<s>
In	O
computing	O
,	O
a	O
compiler	B-Language
is	O
a	O
computer	B-Application
program	I-Application
that	O
transforms	O
source	O
code	O
written	O
in	O
a	O
programming	O
language	O
or	O
computer	O
language	O
(	O
the	O
source	O
language	O
)	O
,	O
into	O
another	O
computer	O
language	O
(	O
the	O
target	O
language	O
,	O
often	O
having	O
a	O
binary	O
form	O
known	O
as	O
object	B-Language
code	I-Language
or	O
machine	B-Language
code	I-Language
)	O
.	O
</s>
<s>
The	O
most	O
common	O
reason	O
for	O
transforming	O
source	O
code	O
is	O
to	O
create	O
an	O
executable	B-Application
program	I-Application
.	O
</s>
<s>
Any	O
program	B-Application
written	O
in	O
a	O
high-level	B-Language
programming	I-Language
language	I-Language
must	O
be	O
translated	O
to	O
object	B-Language
code	I-Language
before	O
it	O
can	O
be	O
executed	O
,	O
so	O
all	O
programmers	O
using	O
such	O
a	O
language	O
use	O
a	O
compiler	B-Language
or	O
an	O
interpreter	B-Application
.	O
</s>
<s>
Thus	O
,	O
compilers	B-Language
are	O
very	O
important	O
to	O
programmers	O
.	O
</s>
<s>
Improvements	O
to	O
a	O
compiler	B-Language
may	O
lead	O
to	O
a	O
large	O
number	O
of	O
improved	O
features	O
in	O
executable	B-Application
programs	I-Application
.	O
</s>
<s>
The	O
Production	B-Application
Quality	I-Application
Compiler-Compiler	I-Application
,	O
in	O
the	O
late	O
1970s	O
,	O
introduced	O
the	O
principles	O
of	O
compiler	B-Language
organization	O
that	O
are	O
still	O
widely	O
used	O
today	O
(	O
e.g.	O
,	O
a	O
front-end	O
handling	O
syntax	O
and	O
semantics	O
and	O
a	O
back-end	O
generating	O
machine	B-Language
code	I-Language
)	O
.	O
</s>
<s>
Software	O
for	O
early	O
computers	O
was	O
primarily	O
written	O
in	O
assembly	B-Language
language	I-Language
,	O
and	O
before	O
that	O
directly	O
in	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
It	O
is	O
usually	O
more	O
productive	O
for	O
a	O
programmer	O
to	O
use	O
a	O
high-level	B-Language
language	I-Language
,	O
and	O
programs	O
written	O
in	O
a	O
high-level	B-Language
language	I-Language
can	O
be	O
reused	O
on	B-Architecture
different	I-Architecture
kinds	I-Architecture
of	I-Architecture
computers	I-Architecture
.	O
</s>
<s>
Even	O
so	O
,	O
it	O
took	O
a	O
while	O
for	O
compilers	B-Language
to	O
become	O
established	O
,	O
because	O
they	O
generated	O
code	O
that	O
did	O
not	O
perform	O
as	O
well	O
as	O
hand-written	O
assembler	B-Language
,	O
they	O
were	O
daunting	O
development	O
projects	O
in	O
their	O
own	O
right	O
,	O
and	O
the	O
very	O
limited	O
memory	B-General_Concept
capacity	O
of	O
early	O
computers	O
created	O
many	O
technical	O
problems	O
for	O
practical	O
compiler	B-Language
implementations	O
.	O
</s>
<s>
The	O
first	O
practical	O
compiler	B-Language
was	O
written	O
by	O
Corrado	O
Böhm	O
in	O
1951	O
for	O
his	O
PhD	O
thesis	O
,	O
one	O
of	O
the	O
first	O
computer	O
science	O
doctorates	O
awarded	O
anywhere	O
in	O
the	O
world	O
.	O
</s>
<s>
The	O
first	O
implemented	O
compiler	B-Language
was	O
written	O
by	O
Grace	O
Hopper	O
,	O
who	O
also	O
coined	O
the	O
term	O
"	O
compiler	B-Language
"	O
,	O
referring	O
to	O
her	O
A-0	B-Language
system	I-Language
which	O
functioned	O
as	O
a	O
loader	O
or	O
linker	B-Application
,	O
not	O
the	O
modern	O
notion	O
of	O
a	O
compiler	B-Language
.	O
</s>
<s>
The	O
first	O
Autocode	B-Language
and	O
compiler	B-Language
in	O
the	O
modern	O
sense	O
were	O
developed	O
by	O
Alick	O
Glennie	O
in	O
1952	O
at	O
the	O
University	O
of	O
Manchester	O
for	O
the	O
Mark	B-Device
1	I-Device
computer	O
.	O
</s>
<s>
The	O
FORTRAN	B-Application
team	O
led	O
by	O
John	O
W	O
.	O
Backus	O
at	O
IBM	O
introduced	O
the	O
first	O
commercially	O
available	O
compiler	B-Language
,	O
in	O
1957	O
,	O
which	O
took	O
18	O
person-years	O
to	O
create	O
.	O
</s>
<s>
The	O
first	O
ALGOL	B-Language
58	I-Language
compiler	B-Language
was	O
completed	O
by	O
the	O
end	O
of	O
1958	O
by	O
Friedrich	O
L	O
.	O
Bauer	O
,	O
Hermann	O
Bottenbruch	O
,	O
Heinz	O
Rutishauser	O
,	O
and	O
Klaus	O
Samelson	O
for	O
the	O
Z22	B-Device
computer	O
.	O
</s>
<s>
had	O
been	O
working	O
on	O
compiler	B-Language
technology	O
for	O
the	O
Sequentielle	O
Formelübersetzung	O
(	O
i.e.	O
</s>
<s>
sequential	O
formula	B-Application
translation	I-Application
)	O
in	O
the	O
previous	O
years	O
.	O
</s>
<s>
By	O
1960	O
,	O
an	O
extended	O
Fortran	B-Application
compiler	B-Language
,	O
ALTAC	O
,	O
was	O
available	O
