treelang) compiler. At the moment this manual is not
incorporated into the main GCC manual as it is incomplete. It
corresponds to the GCC-4.2.4 version of treelang.
Published by the Free Software Foundation 51 Franklin Street, Fifth Floor Boston, MA 02110-1301 USA
Copyright © 1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License”, the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled “GNU Free Documentation License”.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.
This manual documents how to run, install and maintain treelang.
It also documents the features and incompatibilities in the GCC-4.2.4
version of treelang.
--- The Detailed Node Listing ---
Other Languages
treelang internals
treelang compiler interfaces
treelang main compiler
Reporting Bugs
Copyright © 1989, 1991 Free Software Foundation, Inc.
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
Everyone is permitted to copy and distribute verbatim copies
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Permission is granted to copy, distribute and/or modify this document
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with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
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If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
Treelang was based on 'toy' by Richard Kenner, and also uses code from the GCC core code tree. Tim Josling first created the language and documentation, based on the GCC Fortran compiler's documentation framework. Treelang was updated to use the TreeSSA infrastructure by James A. Morrison.
If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.
Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the Free Software Foundation, and others.
The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.
To make this approach work, you must insist on numbers that you can compare, such as, “We will donate ten dollars to the Frobnitz project for each disk sold.” Don't be satisfied with a vague promise, such as “A portion of the profits are donated,” since it doesn't give a basis for comparison.
Even a precise fraction “of the profits from this disk” is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.
Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.
By establishing the idea that supporting further development is “the proper thing to do” when distributing free software for a fee, we can assure a steady flow of resources into making more free software.
Copyright © 1994 Free Software Foundation, Inc.
Verbatim copying and redistribution of this section is permitted
without royalty; alteration is not permitted.
Treelang is a sample language, useful only to help people understand how to implement a new language front end to GCC. It is not a useful language in itself other than as an example or basis for building a new language. Therefore only language developers are likely to have an interest in it.
This manual assumes familiarity with GCC, which you can obtain by using it and by reading the manuals ‘Using the GNU Compiler Collection (GCC)’ and ‘GNU Compiler Collection (GCC) Internals’.
To install treelang, follow the GCC installation instructions, taking care to ensure you specify treelang in the configure step by adding treelang to the list of languages specified by --enable-languages, e.g. ‘--enable-languages=all,treelang’.
If you're generally curious about the future of
treelang, see Projects.
If you're curious about its past,
see Contributors.
To see a few of the questions maintainers of treelang have,
and that you might be able to answer,
see Open Questions.
GNU Treelang, or treelang, is designed initially as a free
replacement for, or alternative to, the 'toy' language, but which is
amenable to inclusion within the GCC source tree.
treelang is largely a cut down version of C, designed to showcase
the features of the GCC code generation back end. Only those features
that are directly supported by the GCC code generation back end are
implemented. Features are implemented in a manner which is easiest and
clearest to implement. Not all or even most code generation back end
features are implemented. The intention is to add features incrementally
until most features of the GCC back end are implemented in treelang.
The main features missing are structures, arrays and pointers.
A sample program follows:
// function prototypes // function 'add' taking two ints and returning an int external_definition int add(int arg1, int arg2); external_definition int subtract(int arg3, int arg4); external_definition int first_nonzero(int arg5, int arg6); external_definition int double_plus_one(int arg7); // function definition add { // return the sum of arg1 and arg2 return arg1 + arg2; } subtract { return arg3 - arg4; } double_plus_one { // aaa is a variable, of type integer and allocated at the start of // the function automatic int aaa; // set aaa to the value returned from add, when passed arg7 and arg7 as // the two parameters aaa=add(arg7, arg7); aaa=add(aaa, aaa); aaa=subtract(subtract(aaa, arg7), arg7) + 1; return aaa; } first_nonzero { // C-like if statement if (arg5) { return arg5; } else { } return arg6; }
Treelang programs consist of whitespace, comments, keywords and names.
A B
x = 1; // Set X to 1
a A _a a_ IF_X
are all different names.
Declarations are built up from the lexical elements described above. A file may contain one of more declarations.
static int add (int a, int b)
Example:
int temp1 = 1;
A variable declaration can be outside a function, or at the start of a function.
This defines the scope, duration and visibility of a function or variable
This defines the data type of a variable or the return type of a function.
The variable declarations must not have initializations.
Values without a unary plus or minus are considered to be unsigned.
e.g. 1 +2 -3
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.
