treelang.texi

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indicate variable is permanent, or function has file scope only
@item automatic
indicate that variable is allocated for the life of the current scope
@item external_reference
indicate that variable or function is defined in another file
@item external_definition
indicate that variable or function is to be accessible from other files
@item int
variable is an integer (same as C int) 
@item char
variable is a character (same as C char)
@item unsigned
variable is unsigned. If this is not present, the variable is signed
@item return
start function return statement
@item void
used as function type to indicate function returns nothing
@end itemize


@item
Names consist of any letter or "_" followed by any number of letters,
numbers, or "_".  "$" is not allowed in a name.  All names must be globally
unique, i.e. may not be used twice in any context, and must
not be a keyword.  Names and keywords are case sensitive.  For example:

@smallexample
a A _a a_ IF_X
@end smallexample

are all different names.

@end itemize

@node Parsing Syntax, Compiler Overview, Lexical Syntax, Top
@chapter Parsing Syntax
@cindex Parsing Syntax

Declarations are built up from the lexical elements described above.  A
file may contain one of more declarations.

@itemize @bullet

@item
declaration: variable declaration OR function prototype OR function declaration

@item
Function Prototype: storage type NAME ( optional_parameter_list )

@smallexample
static int add (int a, int b)
@end smallexample

@item
variable_declaration: storage type NAME initial;

Example:

@smallexample
int temp1 = 1;
@end smallexample

A variable declaration can be outside a function, or at the start of a
function.

@item
storage: automatic OR static OR external_reference OR external_definition

This defines the scope, duration and visibility of a function or variable

@enumerate 1

@item 
automatic: This means a variable is allocated at start of the current scope and
released when the current scope is exited.  This can only be used for variables
within functions.  It cannot be used for functions.

@item 
static: This means a variable is allocated at start of program and
remains allocated until the program as a whole ends.  For a function, it
means that the function is only visible within the current file.

@item
external_definition: For a variable, which must be defined outside a
function, it means that the variable is visible from other files.  For a
function, it means that the function is visible from another file.

@item
external_reference: For a variable, which must be defined outside a
function, it means that the variable is defined in another file.  For a
function, it means that the function is defined in another file.

@end enumerate

@item
type: int OR unsigned int OR char OR unsigned char OR void

This defines the data type of a variable or the return type of a function.

@enumerate a

@item 
int: The variable is a signed integer.  The function returns a signed integer.

@item 
unsigned int: The variable is an unsigned integer.  The function returns an unsigned integer.

@item 
char: The variable is a signed character.  The function returns a signed character.

@item 
unsigned char: The variable is an unsigned character.  The function returns an unsigned character.

@end enumerate

@item
parameter_list OR parameter [, parameter]...

@item
parameter: variable_declaration ,

The variable declarations must not have initializations.

@item 
initial: = value

@item
value: integer_constant

Values without a unary plus or minus are considered to be unsigned.
@smallexample
e.g.@: 1 +2 -3
@end smallexample

@item
function_declaration: name @{ variable_declarations statements @}

A function consists of the function name then the declarations (if any)
and statements (if any) within one pair of braces.

The details of the function arguments come from the function
prototype.  The function prototype must precede the function declaration
in the file.

@item
statement: if_statement OR expression_statement OR return_statement

@item
if_statement: if ( expression ) @{ variable_declarations statements @}
else @{ variable_declarations statements @}

The first lot of statements is executed if the expression is
nonzero.  Otherwise the second lot of statements is executed.  Either
list of statements may be empty, but both sets of braces and the else must be present. 

@smallexample
if (a==b) 
@{
// @r{nothing}
@}
else
@{
a=b;
@}
@end smallexample

@item
expression_statement: expression;

The expression is executed, including any side effects.

@item
return_statement: return expression_opt;

Returns from the function. If the function is void, the expression must
be absent, and if the function is not void the expression must be
present.

@item
expression: variable OR integer_constant OR expression + expression
OR expression - expression OR expression == expression OR ( expression )
OR variable = expression OR function_call

An expression can be a constant or a variable reference or a
function_call.  Expressions can be combined as a sum of two expressions
or the difference of two expressions, or an equality test of two
expressions.  An assignment is also an expression.  Expressions and operator
precedence work as in C.

@item
function_call: function_name ( optional_comma_separated_expressions )

This invokes the function, passing to it the values of the expressions
as actual parameters.