on	O
the	O
Philco	O
2000	O
,	O
so	O
it	O
is	O
probable	O
that	O
a	O
Fortran	B-Application
program	B-Application
was	O
compiled	B-Language
for	O
both	O
IBM	O
and	O
Philco	O
computer	B-General_Concept
architectures	I-General_Concept
in	O
mid-1960	O
.	O
</s>
<s>
The	O
first	O
known	O
demonstrated	O
cross-platform	B-Operating_System
high-level	B-Language
language	I-Language
was	O
COBOL	B-Application
.	O
</s>
<s>
In	O
a	O
demonstration	O
in	O
December	O
1960	O
,	O
a	O
COBOL	B-Application
program	B-Application
was	O
compiled	B-Language
and	O
executed	O
on	O
both	O
the	O
UNIVAC	B-Device
II	I-Device
and	O
the	O
RCA	O
501	O
.	O
</s>
<s>
Like	O
any	O
other	O
software	O
,	O
there	O
are	O
benefits	O
from	O
implementing	O
a	O
compiler	B-Language
in	O
a	O
high-level	B-Language
language	I-Language
.	O
</s>
<s>
In	O
particular	O
,	O
a	O
compiler	B-Language
can	O
be	O
self-hosted	O
–	O
that	O
is	O
,	O
written	O
in	O
the	O
programming	O
language	O
it	O
compiles	B-Language
.	O
</s>
<s>
Building	O
a	O
self-hosting	O
compiler	B-Language
is	O
a	O
bootstrapping	B-Application
problem	O
,	O
i.e.	O
</s>
<s>
the	O
first	O
such	O
compiler	B-Language
for	O
a	O
language	O
must	O
be	O
either	O
hand	O
written	O
machine	B-Language
code	I-Language
,	O
compiled	B-Language
by	O
a	O
compiler	B-Language
written	O
in	O
another	O
language	O
,	O
or	O
compiled	B-Language
by	O
running	O
the	O
compiler	B-Language
's	O
source	O
on	O
itself	O
in	O
an	O
interpreter	B-Application
.	O
</s>
<s>
Corrado	O
Böhm	O
developed	O
a	O
language	O
,	O
a	O
machine	O
,	O
and	O
a	O
translation	O
method	O
for	O
compiling	B-Language
that	O
language	O
on	O
the	O
machine	O
in	O
his	O
PhD	O
dissertation	O
dated	O
1951	O
.	O
</s>
<s>
He	O
not	O
only	O
described	O
a	O
complete	O
compiler	B-Language
,	O
but	O
also	O
defined	O
for	O
the	O
first	O
time	O
that	O
compiler	B-Language
in	O
its	O
own	O
language	O
.	O
</s>
<s>
The	O
Navy	B-Language
Electronics	I-Language
Laboratory	I-Language
International	I-Language
ALGOL	I-Language
Compiler	I-Language
or	O
NELIAC	B-Language
was	O
a	O
dialect	O
and	O
compiler	B-Language
implementation	O
of	O
the	O
ALGOL	B-Language
58	I-Language
programming	I-Language
language	I-Language
developed	O
by	O
the	O
Naval	O
Electronics	O
Laboratory	O
in	O
1958	O
.	O
</s>
<s>
NELIAC	B-Language
was	O
the	O
brainchild	O
of	O
Harry	O
Huskey	O
–	O
then	O
Chairman	O
of	O
the	O
ACM	O
and	O
a	O
well	O
known	O
computer	O
scientist	O
(	O
and	O
later	O
academic	O
supervisor	O
of	O
Niklaus	O
Wirth	O
)	O
,	O
and	O
supported	O
by	O
Maury	O
Halstead	O
,	O
the	O
head	O
of	O
the	O
computational	O
center	O
at	O
NEL	O
.	O
</s>
<s>
The	O
earliest	O
version	O
was	O
implemented	O
on	O
the	O
prototype	O
USQ-17	B-Device
computer	O
(	O
called	O
the	O
Countess	O
)	O
at	O
the	O
laboratory	O
.	O
</s>
<s>
It	O
was	O
the	O
world	O
's	O
first	O
self-compiling	O
compiler	B-Language
–	O
the	O
compiler	B-Language
was	O
first	O
coded	O
in	O
simplified	O
form	O
in	O
assembly	B-Language
language	I-Language
(	O
the	O
bootstrap	O
)	O
,	O
then	O
re-written	O
in	O
its	O
own	O
language	O
and	O
compiled	B-Language
by	O
the	O
bootstrap	O
,	O
and	O
finally	O
re-compiled	O
by	O
itself	O
,	O
making	O
the	O
bootstrap	O
obsolete	O
.	O
</s>
<s>
Another	O
early	O
self-hosting	O
compiler	B-Language
was	O
written	O
for	O
Lisp	B-Language
by	O
Tim	O
Hart	O
and	O
Mike	O
Levin	O
at	O
MIT	O
in	O
1962	O
.	O
</s>
<s>
They	O
wrote	O
a	O
Lisp	B-Language
compiler	B-Language
in	O
Lisp	B-Language
,	O
testing	O
it	O
inside	O
an	O
existing	O
Lisp	B-Language
interpreter	B-Application
.	O
</s>
<s>
Once	O
they	O
had	O
improved	O
the	O
compiler	B-Language
to	O
the	O
point	O
where	O
it	O
could	O
compile	B-Language
its	O
own	O
source	O
code	O
,	O
it	O
was	O
self-hosting	O
.	O
</s>
<s>
The	O
compiler	B-Language
as	O
it	O
exists	O
on	O
the	O
standard	O
compiler	B-Language
tape	O
is	O
a	O
machine	B-Language
language	I-Language
program	B-Application
that	O
was	O
obtained	O
by	O
having	O
the	O
S-expression	B-Protocol
definition	O
of	O
the	O
compiler	B-Language
work	O
on	O
itself	O
through	O
the	O
interpreter	B-Application
.	O
</s>
<s>
This	O
technique	O
is	O
only	O
possible	O
when	O
an	O
interpreter	B-Application
already	O
exists	O
for	O
the	O
very	O
same	O
language	O
that	O
is	O
to	O
be	O
compiled	B-Language
.	O
</s>
<s>
It	O
borrows	O
directly	O
from	O
the	O
notion	O
of	O
running	O
a	O
