Chapter 22: A Hello, World!linear-chapter22

Co-authored-by: godalming123 <68993177+godalming123@users.noreply.github.com>
Co-authored-by: Tamás Hadházy <85063808+Hels15@users.noreply.github.com>

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+# Chapter 22: A Hello, World!
+
+It's high time for strings and printing!  In this chapter we look at what it
+takes to make a basic string - not really a full fledged String class, that
+will wait for another chapter.
+
+```java
+sys.io.p("Hello, World!");
+```
+
+Yeah!  A 1-liner `Hello, World`!  This example can be found in
+`examples/A_helloWorld.smp`.
+
+
+Let's break it down.
+
+- `sys`: is Just A Variable Name.  Like other variables, it is looked up in the
+  current scope... which looks completely empty.  Every Simple program now starts
+  with the `sys` variable in-scope.  `sys` itself a normal `struct`
+  and is defined in `src/main/smp/sys.smp`.
+- `sys.io.p`: lookup the `io` field in `sys` struct, which yields the `sys.io`
+  struct; then a lookup of `p` (short for "print") in the `sys.io` struct.
+  This returns a function.
+- `p("Hello, World!")`: call the function just loaded from the `p` field, with
+   `"Hello, World!"` as a string argument.
+- `"Hello, World!"`: New syntax that yields a constant array of `u8` (chars).
+  The array is otherwise a normal range-checked array, with a leading `#`
+  length field.  The array backing data eventually ends up in the *constant
+  pool*, and from there in an ELF file's RODATA section.
+
+
+    You can also read [this chapter](https://github.com/SeaOfNodes/Simple/tree/linear-chapter22) in a linear Git revision history on the [linear](https://github.com/SeaOfNodes/Simple/tree/linear) branch and [compare](https://github.com/SeaOfNodes/Simple/compare/linear-chapter21...linear-chapter22) it to the previous chapter.
+
+
+## Nested Types and Static Fields
+
+Simple now supports nested types - this is a name-space only change, so no new
+semantics - just the ability to nest type definitions.
+
+```java
+struct Outer { 
+    struct Inner {
+        int in;
+        int pi = 3.14;
+    };
+    int out;
+};
+```
+
+Any final field set in the declaration (as opposed to the constructor) is
+automatically a "static" field - it is final for all instances, and does not
+need to be stored in each instances.  Instead one copy is kept in a global
+space (not really a *class* object yet) and loaded from there.
+
+In this example:
+- Instances of the `Outer` struct have a single `int` field `out`.
+- Instances of the `Inner` struct have a single `int` field `in`.
+- There exists a `Outer.Inner.pi` field in a global space with value `3.14`.
+
+The `sys` struct is another example, with nested `libc` and `io` structs at
+least.  All the fields in these structs are all final, hence static.
+
+
+
+## FFI and I/O Calls
+
+We can now call external / FFI functions, and they are declared like any other
+variable except being assigned "C".  Here is the binding for the libc `write`
+call:
+
+`{ i32 fd, i32 buf, i32 len -> i32 } write = "C";`
+
+Any final variable assigned as "C" will be considered defined externally.  The
+syntax of this might change slightly to avoid ambiguity with assign the same
+"C" string.
+
+There is no linker support for getting the correct signature or namespace; the
+name `write` has to be unique when linking.
