Add Fluid Compiler design doc (#7178)

* Add fluid_compiler.md

* Paragraphing

doc/design/fluid.md

@@ -105,18 +105,10 @@ There are two ways to execute a Fluid program.  When a program is executed, it c
 
 There is a C++ class [`Executor`](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/executor.h), which runs a `ProgramDesc`, similar to how an interpreter runs a Python program.
 
-Fluid is moving towards the direction of a compiler, which is explain in more detail later in this article.
+Fluid is moving towards the direction of a compiler, which is explain in [fluid_compiler.md](fluid_compiler.md).
 
 ## Backward Compatibility of Fluid
 
 Given all the advantages from the removal of the concept of a *model*, hardware manufacturers might still prefer the existence of the concept of a model, so it would be easier for them to support multiple frameworks all at once and could run a trained model during inference.  For example, Nervana, a startup company acquired by Intel, has been working on an XPU that reads the models in the format known as [n-graph](https://github.com/NervanaSystems/ngraph).  Similarly, [Movidius](https://www.movidius.com/) is producing a mobile deep learning chip that reads and runs graphs of operators.  The well-known [ONNX](https://github.com/onnx/onnx) is also a file format of graphs of operators.
 
 For Fluid, we can write a converter that extracts the parts in the `ProgramDesc` protobuf message, converts them into a graph of operators, and exports the graph into the ONNX or n-graph format.
-
-## Towards a Deep Learning Language and the Compiler
-
-We can change the `if-then-else` and loop structure a little bit in the above Fluid example programs, to make it into a new programming language, different than Python.
-
-Even if we do not invent a new language, as long as we get the `ProgramDesc` message filled in, we can write a transpiler, which translates each invocation to an operator, into a C++ call to a kernel function of that operator. For example, a transpiler that weaves the CUDA kernels outputs an NVIDIA-friendly C++ program, which can be built using `nvcc`.  Another transpiler could generate MKL-friendly code that should be built using `icc` from Intel.  More interestingly, we can translate a Fluid program into its distributed version of two `ProgramDesc` messages, one for running on the trainer process, and the other one for the parameter server.  For more details of the last example, the [concurrent programming design](concurrent_programming.md) document would be a good pointer.  The following figure explains the proposed two-stage process:
-
-![](fluid-compiler.png)

doc/design/fluid_compiler.md

@@ -0,0 +1,110 @@
+# PaddlePaddle Fluid: Towards a Compiled Programming Language
+
+As described in [fluid.md](fluid.md), when a Fluid application program
+runs, it generates a `ProgramDesc` protobuf message as an intermediate
+representation of itself.  The C++ class `Executor` can run this
+protobuf message as an interpreter.  This article describes the Fluid
+compiler.
+
+![](fluid-compiler.png)
+
+## ProgramDesc
+
+Before we go deeper into the idea of compiled language, let us take a
+look at a simple example Fluid application.
+
+```python
+import "fluid"
+
+func paddlepaddle() {
+  X = fluid.read(...)
+  W = fluid.Tensor(...)
+  Y = fluid.mult(X, W)
+}
+```
+
+This program consists of a [block](block.md) of three operators --
+`read`, `assign`, and `mult`.  Its `ProgramDesc` message looks like
+the following
+
+```protobuf
+message ProgramDesc {
+  block[0] = Block {
+    vars = [X, W, Y],
+    ops = [
+      read(output = X)
+      assign(input = ..., output = W)
+      mult(input = {X, W}, output = Y)
+    ],
+  }
+}
+```
+ 
+## Transpilers
+
+We can write a transpiler program that takes a `ProgramDesc`, e.g.,
+the above one, and outputs another `ProgramDesc`.  Let us take some
+examples:
+
+1. *Memory optimization transpiler*: We can write a transpiler that
+   inserts some `FreeMemoryOp`s in the above example `ProgramDesc` so
+   to free memory early, before the end of an iteration, so to keep a
+   small memory footprint.
+
+1. *Distributed training transpiler*: We can write a transpiler that
+   converts a`ProgramDesc` into its distributed version of two
+   `ProgramDesc`s -- one for running by the trainer processes and the
+   other for the parameter server.
+
+In the rest of this article, we talk about a special kind of
+transpiler, *Native code generator*, which takes a `ProgramDesc` and
+generates a `.cu` (or `.cc`) file, which could be built by C++
+compilers (gcc, nvcc, icc) into binaries.
+
+## Native Code Generator
+
+For the above example, the native code generator transpiler, say, the
+CUDA code generator, should generate a `main` function:
+
+```c++
+void main() {
+  auto X = fluid_cuda_read(...);
+  auto W = fluid_cuda_create_tensor(...);
+  auto Y = fluid_cuda_mult(X, W);
+}
+```
+
+and the definitions of functions `fluid_cuda_read`,
+`fluid_cuda_create_tensor`, and `fluid_cuda_mult`.  Please be aware
+that each function could just define a C++ instance of an operator and
+run it.  For example
+
+```c++
+paddle::Tensor fluid_cuda_read(...) {
+  paddle::Tensor t;
+  paddle::operator::Read r(&t, ...);
+  r.Run();
+  return t;
+}
+```
+
+For computational operators that have multiple *kernels*, each for a
+specific hardware platform, for example, the `mult` operator, the
+generated code should call its CUDA kernel:
+
+```c++
+paddle::Tensor fluid_cuda_mult(const paddle::Tensor& a, 
+                               const paddle::Tensor& b) {
+  paddle::Tensor t;
+  paddle::operator::Mult m(a, b, ...);
+  Mult.Run(cuda_context);
+}
+```
+
+where `cuda_context` could be a global variable of type
+`paddle::CUDADeviceContext`.
+
+## Multi-Block Code Generation
+
+Most Fluid application programs may have more than one blocks.  To
+execute them, we need to trace [scopes](scope.md).