program	B-Application
on	O
itself	O
as	O
input	O
,	O
which	O
is	O
also	O
used	O
in	O
various	O
proofs	O
in	O
theoretical	O
computer	O
science	O
,	O
such	O
as	O
the	O
proof	O
that	O
the	O
halting	O
problem	O
is	O
undecidable	O
.	O
</s>
<s>
Forth	B-Application
is	O
an	O
example	O
of	O
a	O
self-hosting	O
compiler	B-Language
.	O
</s>
<s>
The	O
self	O
compilation	B-Language
and	O
cross	B-Application
compilation	I-Application
features	O
of	O
Forth	B-Application
are	O
synonymous	O
with	O
metacompilation	B-Application
and	O
metacompilers	B-Language
.	O
</s>
<s>
Like	O
Lisp	B-Language
,	O
Forth	B-Application
is	O
an	O
extensible	B-Application
programming	I-Application
language	I-Application
.	O
</s>
<s>
It	O
is	O
the	O
extensible	B-Application
programming	I-Application
language	I-Application
features	O
of	O
Forth	B-Application
and	O
Lisp	B-Language
that	O
enable	O
them	O
to	O
generate	O
new	O
versions	O
of	O
themselves	O
or	O
port	O
themselves	O
to	O
new	O
environments	O
.	O
</s>
<s>
A	O
parser	B-Language
is	O
an	O
important	O
component	O
of	O
a	O
compiler	B-Language
.	O
</s>
<s>
It	O
parses	B-Language
the	O
source	O
code	O
of	O
a	O
computer	O
programming	O
language	O
to	O
create	O
some	O
form	O
of	O
internal	O
representation	O
.	O
</s>
<s>
Programming	O
languages	O
tend	O
to	O
be	O
specified	O
in	O
terms	O
of	O
a	O
context-free	O
grammar	O
because	O
fast	O
and	O
efficient	O
parsers	B-Language
can	O
be	O
written	O
for	O
them	O
.	O
</s>
<s>
Parsers	B-Language
can	O
be	O
written	O
by	O
hand	O
or	O
generated	O
by	O
a	O
parser	B-Language
generator	I-Language
.	O
</s>
<s>
Block	O
structure	O
was	O
introduced	O
into	O
computer	O
programming	O
languages	O
by	O
the	O
ALGOL	B-Language
project	O
(	O
1957	O
–	O
1960	O
)	O
,	O
which	O
,	O
as	O
a	O
consequence	O
,	O
also	O
featured	O
a	O
context-free	O
grammar	O
to	O
describe	O
the	O
resulting	O
ALGOL	B-Language
syntax	O
.	O
</s>
<s>
Context-free	O
grammars	O
are	O
simple	O
enough	O
to	O
allow	O
the	O
construction	O
of	O
efficient	O
parsing	B-Language
algorithms	O
which	O
,	O
for	O
a	O
given	O
string	O
,	O
determine	O
whether	O
and	O
how	O
it	O
can	O
be	O
generated	O
from	O
the	O
grammar	O
.	O
</s>
<s>
If	O
a	O
programming	O
language	O
designer	O
is	O
willing	O
to	O
work	O
within	O
some	O
limited	O
subsets	O
of	O
context-free	O
grammars	O
,	O
more	O
efficient	O
parsers	B-Language
are	O
possible	O
.	O
</s>
<s>
The	O
LR	B-Application
parser	I-Application
(	O
left	O
to	O
right	O
)	O
was	O
invented	O
by	O
Donald	O
Knuth	O
in	O
1965	O
in	O
a	O
paper	O
,	O
"	O
On	O
the	O
Translation	O
of	O
Languages	O
from	O
Left	O
to	O
Right	O
"	O
.	O
</s>
<s>
An	O
LR	B-Application
parser	I-Application
is	O
a	O
parser	B-Language
that	O
reads	O
input	O
from	O
Left	O
to	O
right	O
(	O
as	O
it	O
would	O
appear	O
if	O
visually	O
displayed	O
)	O
and	O
produces	O
a	O
Rightmost	O
derivation	O
.	O
</s>
<s>
The	O
term	O
LR(k )	O
parser	B-Language
is	O
also	O
used	O
,	O
where	O
k	O
refers	O
to	O
the	O
number	O
of	O
unconsumed	O
lookahead	O
input	O
symbols	O
that	O
are	O
used	O
in	O
making	O
parsing	B-Language
decisions	O
.	O
</s>
<s>
Knuth	O
proved	O
that	O
LR(k )	O
grammars	O
can	O
be	O
parsed	B-Language
with	O
an	O
execution	O
time	O
essentially	O
proportional	O
to	O
the	O
length	O
of	O
the	O
program	B-Application
,	O
and	O
that	O
every	O
LR(k )	O
grammar	O
for	O
k1	O
can	O
be	O
mechanically	O
transformed	O
into	O
an	O
LR(1 )	O
grammar	O
for	O
the	O
same	O
language	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
it	O
is	O
only	O
necessary	O
to	O
have	O
one	O
symbol	O
lookahead	O
to	O
parse	B-Language
any	O
deterministic	O
context-free	O
grammar	O
(	O
DCFG	O
)	O
.	O
</s>
<s>
Korenjak	O
(	O
1969	O
)	O
was	O
the	O
first	O
to	O
show	O
parsers	B-Language
for	O
programming	O
languages	O
could	O
be	O
produced	O
using	O
these	O
techniques	O
.	O
</s>
<s>
Frank	O
DeRemer	O
devised	O
the	O
more	O
practical	O
Simple	B-Application
LR	I-Application
(	O
SLR	O
)	O
and	O
Look-ahead	B-Application
LR	I-Application
(	O
LALR	B-Application
)	O
techniques	O
,	O
published	O
in	O
his	O
PhD	O
dissertation	O
at	O
MIT	O
in	O
1969	O
.	O
</s>
<s>
In	O
practice	O
,	O
LALR	B-Application
offers	O
a	O
good	O
solution	O
;	O
the	O
added	O
power	O
of	O
LALR(1 )	O
parsers	B-Language
over	O
SLR(1 )	O
parsers	B-Language
(	O
that	O
is	O
,	O
LALR(1 )	O
can	O
parse	B-Language
more	O
complex	O
grammars	O
than	O
SLR(1 )	O
)	O
