Compiler

“ We also noted that a consequence of lexical scoping is that the only code which can reference the value of a variable in a given environment is code which is closed in that environment or which receives the value as an argument; this in turn implies that a compiler can structure a run-time environment in any arbitrary fashion, because it will compile all the code which can reference that environment, and so can arrange for that code to reference it in the appropriate manner. ”

“ The first level is the fully applicative level, to which most source-language constructs are easily reduced; the second is the continuation-passing style level, which is easily reduced to machine language. We envision building a compiler in three stages: (a) reduction of a user language to basic SCHEME, whether by macros, a parser of algebraic syntax, or some other means; (b) optimization by means of SCHEME-level source-to-source transformations, and conversion to continuation-passing style; and (c) generation of code for a particular machine. ”

“ For he has what no compiler as such can have—because the moment he has it he ceases to be a compiler, and becomes an artist—the sense of grasp, the power to put his finger, and to keep it, on the central pulse and nerve of the story. ”

“ KEY WORDS (Continue on reverse side if necessary and identify by block number) compiler optimization tail-recursion code generation lambda calculus LISP lexical scoping macros continuations 18. ABSTRACT (Continue on reverse side if necessary and identify by block number) We have developed a compiler for the lexically-scoped dialect of LISP known as SCHEME. The compiler knows relatively little about specific data manipulation primitives such as arithmetic operators, but concentrates on general issues of environment and contral. ”

“ What is new, we think, is the serious proposal that a practical interpreter and compiler can be designed for a language in which such models serve as the sole definitions of these imperative constructs. ”

“ They use a technique from category theory known as factorization to isolate trivial expressions. As far as they go, their work is similar to ours; they have written a compiler for LISP code, producing output code which uses continuations. However, they indicate that they cannot interface compiled and interpreted code correctly. Moreover, while they use continuations, they do not make general use of closures, and in fact there is no clue that closures are permitted in their source language, or that functions are permissible as data objects. ”

“ The reader who wants a real feel for the techniques involved should read the entire document, invent three new SCHEME constructs and write macros for them, and then reimplement the compiler for another run-time environment. ”

“ Such a subset would only restrict the amount of information available to a compiler and would, of necessity, produce a less efficient code. The computer would produce even poorer code if forced to host on a small-target machine.
ACCEPTABILITY No language is going to be of value unless it is used. There are many tangible and intangible influences which will determine its final acceptability. The large scale acceptability of compilers is certainly the most influential, tangible asset. ”

“ Compiler writers have often simplified their job at the expense of the procedure call by adopting certain standard protocols. One of these is that the called procedure should save all registers that it uses. This is in turn often simplified to "save all registers". It is seldom the case that all of these registers actually need to be saved; indeed, in statement-oriented languages such as FORTRAN with little global optimization by the compiler, often no registers need be saved across a procedure call! ”

“ Any man who will read Sheridan, Walker and Jones, will be satisfied not only that there is no uniformity in what is called the best pronunciation, but that such attempts as have hitherto been made to ascertain and establish a standard, render it impossible there ever should be one; and that every succeeding compiler only multiplies the obstacles to the accomplishment of his own wishes. Every compiler has some peculiarities in his scheme ”

“ When the language contains all the essential power, when few can tell the difference between the base language and library definitions, and when the introduction of new data types and routines does not impact the compiler and the language standards, then there is little incentive to proliferate languages. Similarly, if type definitions are processed entirely at compiler time, and the language allows full program specification of the internal representation, there need be no penalty in run time efficiency for using defined types. ”

“ The compiled function objects will reside in MacLISP value cells, and SCHEME functions refer to each other through these cells.
The second set of SPECIAL declarations (variables whose names begin and end with a "*") specify variables used globally by RABBIT. These fall into three categories: variables containing properties of the SCHEME interpreter which are parameters for the compiler (e.g. **ARGUMENT-REGISTERS**) ; switches, primarily for debugging purposes, used to control certain compiler operations (e.g.
FUDGE*) ”

“ The large systems can support cross compilers which are much more powerful than those self-hosted on a smaller target machine. While this is not yet a universal procedure, it certainly provides a further opportunity to bring to bear tools to promote efficient programs. In such a case, the requirement on the high order language is to give as much information to the compiler as possible without overly restricting its options. ”

“ Another objection relating to speed is that the macros produce convoluted code involving the temporary creation and subsequent invocation of many closures. we feel, first of all, that the macro writer should concern himself more with producing correct code than fast code. Furthermore, convolutedness can be eliminated by a few simple optimization techniques in the compiler, to be discussed below. ”

“ They aid in understanding and determining the correctness of programs. They can also be used as additional information by the compiler in deciding what storage and allocation to use (e.g., half words may be more efficient for integers in the range 0 to 1000) ; Range specifications also offer the opportunity for the translator to automatically insert range tests for run-time or debug-time validation of the program logic. With variable ranges specified in the program, it becomes possible to perform many subscript bounds checks at compile-time. ”