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.
if (a==b)
{
// nothing
}
else
{
a=b;
}
The expression is executed, including any side effects.
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.
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.
This invokes the function, passing to it the values of the expressions as actual parameters.
treelang is run as part of the GCC compiler.
gdb).
ld command. However, the gcc command, as with most
compiler commands, automatically performs the linking step by calling on
ld directly, unless asked to not do so by the user.)
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.
(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.)
treelang consists of several components:
gcc command, which also might be
installed as the system's cc command.
(In many cases, cc refers to the
system's “native” C compiler, which
might be a non-GNU compiler, or an older version
of GCC considered more stable or that is
used to build the operating system kernel.)
treelang command itself.
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
treelang compilation phase. This is the same library that the
main C compiler uses (libc).
tree1.
Note that tree1 does not generate machine code directly—it
generates assembly code that is a more readable form
of machine code, leaving the conversion to actual machine code
to an assembler, usually named as.
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, gcc determines which actions to perform, including
preprocessing, compiling (in a variety of possible languages), assembling,
and linking.
For example, the command ‘gcc foo.c’ drives the file
foo.c through the preprocessor cpp, then
the C compiler (internally named
cc1), then the assembler (usually as), then the linker
(ld), producing an executable program named a.out (on
UNIX systems).
As another example, the command ‘gcc foo.tree’ would do much the
same as ‘gcc foo.c’, but instead of using the C compiler named
cc1, gcc would use the treelang compiler (named
tree1). However there is no preprocessor for treelang.
In a GNU Treelang installation, gcc recognizes Treelang source
files by name just like it does C and C++ source files. It knows to use
the Treelang compiler named tree1, instead of cc1 or
cc1plus, to compile Treelang files. If a file's name ends in
.tree then GCC knows that the program is written in treelang. You
can also manually override the language.
Non-Treelang-related operation of gcc is generally
unaffected by installing the GNU Treelang version of gcc.
However, without the installed version of gcc being the
GNU Treelang version, gcc will not be able to compile
and link Treelang programs.
The command ‘gcc -v x.tree’ where ‘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 tree1 program represents most of what is unique to GNU
Treelang; tree1 is a combination of two rather large chunks of
code.
One chunk is the so-called 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 cc1,
cc1plus, and 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.
The other chunk of 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 Treelang Front End (TFE). The cc1 and
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 ‘-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).
A GNU Treelang installation includes a modified version of the gcc
command.
In a non-Treelang installation, gcc recognizes C, C++,
and Objective-C source files.
In a GNU Treelang installation, gcc also recognizes Treelang source
files and accepts Treelang-specific command-line options, plus some
command-line options that are designed to cater to Treelang users
but apply to other languages as well.
See Programming Languages Supported by GCC,
for information on the way different languages are handled
by the GCC compiler (gcc).
You can use this, combined with the output of the ‘gcc -v x.tree’ command to get the options applicable to treelang. Treelang programs must end with the suffix ‘.tree’.
Treelang programs are not by default run through the C
preprocessor by gcc. There is no reason why they cannot be run through the
preprocessor manually, but you would need to prevent the preprocessor
from generating #line directives, using the ‘-P’ option, otherwise
tree1 will not accept the input.
The GNU Treelang compiler, treelang, supports programs written
in the GNU Treelang language.
The output of treelang programs looks like C program code to the linker and everybody else, so you should be able to freely mix treelang and C (and C++) code, with one proviso.
C promotes small integer types to 'int' when used as function parameters and return values in non-prototyped functions. Since treelang has no non-prototyped functions, the treelang compiler does not do this.
To create a compiler that integrates into GCC, you need create many files. Some of the files are integrated into the main GCC makefile, to build the various parts of the compiler and to run the test suite. Others are incorporated into various GCC programs such as gcc.c. Finally you must provide the actual programs comprising your compiler.
This need not be present if the language is incorporated into the main GCC tree, as the main GCC directory has this file.
This need not be present if the language is incorporated into the main GCC tree, as the main GCC directory has this file.
Makefile in turn is the main instruction to actually build everything. The build instructions are held in the main GCC manual and web site so they are not repeated here.
There are some comments at the top which will help you understand what you need to do.
There are make commands to build things, remove generated files with various degrees of thoroughness, count the lines of code (so you know how much progress you are making), build info and html files from the texinfo source, run the tests etc.
The GCC compiler consists of a driver, which then executes the various compiler phases based on the instructions in the specs files.
Typically a program's language will be identified from its suffix (e.g., .tree) for treelang programs.
