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diff --git a/native_client_sdk/src/doc/nacl-and-pnacl.rst b/native_client_sdk/src/doc/nacl-and-pnacl.rst index 7eb07fb..b83f0a5 100644 --- a/native_client_sdk/src/doc/nacl-and-pnacl.rst +++ b/native_client_sdk/src/doc/nacl-and-pnacl.rst @@ -12,103 +12,102 @@ This document describes the differences between **Native Client** and :backlinks: none :depth: 2 +.. _native-client-nacl: + Native Client (NaCl) ==================== Native Client enables the execution of native code securely inside web applications through the use of advanced `Software Fault Isolation (SFI) -techniques <http://research.google.com/pubs/pub35649.html>`_. Since its launch in -2011, Native Client has provided developers with the ability to harness a -client machine's computational power to a much fuller extent than traditional -web technologies, by running compiled C and C++ code at near-native speeds and -taking advantage of multiple cores with shared memory. - -While Native Client provides operating system independence, it requires -developers to generate architecture-specific executable -(**nexe**) modules for each hardware platform. This is not only inconvenient -for developers, but architecture-specific machine code is not portable and thus -not well-suited for the open web. The traditional method of application -distribution on the web is through a self-contained bundle of HTML, CSS, -JavaScript, and other resources (images, etc.) that can be hosted on a server -and run inside a web browser. With this type of distribution, a website -created today should still work years later, on all platforms. -Architecture-specific executables are clearly not a good fit for distribution -on the web. As a consequence, Native Client has been restricted to -applications and browser extensions that are installed through the +techniques <http://research.google.com/pubs/pub35649.html>`_. Native Client +allows you to harness a client machine's computational power to a fuller extent +than traditional web technologies. It does this by running compiled C and C++ +code at near-native speeds, and exposing a CPU's full capabilities, including +SIMD vectors and multiple-core processing with shared memory. + +While Native Client provides operating system independence, it requires you to +generate architecture-specific executables (**nexe**) for each hardware +platform. This is neither portable nor convenient, making it ill-suited for the +open web. + +The traditional method of application distribution on the web is through self- +contained bundles of HTML, CSS, JavaScript, and other resources (images, etc.) +that can be hosted on a server and run inside a web browser. With this type of +distribution, a website created today should still work years later, on all +platforms. Architecture-specific executables are clearly not a good fit for +distribution on the web. Consequently, Native Client has been until recently +restricted to applications and browser extensions that are installed through the Chrome Web Store. +.. _portable-native-client-pnacl: + Portable Native Client (PNaCl) ============================== PNaCl solves the portability problem by splitting the compilation process into two parts: -#. compiling the source code to a portable bitcode format, and -#. translating the bitcode to a host-specific executable just before execution. - -PNaCl enables developers to distribute **portable executables** (**pexe**) -modules that the hosting environment (in other words, the Chrome browser) can -translate to native code before executing. This portability aligns Native Client -with existing open web technologies such as JavaScript. A developer can -distribute a **pexe** as part of an application (along with HTML, CSS, and -JavaScript), and the user's machine is simply able to run it. - -With PNaCl, a developer generates a single **pexe** from source code, -rather than multiple platform-specific nexes. The **pexe** provides both -architecture- and OS-independence. Since the **pexe** uses an abstract, -architecture-independent format, it does not suffer from the portability -problem described above. Future versions of hosting environments should -have no problem executing the **pexe**, even on new architectures. -Moreover, if an existing architecture is subsequently enhanced, the -**pexe** doesn't even have to be recompiled. In some cases the -client-side translation will automatically be able to take advantage of -the new capabilities. A **pexe** module can be part of any web -application. It does not have to be distributed through the Chrome Web -Store. In short, PNaCl combines the portability of existing web technologies -with the performance and security benefits of Native Client. +#. compiling the source code to a bitcode executable (pexe), and +#. translating the bitcode to a host-specific executable as soon as the module + loads in the browser but before any code