is	O
useful	O
,	O
and	O
,	O
though	O
LALR(1 )	O
is	O
not	O
comparable	O
with	O
LL(1 )	O
(	O
See	O
below	O
)	O
(LALR(1 )	O
cannot	O
parse	B-Language
all	O
LL(1 )	O
grammars	O
)	O
,	O
most	O
LL(1 )	O
grammars	O
encountered	O
in	O
practice	O
can	O
be	O
parsed	B-Language
by	O
LALR(1 )	O
.	O
</s>
<s>
LR(1 )	O
grammars	O
are	O
more	O
powerful	O
again	O
than	O
LALR(1 )	O
;	O
however	O
,	O
an	O
LR(1 )	O
grammar	O
requires	O
a	O
canonical	B-Application
LR	I-Application
parser	I-Application
which	O
would	O
be	O
extremely	O
large	O
in	O
size	O
and	O
is	O
not	O
considered	O
practical	O
.	O
</s>
<s>
The	O
syntax	O
of	O
many	O
programming	O
languages	O
are	O
defined	O
by	O
grammars	O
that	O
can	O
be	O
parsed	B-Language
with	O
an	O
LALR(1 )	O
parser	B-Language
,	O
and	O
for	O
this	O
reason	O
LALR	B-Application
parsers	I-Application
are	O
often	O
used	O
by	O
compilers	B-Language
to	O
perform	O
syntax	B-Language
analysis	I-Language
of	O
source	O
code	O
.	O
</s>
<s>
A	O
recursive	B-Application
ascent	I-Application
parser	I-Application
implements	O
an	O
LALR	B-Application
parser	I-Application
using	O
mutually-recursive	O
functions	O
rather	O
than	O
tables	O
.	O
</s>
<s>
Thus	O
,	O
the	O
parser	B-Language
is	O
directly	O
encoded	O
in	O
the	O
host	O
language	O
similar	O
to	O
recursive	B-Application
descent	I-Application
.	O
</s>
<s>
Direct	O
encoding	O
usually	O
yields	O
a	O
parser	B-Language
which	O
is	O
faster	O
than	O
its	O
table-driven	O
equivalent	O
for	O
the	O
same	O
reason	O
that	O
compilation	B-Language
is	O
faster	O
than	O
interpretation	O
.	O
</s>
<s>
It	O
is	O
also	O
(	O
in	O
principle	O
)	O
possible	O
to	O
hand	O
edit	O
a	O
recursive	B-Application
ascent	I-Application
parser	I-Application
,	O
whereas	O
a	O
tabular	O
implementation	O
is	O
nigh	O
unreadable	O
to	O
the	O
average	O
human	O
.	O
</s>
<s>
Recursive	B-Application
ascent	I-Application
was	O
first	O
described	O
by	O
Thomas	O
Pennello	O
in	O
his	O
article	O
"	O
Very	O
fast	O
LR	B-Application
parsing	I-Application
"	O
in	O
1986	O
.	O
</s>
<s>
An	O
LL	B-Language
parser	I-Language
parses	B-Language
the	O
input	O
from	O
Left	O
to	O
right	O
,	O
and	O
constructs	O
a	O
Leftmost	O
derivation	O
of	O
the	O
sentence	O
(	O
hence	O
LL	O
,	O
as	O
opposed	O
to	O
LR	O
)	O
.	O
</s>
<s>
The	O
class	O
of	O
grammars	O
which	O
are	O
parsable	B-Language
in	O
this	O
way	O
is	O
known	O
as	O
the	O
LL	O
grammars	O
.	O
</s>
<s>
LL	O
grammars	O
are	O
an	O
even	O
more	O
restricted	O
class	O
of	O
context-free	O
grammars	O
than	O
LR	B-Application
grammars	I-Application
.	O
</s>
<s>
Nevertheless	O
,	O
they	O
are	O
of	O
great	O
interest	O
to	O
compiler	B-Language
writers	O
,	O
because	O
such	O
a	O
parser	B-Language
is	O
simple	O
and	O
efficient	O
to	O
implement	O
.	O
</s>
<s>
LL(k )	O
grammars	O
can	O
be	O
parsed	B-Language
by	O
a	O
recursive	B-Application
descent	I-Application
parser	I-Application
which	O
is	O
usually	O
coded	O
by	O
hand	O
,	O
although	O
a	O
notation	O
such	O
as	O
META	B-Application
II	I-Application
might	O
alternatively	O
be	O
used	O
.	O
</s>
<s>
The	O
design	O
of	O
ALGOL	B-Language
sparked	O
investigation	O
of	O
recursive	B-Application
descent	I-Application
,	O
since	O
the	O
ALGOL	B-Language
language	I-Language
itself	O
is	O
recursive	O
.	O
</s>
<s>
The	O
concept	O
of	O
recursive	B-Application
descent	I-Application
parsing	I-Application
was	O
discussed	O
in	O
the	O
January	O
1961	O
issue	O
of	O
Communications	O
of	O
the	O
ACM	O
in	O
separate	O
papers	O
by	O
A.A.	O
Grau	O
and	O
Edgar	O
T	O
.	O
"	O
Ned	O
"	O
Irons	O
.	O
</s>
<s>
Richard	O
Waychoff	O
and	O
colleagues	O
also	O
implemented	O
recursive	B-Application
descent	I-Application
in	O
the	O
Burroughs	O
ALGOL	B-Language
compiler	B-Language
in	O
March	O
1961	O
,	O
the	O
two	O
groups	O
used	O
different	O
approaches	O
but	O
were	O
in	O
at	O
least	O
informal	O
contact	O
.	O
</s>
<s>
Recursive	B-Application
descent	I-Application
was	O
popularised	O
by	O
Niklaus	O
Wirth	O
with	O
PL/0	B-Language
,	O
an	O
educational	B-Language
programming	I-Language
language	I-Language
used	O
to	O
teach	O
compiler	B-Language
construction	O
in	O
the	O
1970s	O
.	O
</s>
<s>
LR	B-Application
parsing	I-Application
can	O
handle	O
a	O
larger	O
range	O
of	O
languages	O
than	O
LL	B-Language
parsing	I-Language
,	O
and	O
is	O
also	O
better	O
at	O
error	O
reporting	O
(	O
This	O
is	O
disputable	O
,	O
REFERENCE	O
is	O
required	O
