Mono的Google Native Client(NaCl)技术支持

Native Client是Google在浏览器领域推出的一个开源技术，它允许在浏览器内编译Web应用程序，并执行原生的编译好的代码。Native Client有以下几个优势（参考Google官方英文介绍）：

• 为Web提供更多的图形，音频以及其他功能：可以直接在web上执行了原生的2D，3D图形渲染程序（对Web游戏很有用），播放音视频，响应鼠标键盘事件，多线程执行代码等等，而这一切，不需要浏览器安装任何插件。
• 良好的可移植性：一个Web程序，只需要开发一份代码，即可以在所有平台（包括Windows，linux，Mac等）运行。
• 高安全性：安装不被信任的桌面程序一级浏览器插件，可能带来很高的安全风险（如程序携带木马，病毒）。而Native client使用了双层沙盒（sandbox）设计来保护用户的本地资源。Native Client的架构可以保证web要应用的安全性，并且取得和原声代码相同或相近的性能。
• 方便从桌面迁移：很多开发厂商之前花了大力气开发桌面程序，随着云计算的到来，越来越多的程序会被移植到互联网上，由于NaCl支持直接执行C/C++/Java等代码，Native Client技术可以简化移植过程，减少移植成本。
• 高性能：Native Client可以让web应用已接近桌面程序的性能运行，这就为在浏览器内运行性能苛刻的程序提供了基础，如大型3D游戏。

参照这两篇文章：

漫谈Google的Native Client技术（一）–历史动力篇（Web本地计算发展史）

漫谈Google的Native Client(NaCl)技术（二）–技术篇（兼谈LLVM）

Mono从2.10开始支持NaCI技术（开源Mono框架将C#编程带到iPhone、Android和Wii），Unity 3.5也通过Mono支持NaCI ，Getting Started with Native Client Development。经过近2年的开发，昨天的Mono版本代码中签入了一个重要的NaCI更新，https://github.com/mono/mono/pull/538 ：

• Restore nacl directory for this fork
• Add back temporary __nacl_suspend_thread_if_needed to this fork
• Set up for building on TeamCity.
• Zip up build results for TeamCity
• Remove dependency to gettext
• Simple random implementation for NaCl, so we don't get exceptions.
• Add x86_64 build.
• cleanup.
• Updated README instructions
• Don't use sgen for non-nacl mono build for parity
• brushing off some nacl dust
• Remove hardcoded path
• NaCl working again post M14
• Fixes for nacl
• Updated README
• Merge remote branch 'upstream/master' into merge_head
• Fixups for nacl at head revision
• New master script to build all for NaCl
• Add nacl cross-compile target
• Fix up bugs in x86-codegen for NaCl, use 4.0 for regression tests
• Merge branch 'merge_head'
• Wrap pthread_exit () with mono_gc_pthread_exit () since NACL requires it.
• No longer need BOEHM_DEFINES in io-layer, pthread_exit is wrapped
• Fix NaCl GC deadlock
• - Fix nacl aborting on windows because of failed write calls
• Update for new nacl tool names, also preliminary glibc support.
• First pass nacl glibc work
• More glibc nacl work, builds shared library.
• Fix amd64_alu_reg_imm for NaCl (previous fix was over ambitious)
• Faster GC instrumentation for managed nacl code.
• Compare pointers against 4 byte immediates
• Build with -O2 in Native Client x86-64.
• Merge pull request #1 from pasko/master
• Fix compile and validation errors in optimized build
• Fix conv.ovf.i.un test for 64 bit nacl
• Also fix convert i->i and i->u with no ovf
• enable gc instrumentation with inlined if
• Add volatile qualifier to libgc stop the world.
• Brad's patch to bootstrap NaCl glibc
• Merge pull request #2 from zheka/master
• Added __nacl_thread_suspension_needed to dynamic exports.
• Force memory allocation to use mmap
• Merge pull request #3 from zheka/master
• get rid of the alias to __nacl_thread_suspension_needed
• Always use near_call == TRUE in emit_call_body().
• Merge pull request #4 from bradchen/master
• Make size of immediate explicit in generating CMP instruction.
• Run fsacheck tests with data from high-memory to
• Add 'laddcc'/'lsubcc' IR opcode to deal explicitly with long arguments
• Pass return valuetypes of non power of two size to their nearest power of two, previously defaulted to sizeof(gpointer) which is too small for 5, 6, and 7 byte types on ILP32 machines.
• Fix patch targets to newly installed methods for NaCl
• Make divide by zero checks explicit for Native Client
• Pass TYPEDBYREF types the same as VALUETYPE for amd64 NaCl because that's more appropriate for the ABI
• Fix BB max_offset for x86 NaCl
• Fix spaces/tabs from last commit
• Make explicit-null-checks default for mono in NaCl (not embedded case), option to disable since the environment is mostly unusable
• Add nacl-specific regression tests
• Support 'make check' for self-hosted NaCl Mono in mono/mini
• Self-host NaCl Mono build
• Re-enable dlopen support in NaCl Mono
• Enable 'make check' for NaCl on root build directory:
• Add stderr output on nacl_dyncode_create failure, makes it easier to debug bad code
• Change README contents since those instructions don't work anymore
• Add custom nacl_interp script for building/testing NaCl Mono
• Make scripts work with slightly different paths from pepper_18.
• Use /bin/bash explicitly so the script runs properly
• Change install directory so it doesn't modify the toolchain dir
• Disable AOT loadhook for native client
• Initial NaCl/ARM work.
• Fix gmodule-unix.c (needed config.h for HAVE_DLFCN_H)
• Merge upstream mono
• Fix NaCl build post-merge with upstream master