From e99b4bdfb2d89f4463e126c292332d3cf50eab20 Mon Sep 17 00:00:00 2001
From: Alexis Pereda <alexis@pereda.fr>
Date: Tue, 2 Jul 2019 17:21:34 +0200
Subject: [PATCH] [et] first draft

---
 README.md                     | 41 ++++++++++++++++++++++++++++++
 examples/et/1_first_draft.cpp | 48 +++++++++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 examples/et/1_first_draft.cpp

diff --git a/README.md b/README.md
index ed87e55..705dc2c 100644
--- a/README.md
+++ b/README.md
@@ -13,3 +13,44 @@ make
 ```
 
 then run any example binary, located inside the `build/examples/` directory.
+
+## Remarks
+
+### Assembler code
+
+Some examples are meant to show the assembler code they generate.
+To see that, one can use for example https://godbolt.org/ or GDB.
+With GDB:
+```bash
+gdb -q -ex 'disassemble main' -ex q "${program_to_execute}"
+```
+
+Important note:
+if using `cmake` to produce the Makefile, you must call it with at least:
+```bash
+cmake -DCMAKE_BUILD_TYPE=Release ..
+```
+or the examples will be compiled without the optimization level 2.
+
+### use of `volatile`
+
+When a value is required to show something, it is necessary to prevent the compiler to
+optimize out this value. To achieve this, one can read the value from a file, for example,
+but it will generate more assembler code.
+Another solution is the `volatile` type modifier that forbids optimization like caching or
+other optimizations like:
+```cpp
+int main() {
+	int volatile v0 = 5, v1 = 7;
+	return v0 + v1;
+}
+```
+
+Without `volatile`, the addition is done by the compiler and `main` returns directly `12`.
+With `volatile`, the generated assembler code will contain an `add` instruction, even with `-O2`.
+
+### `std::printf` instead of `std::cout`
+
+C++ iostreams cause lots of assembler code, for that reason, when an example is expected to be
+possibly used by looking at its generated assembler code, iostreams are avoided and replaced by
+`printf`.
diff --git a/examples/et/1_first_draft.cpp b/examples/et/1_first_draft.cpp
new file mode 100644
index 0000000..faea1fd
--- /dev/null
+++ b/examples/et/1_first_draft.cpp
@@ -0,0 +1,48 @@
+#include <cstdio>
+
+struct Add {
+	template<typename T>
+	static auto eval(T lhs, T rhs) {
+		return lhs + rhs;
+	}
+};
+
+struct Mul {
+	template<typename T>
+	static auto eval(T lhs, T rhs) {
+		return lhs * rhs;
+	}
+};
+
+template<typename T>
+class Value {
+	T _value;
+
+public:
+	Value(T value): _value(value) {}
+
+	auto eval() const { return _value; }
+};
+
+template<typename Op, typename LHS, typename RHS>
+struct BinExpr {
+	LHS lhs;
+	RHS rhs;
+
+	auto eval() const {
+		return Op::eval(lhs.eval(), rhs.eval());
+	}
+};
+
+int main() {
+	using SubExpr = BinExpr<Mul, Value<int>, Value<int>>;
+	using MyExpr  = BinExpr<Add, Value<int>, SubExpr>;
+
+	int volatile v0 = 3, v1 = 5, v2 = 7;
+	int result;
+
+	MyExpr expr{{v0}, {{v1}, {v2}}};
+	result = expr.eval();
+
+	std::printf("%d\n", result);
+}