+
+
+## Auto-Import and the "sys." namespace
+
+The `sys` struct includes `libc` bindings, and any default library code
+including e.g. printing support and collections.  As of this chapter it
+includes minimal libc bindings and an easy print:
+
+```java
+// top-level default import
+struct sys {
+    // libc bindings
+    /** https://www.man7.org/linux/man-pages/man2/<libc call>.2.html */
+    struct libc {
+        // fd  buf len -> len
+        {  i32 i64 i32 -> i32 } write = "C";
+        // addr len prot flags fd  off -> void*
+        {  i64  i64 i32  i32   i32 i32 -> i64 } mmap = "C";
+        // mmap flags
+        i32 PROT_EXEC = 4;
+        i32 PROT_READ = 1;
+        i32 PROT_WRITE= 2;
+        i32 PROT_NONE = 0;
+        i32 MAP_PRIVATE = 2;
+        i32 MAP_ANON = 32;
+    };
+    struct io {
+        val p = { u8[~] str ->
+            i64 ptr = str;  // cast array base to i64
+            i32 len = str#; // cast length to signed
+            return sys.libc.write(1,ptr,len);
+        };
+    };
+};
+```
+
+The `sys` import itself is Just Another `struct` like any other struct; it has
+fields and assignments and types declared internally.  Note that these
+assignments are all final and in the struct declaration, hence these are all
+*static* fields.
+
+
+## Casting a Pointer to an `i64`
+
+You can now cast a pointer to an i64, although not the other way around.
+`i64 ptr = str; // cast array base to i64`
+
+For arrays, this cast is to the array *base* skipping the `#` length field, and
+is suitable for passing to external "C" calls.  For structs, this is just the
+struct base.  This is used to pass a length-checked Simple array to the libc
+`write` call in the above `sys.io.p` function.
+
+
+## Arrays of Constants and Constant Arrays
+
+The Simple type system now supports the notion of a "constant array" - an array
+of fixed constants, stored in the ELF file in the RODATA section.  Currently we
+only support strings at the parser level: `"Hello, World!"` makes a `u8[13]`
+array containing 13 ASCII bytes; like all arrays it has a length and will be
+range-checked at some point.
+
+A constant version of a non-constant array can be assigned using the `u8[~]`
+syntax.
+
+```java
+// Make a mutable array of ints
+int N=4;
+i32[] is = new i32[N];
+// Mutate the array
+for( int i=0; i<N; i++ )
+    is[i] = i*i;
+// Function "sum" can read but not write the array
+val sum = { i32[~] nums ->   // Notice the [~] signature marking nums as immutable
+    int sum = 0;
+    for( int i=0; i<nums#; i++ )
+        sum += nums[i];
+    return sum;
+};
+return sum(is);
+```
+
+Note that this affects the deep contents of the array and not the array
+variable itself.
+
+## Quote Characters
+
+The form  ``` `0` ``` makes a `u8` value with the ASCII character `0`.
+
+## Continuous Improvement
+
+In the rush to get executable code from 
+[Chapters 19](https://github.com/SeaOfNodes/Simple/tree/linear-chapter19),
+[Chapters 20](https://github.com/SeaOfNodes/Simple/tree/linear-chapter20), and
+[Chapters 21](https://github.com/SeaOfNodes/Simple/tree/linear-chapter21), we
+wrote a lot of code in a hurry.  That also means we wrote a lot of bugs (sad
+face), and will be paying the price for a few chapters.  In this chapter we
+also found and fixed a number of bugs, here's a sample:
+
+- x86 encoding bugs:
+- - `imul` with immediate forms and `setXX` had incorrect `REX` forms.
+- - x86 `div` was taking `RAX,RDX` as the second *input* register; they are already 
+    hardwired as the *first* input register.
+- - Many x86 ops that kill `flags` did not state so in the port.
+- - Short forms for some x86 ops (e.g. using the `inc` vs `add+1`)
+- Global Code Motion `ScheduleLate` missed visiting values only visible around
+  loop backedges; also was adding extra dependence edges between loads and
+  stores with different aliasing.
+- Inlining would reset the **idepth** (as expected) but then use the existing
+  `CallNode` and `FunNode` that are *folding* but not yet peep'd away - leading
+  to incorrect **idepth** calculations.
+  
+Many more bugs got fixed, but this sample should give an idea of the continuous
+improvements going on.  These happen because we have finally reached a point
+where we can aggressive test again - we are writing Real (Simple) Code (tm) and
+running it... and crashing on these bugs.  Compilers are big, complex, beasties
+and some amount of bugs and bug fixing is to be expected.