execution. + +This portability aligns Native Client with existing open web technologies such +as JavaScript. You can distribute a pexe as part of an application (along with +HTML, CSS, and JavaScript), and the user's machine is simply able to run it. + +With PNaCl, you'll generate a single pexe, rather than multiple platform- +specific nexes. Since the pexe uses an abstract, architecture- and OS- +independent format, it does not suffer from the portability problem described +above. Although, PNaCl can be more efficient on some operating systems than on +others. PNaCl boasts the same level of security as NaCl. Future versions of +hosting environments should have no problem executing the pexe, even on new +architectures. Moreover, if an existing architecture is enhanced, the pexe +doesn't need to be recompiled. In some cases the client-side translation will +automatically take advantage of new capabilities. A pexe can be part of any web +application. It does not have to be distributed through the Chrome Web Store. In +short, PNaCl combines the portability of existing web technologies with the +performance and security benefits of Native Client. PNaCl is a new technology, and as such it still has a few limitations as compared to NaCl. These limitations are described below. +.. _when-to-use-pnacl: + When to use PNaCl ================= PNaCl is the preferred toolchain for Native Client, and the only way to deploy -Native Client modules on the open web. Unless your project is subject to one -of the narrow limitations described below -(see :ref:`When to use NaCl<when-to-use-nacl>`), you should use PNaCl. - -Beginning with version 31, the Chrome browser supports translation of -**pexe** modules and their use in web applications, without requiring -any installation (either of a browser plugin or of the applications -themselves). Native Client and PNaCl are open-source technologies, and -our hope is that they will be added to other hosting platforms in the -future. - -If controlled distribution through the Chrome Web Store is an important part -of your product plan, the benefits of PNaCl are less critical for you. But -you can still use the PNaCl toolchain and distribute your application -through the Chrome Web Store, and thereby take advantage of the -conveniences of PNaCl, such as not having to explicitly compile your application -for all supported architectures. +Native Client modules without the Google Web Store. Unless your project is +subject to one of the narrow limitations described under ":ref:`When to use +NaCl<when-to-use-nacl>`", you should use PNaCl. + +Since version 31, Chrome supports translation of pexe +modules and their use in web applications without requiring installation either +of a browser plug-in or of the applications themselves. Native Client and PNaCl +are open-source technologies, and our hope is that they will be added to other +hosting platforms in the future. + +If controlled distribution through the Chrome Web Store is an important part of +your product plan, the benefits of PNaCl are less critical for you. But you can +still use the PNaCl toolchain and distribute your application through the Chrome +Web Store, and thereby take advantage of the conveniences of PNaCl, such as not +having to explicitly compile your application for all supported architectures. .. _when-to-use-nacl: When to use NaCl ================ -The limitations below apply to the current release of PNaCl. If any of -these limitations are critical for your application, you should use -non-portable NaCl: - -* PNaCl does not support architecture-specific - instructions in an application (i.e., inline assembly), but tries to - offer high-performance portable equivalents. One such example is - PNaCl's :ref:`Portable SIMD Vectors <portable_simd_vectors>`. -* PNaCl only supports static linking with the ``newlib`` - C standard library (the Native Client SDK provides a PNaCl port of - ``newlib``). Dynamic linking and ``glibc`` are not yet supported. - Work is under way to enable dynamic linking in future versions of PNaCl. -* PNaCl does not support some GNU extensions - like taking the address of a label for computed ``goto``, or nested +Use NaCl if any of the following apply to your application: + +* Your application requires architecture-specific instructions such as, for + example, inline assembly. PNaCl tries to offer high-performance portable + equivalents. One such example is PNaCl's :ref:`Portable SIMD Vectors + <portable_simd_vectors>`. +* Your application uses dynamic linking. PNaCl only supports static linking + with a PNaCl port of the ``newlib`` C standard library. Dynamic linking and + ``glibc`` are not yet supported in PNaCl. Work is under way to enable dynamic + linking in future versions of PNaCl. +* Your application uses certain GNU extensions not supported by PNaCl's LLVM + toolchain, like taking the address of a label for computed ``goto``, or nested functions. |