The driver (gcc.c) will then drive (exec) in turn a preprocessor, the main compiler, the assembler and the link editor. Options to GCC allow you to override all of this. In the case of treelang programs there is no preprocessor, and mostly these days the C preprocessor is run within the main C compiler rather than as a separate process, apparently for reasons of speed.
You will be using the standard assembler and linkage editor so these are ignored from now on.
You have to write your own preprocessor if you want one. This is usually totally language specific. The main point to be aware of is to ensure that you find some way to pass file name and line number information through to the main compiler so that it can tell the back end this information and so the debugger can find the right source line for each piece of code. That is all there is to say about the preprocessor except that the preprocessor will probably not be the slowest part of the compiler and will probably not use the most memory so don't waste too much time tuning it until you know you need to do so.
The main compiler for treelang consists of toplev.c from the main GCC compiler, the parser, lexer and back end interface routines, and the back end routines themselves, of which there are many.
toplev.c does a lot of work for you and you should almost certainly use it.
Writing this code is the hard part of creating a compiler using GCC. The back end interface documentation is incomplete and the interface is complex.
There are three main aspects to interfacing to the other GCC code.
In treelang this is handled mainly in tree1.c and partly in treetree.c. Peruse toplev.c for details of what you need to do.
Interfacing to the garbage collection. In treelang this is mainly in tree1.c.
Memory allocation in the compiler should be done using the ggc_alloc and kindred routines in ggc*.*. At the end of every 'function' in your language, toplev.c calls the garbage collection several times. The garbage collection calls mark routines which go through the memory which is still used, telling the garbage collection not to free it. Then all the memory not used is freed.
What this means is that you need a way to hook into this marking process. This is done by calling ggc_add_root. This provides the address of a callback routine which will be called duing garbage collection and which can call ggc_mark to save the storage. If storage is only used within the parsing of a function, you do not need to provide a way to mark it.
Note that you can also call ggc_mark_tree to mark any of the back end internal 'tree' nodes. This routine will follow the branches of the trees and mark all the subordinate structures. This is useful for example when you have created a variable declaration that will be used across multiple functions, or for a function declaration (from a prototype) that may be used later on. See the next item for more on the tree nodes.
In treelang this is done in treetree.c. A typedef called 'tree' which is defined in tree.h and tree.def in the GCC directory and largely implemented in tree.c and stmt.c forms the basic interface to the compiler back end.
In general you call various tree routines to generate code, either directly or through toplev.c. You build up data structures and expressions in similar ways.
You can read some documentation on this which can be found via the GCC main web page. In particular, the documentation produced by Joachim Nadler and translated by Tim Josling can be quite useful. the C compiler also has documentation in the main GCC manual (particularly the current CVS version) which is useful on a lot of the details.
In time it is hoped to enhance this document to provide a more comprehensive overview of this topic. The main gap is in explaining how it all works together.
If you know GCC well, please consider looking at the file treetree.c and resolving any questions marked "???".
You can report bugs to gcc-bugs@gcc.gnu.org or bug-gcc@gnu.org. Please make sure bugs are real before reporting them. Follow the guidelines in the main GCC manual for submitting bug reports.
If you would like to write bug fixes or improvements for the GNU Treelang compiler, that is very helpful. Send suggested fixes to gcc-patches@gcc.gnu.org.
If you need help installing, using or changing GNU Treelang, there are two ways to find it:
If you want to contribute to treelang by doing research,
design, specification, documentation, coding, or testing,
the following information should give you some ideas.
Send a message to gcc@gcc.gnu.org if you plan to add a feature.
The main requirement for treelang is to add features and to add documentation. Features are things that the GCC back end can do but which are not reflected in treelang. Examples include structures, unions, pointers, arrays.
as command: Compiler Overviewas: Compiler Overviewgcc: TREELANG and GCCgcc: Compiler Overviewgdb: Compiler Overviewld: Compiler Overviewtreelang: Compiler Overviewtreelang: Compiler OverviewGCC, back end: Compiler Overviewgcc, command: TREELANG and GCCgcc, command: Compiler Overviewgcc, command as driver: Compiler Overviewgcc, not recognizing Treelang source: Compiler Overviewgdb, command: Compiler Overviewld command: Compiler Overviewtree1: Compiler Overviewtree1, program: Compiler Overviewtreelang, command: Compiler Overviewtreelang, components of: Compiler Overviewtreelang, front end: Compiler Overview