add a readme

Author: pseudocc

README.md

@@ -0,0 +1,96 @@
+# LLVM-C EXAMPLE
+
+works well on ubuntu 20.04.
+
+## Installation
+
+LLVM and Clang.
+
+```bash
+sudo apt install llvm-12
+sudo apt install clang-12
+```
+
+## Build
+
+with GNU Make.
+
+```bash
+make
+```
+
+## Run from LLVM IR Bitcode
+
+Run fac.
+
+```bash
+./fac 8
+# fac(8) = 40320
+```
+
+Run fib.
+
+```bash
+./fib 13
+# fib(13) = 233
+```
+
+## LLVM IR Text Format
+
+```bash
+make math.ll
+cat math.ll
+```
+
+## Fibonacci
+
+this contains basic operations on parameters and local variables.
+also if statement and while loop for control flow.
+
+0. compare
+0. loop
+0. assignment
+0. arithmetic
+
+```c
+int fib(int n) {
+  int a, b, t;
+  if (n < 1)
+    return -1;
+  if (n < 3)
+    return 1;
+  a = 1;
+  b = 1;
+  while (n-- > 2) {
+    t = a + b;
+    a = b;
+    b = t;
+  }
+  return b;
+}
+```
+
+## Factorial
+
+this contains recursive call and if statement and switch block.
+using a PHI instruction to catch all return value of each block.
+
+0. compare
+0. function call
+0. arithmetic
+0. switch
+0. PHI instruction
+
+```c
+int fac(int n) {
+  if (n < 0)
+    return -fac(-n);
+  switch (n) {
+  case 0:
+  case 1:
+    return 1;
+  default:
+    return n * fac(n - 1);
+  }
+}
+```