)	O
,	O
i.e.	O
</s>
<s>
In	O
1970	O
,	O
Jay	O
Earley	O
invented	O
what	O
came	O
to	O
be	O
known	O
as	O
the	O
Earley	B-Algorithm
parser	I-Algorithm
.	O
</s>
<s>
Earley	B-Algorithm
parsers	I-Algorithm
are	O
appealing	O
because	O
they	O
can	O
parse	B-Language
all	O
context-free	O
languages	O
reasonably	O
efficiently	O
.	O
</s>
<s>
John	O
Backus	O
proposed	O
"	O
metalinguistic	O
formulas	O
"	O
to	O
describe	O
the	O
syntax	O
of	O
the	O
new	B-Language
programming	I-Language
language	I-Language
IAL	B-Language
,	O
known	O
today	O
as	O
ALGOL	B-Language
58	I-Language
(	O
1959	O
)	O
.	O
</s>
<s>
Backus	O
's	O
work	O
was	O
based	O
on	O
the	O
Post	B-Application
canonical	I-Application
system	I-Application
devised	O
by	O
Emil	O
Post	O
.	O
</s>
<s>
Further	O
development	O
of	O
ALGOL	B-Language
led	O
to	O
ALGOL	B-Language
60	I-Language
;	O
in	O
its	O
report	O
(	O
1963	O
)	O
,	O
Peter	O
Naur	O
named	O
Backus	O
's	O
notation	O
Backus	O
normal	O
form	O
(	O
BNF	O
)	O
,	O
and	O
simplified	O
it	O
to	O
minimize	O
the	O
character	O
set	O
used	O
.	O
</s>
<s>
Niklaus	O
Wirth	O
defined	O
extended	O
Backus	O
–	O
Naur	O
form	O
(	O
EBNF	O
)	O
,	O
a	O
refined	O
version	O
of	O
BNF	O
,	O
in	O
the	O
early	O
1970s	O
for	O
PL/0	B-Language
.	O
</s>
<s>
Both	O
EBNF	O
and	O
ABNF	O
are	O
widely	O
used	O
to	O
specify	O
the	O
grammar	O
of	O
programming	O
languages	O
,	O
as	O
the	O
inputs	O
to	O
parser	B-Language
generators	I-Language
,	O
and	O
in	O
other	O
fields	O
such	O
as	O
defining	O
communication	O
protocols	O
.	O
</s>
<s>
A	O
parser	B-Language
generator	I-Language
generates	O
the	O
lexical-analyser	O
portion	O
of	O
a	O
compiler	B-Language
.	O
</s>
<s>
It	O
is	O
a	O
program	B-Application
that	O
takes	O
a	O
description	O
of	O
a	O
formal	O
grammar	O
of	O
a	O
specific	O
programming	O
language	O
and	O
produces	O
a	O
parser	B-Language
for	O
that	O
language	O
.	O
</s>
<s>
That	O
parser	B-Language
can	O
be	O
used	O
in	O
a	O
compiler	B-Language
for	O
that	O
specific	O
language	O
.	O
</s>
<s>
The	O
parser	B-Language
detects	O
and	O
identifies	O
the	O
reserved	O
words	O
and	O
symbols	O
of	O
the	O
specific	O
language	O
from	O
a	O
stream	O
of	O
text	O
and	O
returns	O
these	O
as	O
tokens	O
to	O
the	O
code	O
which	O
implements	O
the	O
syntactic	O
validation	O
and	O
translation	O
into	O
object	B-Language
code	I-Language
.	O
</s>
<s>
This	O
second	O
part	O
of	O
the	O
compiler	B-Language
can	O
also	O
be	O
created	O
by	O
a	O
compiler-compiler	B-Language
using	O
a	O
formal	O
rules-of-precedence	O
syntax-description	O
as	O
input	O
.	O
</s>
<s>
The	O
first	O
compiler-compiler	B-Language
to	O
use	O
that	O
name	O
was	O
written	O
by	O
Tony	O
Brooker	O
in	O
1960	O
and	O
was	O
used	O
to	O
create	O
compilers	B-Language
for	O
the	O
Atlas	B-Device
computer	I-Device
at	O
the	O
University	O
of	O
Manchester	O
,	O
including	O
the	O
Atlas	B-Device
Autocode	B-Language
compiler	B-Language
.	O
</s>
<s>
However	O
it	O
was	O
rather	O
different	O
from	O
modern	O
compiler-compilers	B-Language
,	O
and	O
today	O
would	O
probably	O
be	O
described	O
as	O
being	O
somewhere	O
between	O
a	O
highly	O
customisable	O
generic	O
compiler	B-Language
and	O
an	O
extensible-syntax	B-Application
language	I-Application
.	O
</s>
<s>
The	O
name	O
"	O
compiler-compiler	B-Language
"	O
was	O
far	O
more	O
appropriate	O
for	O
Brooker	O
's	O
system	O
than	O
it	O
is	O
for	O
most	O
modern	O
compiler-compilers	B-Language
,	O
which	O
are	O
more	O
accurately	O
described	O
as	O
parser	B-Language
generators	I-Language
.	O
</s>
<s>
It	O
is	O
almost	O
certain	O
that	O
the	O
"	O
Compiler-Compiler	B-Language
"	O
name	O
has	O
entered	O
common	O
use	O
due	O
to	O
Yacc	B-Application
rather	O
than	O
Brooker	O
's	O
work	O
being	O
remembered	O
.	O
</s>
<s>
In	O
the	O
early	O
1960s	O
,	O
Robert	O
McClure	O
at	O
Texas	O
Instruments	O
invented	O
a	O
compiler-compiler	B-Language
called	O
TMG	B-Application
,	O
the	O
name	O
taken	O
from	O
"	O
transmogrification	O
"	O
.	O
</s>
<s>
In	O
the	O
following	O
years	O
TMG	B-Application
was	O
ported	O
to	O
several	O
UNIVAC	O
and	O
IBM	O
mainframe	O
computers	O
.	O
</s>
<s>
The	O
Multics	B-Application
project	O
,	O
a	O
joint	O
venture	O
between	O
MIT	O
and	O
Bell	O
Labs	O
,	O
was	O
one	O
of	O
the	O
first	O
to	O
develop	O
an	O
operating	B-General_Concept
system	I-General_Concept
in	O
a	O
high-level	B-Language
language	I-Language
.	O
</s>
<s>
PL/I	B-Language
was	O
