System: Refactor main loop

Reduces JIT exits.
Improves runahead performance.

src/common/fastjmp.h

@@ -0,0 +1,33 @@
+// SPDX-FileCopyrightText: 2021 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: (GPL-3.0 OR CC-BY-NC-ND-4.0)
+
+#pragma once
+
+#include "types.h"
+
+#include <cstddef>
+#include <cstdint>
+
+struct fastjmp_buf
+{
+#if defined(_WIN32) && defined(_M_AMD64)
+  static constexpr std::size_t BUF_SIZE = 240;
+#elif defined(_M_ARM64) || defined(__aarch64__)
+  static constexpr std::size_t BUF_SIZE = 168;
+#elif defined(__x86_64__)
+  static constexpr std::size_t BUF_SIZE = 64;
+#elif defined(_M_IX86) || defined(__i386__)
+  static constexpr std::size_t BUF_SIZE = 24;
+#elif defined(__riscv) && __riscv_xlen == 64
+  static constexpr std::size_t BUF_SIZE = 208;
+#else
+#error Unknown architecture.
+#endif
+
+  alignas(16) std::uint8_t buf[BUF_SIZE];
+};
+
+extern "C" {
+int fastjmp_set(fastjmp_buf* buf);
+[[noreturn]] void fastjmp_jmp(const fastjmp_buf* buf, int ret);
+}