@end itemize

@cindex compilers
@node Compiler Overview, TREELANG and GCC, Parsing Syntax, Top
@chapter Compiler Overview
treelang is run as part of the GCC compiler. 

@itemize @bullet
@cindex source code
@cindex file, source
@cindex code, source
@cindex source file
@item
It reads a user's program, stored in a file and containing instructions
written in the appropriate language (Treelang, C, and so on).  This file
contains @dfn{source code}.

@cindex translation of user programs
@cindex machine code
@cindex code, machine
@cindex mistakes
@item
It translates the user's program into instructions a computer can carry
out more quickly than it takes to translate the instructions in the
first place.  These instructions are called @dfn{machine code}---code
designed to be efficiently translated and processed by a machine such as
a computer.  Humans usually aren't as good writing machine code as they
are at writing Treelang or C, because it is easy to make tiny mistakes
writing machine code.  When writing Treelang or C, it is easy to make
big mistakes. But you can only make one mistake, because the compiler
stops after it finds any problem.

@cindex debugger
@cindex bugs, finding
@cindex @code{gdb}, command
@cindex commands, @code{gdb}
@item
It provides information in the generated machine code
that can make it easier to find bugs in the program
(using a debugging tool, called a @dfn{debugger},
such as @code{gdb}).

@cindex libraries
@cindex linking
@cindex @code{ld} command
@cindex commands, @code{ld}
@item
It locates and gathers machine code already generated to perform actions
requested by statements in the user's program.  This machine code is
organized into @dfn{libraries} and is located and gathered during the
@dfn{link} phase of the compilation process.  (Linking often is thought
of as a separate step, because it can be directly invoked via the
@code{ld} command.  However, the @code{gcc} command, as with most
compiler commands, automatically performs the linking step by calling on
@code{ld} directly, unless asked to not do so by the user.)

@cindex language, incorrect use of
@cindex incorrect use of language
@item
It attempts to diagnose cases where the user's program contains
incorrect usages of the language.  The @dfn{diagnostics} produced by the
compiler indicate the problem and the location in the user's source file
where the problem was first noticed.  The user can use this information
to locate and fix the problem.

The compiler stops after the first error.  There are no plans to fix
this, ever, as it would vastly complicate the implementation of treelang
to little or no benefit.

@cindex diagnostics, incorrect
@cindex incorrect diagnostics
@cindex error messages, incorrect
@cindex incorrect error messages
(Sometimes an incorrect usage of the language leads to a situation where
the compiler can not make any sense of what it reads---while a human
might be able to---and thus ends up complaining about an incorrect
``problem'' it encounters that, in fact, reflects a misunderstanding of
the programmer's intention.)

@cindex warnings
@cindex questionable instructions
@item
There are a few warnings in treelang.  For example an unused static function
generate a warnings when -Wunused-function is specified, similarly an unused
static variable generates a warning when -Wunused-variable are specified.
The only treelang specific warning is a warning when an expression is in a
return statement for functions that return void.
@end itemize

@cindex components of treelang
@cindex @code{treelang}, components of
@code{treelang} consists of several components:

@cindex @code{gcc}, command
@cindex commands, @code{gcc}
@itemize @bullet
@item
A modified version of the @code{gcc} command, which also might be
installed as the system's @code{cc} command.
(In many cases, @code{cc} refers to the
system's ``native'' C compiler, which
might be a non-GNU compiler, or an older version
of @code{GCC} considered more stable or that is
used to build the operating system kernel.)

@cindex @code{treelang}, command
@cindex commands, @code{treelang}
@item
The @code{treelang} command itself.

@item
The @code{libc} run-time library.  This library contains the machine
code needed to support capabilities of the Treelang language that are
not directly provided by the machine code generated by the
@code{treelang} compilation phase.  This is the same library that the
main C compiler uses (libc).

@cindex @code{tree1}, program
@cindex programs, @code{tree1}
@cindex assembler
@cindex @code{as} command
@cindex commands, @code{as}
@cindex assembly code
@cindex code, assembly
@item
The compiler itself, is internally named @code{tree1}.

Note that @code{tree1} does not generate machine code directly---it
generates @dfn{assembly code} that is a more readable form
of machine code, leaving the conversion to actual machine code
to an @dfn{assembler}, usually named @code{as}.
@end itemize

@code{GCC} is often thought of as ``the C compiler'' only,
but it does more than that.
Based on command-line options and the names given for files
on the command line, @code{gcc} determines which actions to perform, including
preprocessing, compiling (in a variety of possible languages), assembling,
and linking.