chosen	O
as	O
the	O
language	O
,	O
but	O
an	O
external	O
supplier	O
could	O
not	O
supply	O
a	O
working	O
compiler	B-Language
.	O
</s>
<s>
The	O
Multics	B-Application
team	O
developed	O
their	O
own	O
subset	O
dialect	O
of	O
PL/I	B-Language
known	O
as	O
Early	O
PL/I	B-Language
(	O
EPL	O
)	O
as	O
their	O
implementation	O
language	O
in	O
1964	O
.	O
</s>
<s>
TMG	B-Application
was	O
ported	O
to	O
GE-600	B-Device
series	I-Device
and	O
used	O
to	O
develop	O
EPL	O
by	O
Douglas	O
McIlroy	O
,	O
Robert	O
Morris	O
,	O
and	O
others	O
.	O
</s>
<s>
Not	O
long	O
after	O
Ken	O
Thompson	O
wrote	O
the	O
first	O
version	O
of	O
Unix	B-Application
for	O
the	O
PDP-7	B-Device
in	O
1969	O
,	O
Douglas	O
McIlroy	O
created	O
the	O
new	O
system	O
's	O
first	O
higher-level	O
language	O
:	O
an	O
implementation	O
of	O
McClure	O
's	O
TMG	B-Application
.	O
</s>
<s>
TMG	B-Application
was	O
also	O
the	O
compiler	B-Language
definition	O
tool	O
used	O
by	O
Ken	O
Thompson	O
to	O
write	O
the	O
compiler	B-Language
for	O
the	O
B	B-Language
language	I-Language
on	O
his	O
PDP-7	B-Device
in	O
1970	O
.	O
</s>
<s>
B	B-Language
was	O
the	O
immediate	O
ancestor	O
of	O
C	B-Language
.	O
</s>
<s>
An	O
early	O
LALR	B-Application
parser	I-Application
generator	I-Application
was	O
called	O
"	O
TWS	O
"	O
,	O
created	O
by	O
Frank	O
DeRemer	O
and	O
Tom	O
Pennello	O
.	O
</s>
<s>
XPL	B-Language
is	O
a	O
dialect	O
of	O
the	O
PL/I	B-Language
programming	I-Language
language	I-Language
,	O
used	O
for	O
the	O
development	O
of	O
compilers	B-Language
for	O
computer	O
languages	O
.	O
</s>
<s>
It	O
was	O
designed	O
and	O
implemented	O
in	O
1967	O
by	O
a	O
team	O
with	O
William	O
M	O
.	O
McKeeman	O
,	O
James	O
J	O
.	O
Horning	O
,	O
and	O
David	O
B	B-Language
.	O
Wortman	O
at	O
Stanford	O
University	O
and	O
the	O
University	O
of	O
California	O
,	O
Santa	O
Cruz	O
.	O
</s>
<s>
XPL	B-Language
featured	O
a	O
relatively	O
simple	O
translator	B-Language
writing	I-Language
system	I-Language
dubbed	O
ANALYZER	O
,	O
based	O
upon	O
a	O
bottom-up	B-Application
compiler	I-Application
precedence	O
parsing	B-Language
technique	O
called	O
MSP	O
(	O
mixed	O
strategy	O
precedence	O
)	O
.	O
</s>
<s>
XPL	B-Language
was	O
bootstrapped	O
through	O
Burroughs	O
Algol	B-Language
onto	O
the	O
IBM	B-Device
System/360	I-Device
computer	O
.	O
</s>
<s>
(	O
Some	O
subsequent	O
versions	O
of	O
XPL	B-Language
used	O
on	O
University	O
of	O
Toronto	O
internal	O
projects	O
utilized	O
an	O
SLR(1 )	O
parser	B-Language
,	O
but	O
those	O
implementations	O
have	O
never	O
been	O
distributed	O
)	O
.	O
</s>
<s>
Yacc	B-Application
is	O
a	O
parser	B-Language
generator	I-Language
(	O
loosely	O
,	O
compiler-compiler	B-Language
)	O
,	O
not	O
to	O
be	O
confused	O
with	O
lex	B-General_Concept
,	O
which	O
is	O
a	O
lexical	B-Application
analyzer	I-Application
frequently	O
used	O
as	O
a	O
first	O
stage	O
by	O
Yacc	B-Application
.	O
</s>
<s>
Yacc	B-Application
was	O
developed	O
by	O
Stephen	O
C	B-Language
.	O
Johnson	O
at	O
AT&T	O
for	O
the	O
Unix	B-Application
operating	I-Application
system	I-Application
.	O
</s>
<s>
It	O
generates	O
an	O
LALR(1 )	O
compiler	B-Language
based	O
on	O
a	O
grammar	O
written	O
in	O
a	O
notation	O
similar	O
to	O
Backus	O
–	O
Naur	O
form	O
.	O
</s>
<s>
Johnson	O
worked	O
on	O
Yacc	B-Application
in	O
the	O
early	O
1970s	O
at	O
Bell	O
Labs	O
.	O
</s>
<s>
He	O
was	O
familiar	O
with	O
TMG	B-Application
and	O
its	O
influence	O
can	O
be	O
seen	O
in	O
Yacc	B-Application
and	O
the	O
design	O
of	O
the	O
C	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
Because	O
Yacc	B-Application
was	O
the	O
default	O
compiler	B-Language
generator	I-Language
on	O
most	O
Unix	B-Application
systems	I-Application
,	O
it	O
was	O
widely	O
distributed	O
and	O
used	O
.	O
</s>
<s>
Derivatives	O
such	O
as	O
GNU	B-Application
Bison	I-Application
are	O
still	O
in	O
use	O
.	O
</s>
<s>
The	O
compiler	B-Language
generated	O
by	O
Yacc	B-Application
requires	O
a	O
lexical	B-Application
analyzer	I-Application
.	O
</s>
<s>
Lexical	B-Application
analyzer	I-Application
generators	O
,	O
such	O
as	O
lex	B-General_Concept
or	O
flex	B-General_Concept
are	O
widely	O
available	O
.	O
</s>
<s>
The	O
IEEE	O
POSIX	O
P1003.2	O
standard	O
defines	O
the	O
functionality	O
and	O
requirements	O
for	O
both	O
Lex	B-General_Concept
and	O
Yacc	B-Application
.	O
</s>
<s>
Coco/R	B-Application
is	O
a	O
parser	B-Language
generator	I-Language