“ If the practice of a few men in the capital is to be the standard, a knowlege of this must be communicated to the whole nation. Who shall [Pg 26] do this? An able compiler perhaps attempts to give this practice in a dictionary; but it is probable that the pronunciation, even at court, or on the stage, is not uniform. The compiler therefore must follow his particular friends and patrons; in which case he is sure to be opposed and the authority of his standard called in question; or he must give two pronunciations as the standard, which leaves the student in the same uncertainty as it found him. ”

“ See also {blue goo}, {gray goo}, {nanobot}.
:nasal demons: /n./ Recognized shorthand on the Usenet group comp.std.c for any unexpected behavior of a C compiler on encountering an undefined construct. During a discussion on that group in early 1992, a regular remarked "When the compiler encounters [a given undefined construct] it is legal for it to make demons fly out of your nose" (the implication is that the compiler may choose any arbitrarily bizarre way to interpret the code without violating the ANSI C standard) . ”

“ It turns out that in most programming languages (COBOL, PL/I, FORTRAN. and LISP in particular) the distinction between lexical and dynamic scoping doesn't matter most of the time anyway. We should leave the compiler free to produce the best possible code; only in cases where structures are explicitly declared to be dynamically referenced should the compiler be forced to leave them on the stack in an otherwise tail-recursive situation.
In general, procedure calls may be usefully thought of as GOTO statements which also pass parameters, and can be uniformly encoded as JUMP instructions. ”

“ A small number of optimization techniques, coupled with the treatment of function calls as GOTO statements, serve to produce code as good as that produced by more traditional compilers. The macro approach enables speedy implementation of new constructs as desired without sacrificing efficiency in the generated code. A fair amount of analysis is devoted to determining whether environments may be stack-allocated or must be heap-allocated. ”

“ The macro facility and the optimization techniques used by the compiler can be conceptually unified. The same properties which make it easy to write the macros make it easy to define optimizations correctly. In the same way that many programming constructs are defined in terms of a small, well-chosen basis set, so a large number of traditional optimization techniques fall out as special cases of the few used in RABBIT. This is no accident. ”

“ The main routine here is PSETQIFY, which takes a list of arguments (pieces of NacLISP code which will generate values when executed at run-time) and a list of corresponding registers. One of two different methods is used depending on the number of values involved, Method Z produces better code (this is obvious only when one understands the properties of the HacLISP compiler which will compile the 1'IacLISP code into PDP-10 machine language) . ”

“ An assembler or compiler prepares a machine language tape suitable for direct interpretation by the computer from a program tape in operator language. Generally speaking, one statement accepted by FRAP produces one instruction for the machine. A single statement written for the PDP-3 compiler, DECAL-3, may cause several instructions to be written. ”

“ Procedurally defined data structures may compile into code as good as would be expected for data defined by the more usual declarative means. Lambda-calculus-theoretic models of such constructs as GOTO, DO loops, call-by-name, etc. may be used directly as macros, the expansion of which may then compile into code as good as that produced by compilers which are designed especially to handle GOTO, DO, etc. The necessary characteristics of such a compiler designed according to this philosophy are discussed. Such a compiler is to be built in the near future as a testing ground for these ideas. ”

“ The practice of translating {hot spot}s from an {HLL} into hand-tuned assembler, as opposed to trying to coerce the compiler into generating better code. Both the term and the practice are becoming uncommon. See {tune}, {bum}, {by hand}; syn. with /v./ {cruft}. 2. More generally, manual construction or patching of data sets that would normally be generated by a translation utility and interpreted by another program, and aren't really designed to be read or modified by humans. ”

“ Within the compiler an environment structure is also represented as a simple list, with the name of a variable occupying the position which its value will occupy in the run-time environment.
For every CLANBDA, CONTINUATION, and CLABELS, a slot called CONSENV is filled in, which is a list representing what the environment structure will look like when the closure (s) for that construct are to be constructed, if any. This is done by walking over the cnode-tree and doing to the environment representation precisely what will be done to the real environment at run time. ”

“ The second DECLARE form defines to the MacLISP compiler a function called DISPLACE for obscure reasons having to do with the implementation of SCHEME macros.
TRANSDUCE does the primary work of processing the input file. when it is done, another timestamp is printed to the output file, so that the real time consumed can be determined; then the runtime statistics are calculated and printed, along with the number of errors if any occurred. The output file is then renamed as "firstname LISP" and closed. ”

“ Many older processor architectures suffer from a serious shortage of general-purpose registers. This is especially a problem for compiler-writers, because their generated code needs places to store temporaries for things like intermediate values in expression evaluation. ”

“ The language is relatively machine independent, representing possible logical constructs rather than specific hardware function, and is translated by a compiler into machine code to be run on a target machine. ”

“ The difficult question of the extent to which a compiler may utilize the materials of another has come especially to the front in the American courts with reference to law digests and reports, within recent years. ”

“ Traditionally, the first program a C coder is supposed to write in a new environment is one that just prints "hello, world" to standard output (and indeed it is the first example program in K&R) . Environments that generate an unreasonably large executable for this trivial test or which require a hairy compiler-linker invocation to generate it are considered to lose (see X) . 3. Greeting uttered by a hacker making an entrance or requesting information from anyone present. "Hello, world! Is the LAN back up yet?"
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