@cindex driver, gcc command as
@cindex @code{gcc}, command as driver
@cindex executable file
@cindex files, executable
@cindex cc1 program
@cindex programs, cc1
@cindex preprocessor
@cindex cpp program
@cindex programs, cpp
For example, the command @samp{gcc foo.c} @dfn{drives} the file
@file{foo.c} through the preprocessor @code{cpp}, then
the C compiler (internally named
@code{cc1}), then the assembler (usually @code{as}), then the linker
(@code{ld}), producing an executable program named @file{a.out} (on
UNIX systems).

@cindex treelang program
@cindex programs, treelang
As another example, the command @samp{gcc foo.tree} would do much the
same as @samp{gcc foo.c}, but instead of using the C compiler named
@code{cc1}, @code{gcc} would use the treelang compiler (named
@code{tree1}). However there is no preprocessor for treelang.

@cindex @code{tree1}, program
@cindex programs, @code{tree1}
In a GNU Treelang installation, @code{gcc} recognizes Treelang source
files by name just like it does C and C++ source files.  It knows to use
the Treelang compiler named @code{tree1}, instead of @code{cc1} or
@code{cc1plus}, to compile Treelang files.  If a file's name ends in
@code{.tree} then GCC knows that the program is written in treelang.  You
can also manually override the language.

@cindex @code{gcc}, not recognizing Treelang source
@cindex unrecognized file format
@cindex file format not recognized
Non-Treelang-related operation of @code{gcc} is generally
unaffected by installing the GNU Treelang version of @code{gcc}.
However, without the installed version of @code{gcc} being the
GNU Treelang version, @code{gcc} will not be able to compile
and link Treelang programs.

@cindex printing version information
@cindex version information, printing
The command @samp{gcc -v x.tree} where @samp{x.tree} is a file which
must exist but whose contents are ignored, is a quick way to display
version information for the various programs used to compile a typical
Treelang source file. 

The @code{tree1} program represents most of what is unique to GNU
Treelang; @code{tree1} is a combination of two rather large chunks of
code.

@cindex GCC Back End (GBE)
@cindex GBE
@cindex @code{GCC}, back end
@cindex back end, GCC
@cindex code generator
One chunk is the so-called @dfn{GNU Back End}, or GBE,
which knows how to generate fast code for a wide variety of processors.
The same GBE is used by the C, C++, and Treelang compiler programs @code{cc1},
@code{cc1plus}, and @code{tree1}, plus others.
Often the GBE is referred to as the ``GCC back end'' or
even just ``GCC''---in this manual, the term GBE is used
whenever the distinction is important.

@cindex GNU Treelang Front End (TFE)
@cindex tree1
@cindex @code{treelang}, front end
@cindex front end, @code{treelang}
The other chunk of @code{tree1} is the majority of what is unique about
GNU Treelang---the code that knows how to interpret Treelang programs to
determine what they are intending to do, and then communicate that
knowledge to the GBE for actual compilation of those programs.  This
chunk is called the @dfn{Treelang Front End} (TFE).  The @code{cc1} and
@code{cc1plus} programs have their own front ends, for the C and C++
languages, respectively.  These fronts ends are responsible for
diagnosing incorrect usage of their respective languages by the programs
the process, and are responsible for most of the warnings about
questionable constructs as well.  (The GBE in principle handles
producing some warnings, like those concerning possible references to
undefined variables, but these warnings should not occur in treelang
programs as the front end is meant to pick them up first).

Because so much is shared among the compilers for various languages,
much of the behavior and many of the user-selectable options for these
compilers are similar.
For example, diagnostics (error messages and
warnings) are similar in appearance; command-line
options like @samp{-Wall} have generally similar effects; and the quality
of generated code (in terms of speed and size) is roughly similar
(since that work is done by the shared GBE).

@node TREELANG and GCC, Compiler, Compiler Overview, Top
@chapter Compile Treelang, C, or Other Programs
@cindex compiling programs
@cindex programs, compiling

@cindex @code{gcc}, command
@cindex commands, @code{gcc}
A GNU Treelang installation includes a modified version of the @code{gcc}
command.