that	O
generates	O
LL(1 )	O
parsers	B-Language
in	O
Modula-2	O
(	O
with	O
plug-ins	O
for	O
other	O
languages	O
)	O
from	O
input	O
grammars	O
written	O
in	O
a	O
variant	O
of	O
EBNF	O
.	O
</s>
<s>
ANTLR	B-Application
is	O
a	O
parser	B-Language
generator	I-Language
that	O
generates	O
LL( *	O
)	O
parsers	B-Language
in	O
Java	O
from	O
input	O
grammars	O
written	O
in	O
a	O
variant	O
of	O
EBNF	O
.	O
</s>
<s>
Metacompilers	B-Language
differ	O
from	O
parser	B-Language
generators	I-Language
,	O
taking	O
as	O
input	O
a	O
program	B-Application
written	O
in	O
a	O
metalanguage	O
.	O
</s>
<s>
Their	O
input	O
consists	O
grammar	O
analyzing	O
formula	O
combined	O
with	O
embedded	O
transform	O
operations	O
that	O
construct	O
abstract	B-Data_Structure
syntax	I-Data_Structure
trees	I-Data_Structure
,	O
or	O
simply	O
output	O
reformatted	O
text	O
strings	O
that	O
may	O
be	O
stack	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
Many	O
can	O
be	O
programmed	O
in	O
their	O
own	O
metalanguage	O
enabling	O
them	O
to	O
compile	B-Language
themselves	O
,	O
making	O
them	O
self-hosting	O
extensible	B-Application
language	I-Application
compilers	B-Language
.	O
</s>
<s>
Many	O
metacompilers	B-Language
build	O
on	O
the	O
work	O
of	O
Dewey	O
Val	O
Schorre	O
.	O
</s>
<s>
His	O
META	B-Application
II	I-Application
compiler	B-Language
,	O
first	O
released	O
in	O
1964	O
,	O
was	O
the	O
first	O
documented	O
metacompiler	B-Language
.	O
</s>
<s>
Able	O
to	O
define	O
its	O
own	O
language	O
and	O
others	O
,	O
META	B-Application
II	I-Application
accepted	O
syntax	O
formula	O
having	O
imbedded	O
output	O
(	O
code	O
production	O
)	O
.	O
</s>
<s>
It	O
also	O
translated	O
to	O
one	O
of	O
the	O
earliest	O
instances	O
of	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Lexical	B-Application
analysis	I-Application
was	O
performed	O
by	O
built	O
token	O
recognizing	O
functions	O
:	O
.ID	O
,	O
.STRING	O
,	O
and	O
.NUMBER	O
.	O
</s>
<s>
TREE-META	B-Application
,	O
a	O
second	O
generation	O
Schorre	O
metacompiler	B-Language
,	O
appeared	O
around	O
1968	O
.	O
</s>
<s>
It	O
extended	O
the	O
capabilities	O
of	O
META	B-Application
II	I-Application
,	O
adding	O
unparse	O
rules	O
separating	O
code	O
production	O
from	O
the	O
grammar	O
analysis	O
.	O
</s>
<s>
Tree	O
transform	O
operations	O
in	O
the	O
syntax	O
formula	O
produce	O
abstract	B-Data_Structure
syntax	I-Data_Structure
trees	I-Data_Structure
that	O
the	O
unparse	O
rules	O
operate	O
on	O
.	O
</s>
<s>
CWIC	O
,	O
described	O
in	O
a	O
1970	O
ACM	O
publication	O
is	O
a	O
third	O
generation	O
Schorre	O
metacompiler	B-Language
that	O
added	O
lexing	B-Application
rules	O
and	O
backtracking	O
operators	O
to	O
the	O
grammar	O
analysis	O
.	O
</s>
<s>
LISP	B-Language
2	I-Language
was	O
married	O
with	O
the	O
unparse	O
rules	O
of	O
TREEMETA	O
in	O
the	O
CWIC	O
generator	O
language	O
.	O
</s>
<s>
With	O
LISP	B-Language
2	I-Language
processing	O
,	O
CWIC	O
can	O
generate	O
fully	O
optimized	O
code	O
.	O
</s>
<s>
CWIC	O
also	O
provided	O
binary	O
code	B-Application
generation	I-Application
into	O
named	O
code	O
sections	O
.	O
</s>
<s>
Single	O
and	O
multipass	O
compiles	B-Language
could	O
be	O
implemented	O
using	O
CWIC	O
.	O
</s>
<s>
CWIC	O
compiled	B-Language
to	O
8-bit	O
byte-addressable	O
machine	B-Language
code	I-Language
instructions	O
primarily	O
designed	O
to	O
produce	O
IBM	B-Device
System/360	I-Device
code	O
.	O
</s>
<s>
One	O
important	O
feature	O
would	O
be	O
the	O
abstraction	O
of	O
the	O
target	O
processor	O
instruction	O
set	O
,	O
generating	O
to	O
a	O
pseudo	O
machine	B-Language
instruction	I-Language
set	O
,	O
macros	O
,	O
that	O
could	O
be	O
separately	O
defined	O
or	O
mapped	O
to	O
a	O
real	O
machine	O
's	O
instructions	O
.	O
</s>
<s>
Optimizations	O
applying	O
to	O
sequential	O
instructions	O
could	O
then	O
be	O
applied	O
to	O
the	O
pseudo	O
instruction	O
before	O
their	O
expansion	O
to	O
target	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
A	O
cross	B-Application
compiler	I-Application
runs	O
in	O
one	O
environment	O
but	O
produces	O
object	B-Language
code	I-Language
for	O
another	O
.	O
</s>
<s>
Cross	B-Application
compilers	I-Application
are	O
used	O
for	O
embedded	O
development	O
,	O
where	O
the	O
target	O
computer	O
has	O
limited	O
capabilities	O
.	O
</s>
<s>
An	O
early	O
example	O
of	O
cross	B-Application
compilation	I-Application
was	O
AIMICO	O
