No);
+ emit8(0xff);
+ emit_modrm_slash(1, op);
+ } else {
+ VERIFY_NOT_REACHED();
+ }
+
+ if (overflow_label.has_value()) {
+ jump_if(Condition::Overflow, *overflow_label);
+ }
+ }
+
  void add(Operand dst, Operand src)
  {
  if (dst.is_register_or_memory() && src.type == Operand::Type::Reg) {

diff --git a/Userland/Libraries/LibJS/JIT/Compiler.cpp b/Userland/Libraries/LibJS/JIT/Compiler.cpp
@@ -396,9 +396,41 @@ static Value cxx_decrement(VM& vm, Value value)
 void Compiler::compile_decrement(Bytecode::Op::Decrement const&)
 {
  load_accumulator(ARG1);
+
+ Assembler::Label end {};
+ Assembler::Label slow_case {};
+
+ branch_if_int32(ARG1, [&] {
+ // GPR0 = ARG1;
+ m_assembler.mov(
+ Assembler::Operand::Register(GPR0),
+ Assembler::Operand::Register(ARG1));
+
+ // GPR0--;
+ m_assembler.dec32(
+ Assembler::Operand::Register(GPR0),
+ slow_case);
+
+ // accumulator = GPR0 | SHIFTED_INT32_TAG;
+ m_assembler.mov(
+ Assembler::Operand::Register(GPR1),
+ Assembler::Operand::Imm(SHIFTED_INT32_TAG));
+ m_assembler.bitwise_or(
+ Assembler::Operand::Register(GPR0),
+ Assembler::Operand::Register(GPR1));
+
+ // accumulator = GPR0;
+ store_accumulator(GPR0);
+
+ m_assembler.jump(end);
+ });
+
+ slow_case.link(m_assembler);
  native_call((void*)cxx_decrement);
  store_accumulator(RET);
  check_exception();
+
+ end.link(m_assembler);
 }
 
 void Compiler::check_exception()