,	O
where	O
a	O
FLOW-MATIC	O
program	B-Application
on	O
a	O
UNIVAC	B-Device
II	I-Device
was	O
used	O
to	O
generate	O
assembly	B-Language
language	I-Language
for	O
the	O
IBM	B-Device
705	I-Device
,	O
which	O
was	O
then	O
assembled	O
on	O
the	O
IBM	O
computer	O
.	O
</s>
<s>
The	O
ALGOL	B-General_Concept
68C	I-General_Concept
compiler	B-Language
generated	O
ZCODE	B-General_Concept
output	O
,	O
that	O
could	O
then	O
be	O
either	O
compiled	B-Language
into	O
the	O
local	O
machine	B-Language
code	I-Language
by	O
a	O
ZCODE	B-General_Concept
translator	O
or	O
run	O
interpreted	O
.	O
</s>
<s>
ZCODE	B-General_Concept
is	O
a	O
register-based	O
intermediate	O
language	O
.	O
</s>
<s>
This	O
ability	O
to	O
interpret	O
or	O
compile	B-Language
ZCODE	B-General_Concept
encouraged	O
the	O
porting	O
of	O
ALGOL	B-General_Concept
68C	I-General_Concept
to	O
numerous	O
different	O
computer	O
platforms	O
.	O
</s>
<s>
Compiler	B-Application
optimization	I-Application
is	O
the	O
process	O
of	O
improving	O
the	O
quality	O
of	O
object	B-Language
code	I-Language
without	O
changing	O
the	O
results	O
it	O
produces	O
.	O
</s>
<s>
The	O
developers	O
of	O
the	O
first	O
FORTRAN	B-Application
compiler	B-Language
aimed	O
to	O
generate	O
code	O
that	O
was	O
better	O
than	O
the	O
average	O
hand-coded	O
assembler	B-Language
,	O
so	O
that	O
customers	O
would	O
actually	O
use	O
their	O
product	O
.	O
</s>
<s>
In	O
one	O
of	O
the	O
first	O
real	O
compilers	B-Language
,	O
they	O
often	O
succeeded	O
.	O
</s>
<s>
Later	O
compilers	B-Language
,	O
like	O
IBM	O
's	O
Fortran	B-Application
IV	O
compiler	B-Language
,	O
placed	O
more	O
priority	O
on	O
good	O
diagnostics	O
and	O
executing	O
more	O
quickly	O
,	O
at	O
the	O
expense	O
of	O
object	B-Language
code	I-Language
optimization	O
.	O
</s>
<s>
It	O
was	O
n't	O
until	O
the	O
IBM	B-Device
System/360	I-Device
series	I-Device
that	O
IBM	O
provided	O
two	O
separate	O
compilers	B-Language
—	O
a	O
fast-executing	O
code	O
checker	O
,	O
and	O
a	O
slower	O
,	O
optimizing	O
one	O
.	O
</s>
<s>
Allen	O
's	O
1966	O
paper	O
,	O
Program	B-Application
Optimization	O
,	O
introduced	O
the	O
use	O
of	O
graph	B-Application
data	I-Application
structures	I-Application
to	O
encode	O
program	B-Application
content	O
for	O
optimization	O
.	O
</s>
<s>
Her	O
1970	O
papers	O
,	O
Control	O
Flow	O
Analysis	O
and	O
A	O
Basis	O
for	O
Program	B-Application
Optimization	O
established	O
intervals	O
as	O
the	O
context	O
for	O
efficient	O
and	O
effective	O
data	O
flow	O
analysis	O
and	O
optimization	O
.	O
</s>
<s>
Her	O
1971	O
paper	O
with	O
Cocke	O
,	O
A	O
Catalogue	O
of	O
Optimizing	B-Application
Transformations	I-Application
,	O
provided	O
the	O
first	O
description	O
and	O
systematization	O
of	O
optimizing	B-Application
transformations	I-Application
.	O
</s>
<s>
Her	O
1976	O
paper	O
with	O
Cocke	O
describes	O
one	O
of	O
the	O
two	O
main	O
analysis	O
strategies	O
used	O
in	O
optimizing	B-Application
compilers	I-Application
today	O
.	O
</s>
<s>
Allen	O
developed	O
and	O
implemented	O
her	O
methods	O
as	O
part	O
of	O
compilers	B-Language
for	O
the	O
IBM	O
7030	O
Stretch-Harvest	O
and	O
the	O
experimental	O
Advanced	B-Device
Computing	I-Device
System	I-Device
.	O
</s>
<s>
She	O
went	O
on	O
to	O
establish	O
and	O
lead	O
the	O
PTRAN	O
project	O
on	O
the	O
automatic	O
parallel	O
execution	O
of	O
FORTRAN	B-Application
programs	O
.	O
</s>
<s>
Her	O
PTRAN	O
team	O
developed	O
new	O
parallelism	O
detection	O
schemes	O
and	O
created	O
the	O
concept	O
of	O
the	O
program	B-Application
dependence	O
graph	O
,	O
the	O
primary	O
structuring	O
method	O
used	O
by	O
most	O
parallelizing	O
compilers	B-Language
.	O
</s>
<s>
Programming	O
Languages	O
and	O
their	O
Compilers	B-Language
by	O
John	O
Cocke	O
and	O
Jacob	O
T	O
.	O
Schwartz	O
,	O
published	O
early	O
in	O
1970	O
,	O
devoted	O
more	O
than	O
200	O
pages	O
to	O
optimization	O
algorithms	O
.	O
</s>
<s>
It	O
was	O
used	O
in	O
the	O
XPL	B-Language
compiler	B-Language
that	O
McKeeman	O
helped	O
develop	O
.	O
</s>
<s>
Capex	O
Corporation	O
developed	O
the	O
"	O
COBOL	B-Application
Optimizer	O
"	O
in	O
the	O
mid-1970s	O
for	O
COBOL	B-Application
.	O
</s>
<s>
This	O
type	O
of	O
optimizer	O
depended	O
,	O
in	O
this	O
case	O
,	O
upon	O
knowledge	O
of	O
"	O
weaknesses	O
"	O
in	O
the	O
standard	O
IBM	O
COBOL	B-Application
compiler	B-Language
,	O
and	O
actually	O
replaced	O
(	O
or	O
patched	O
)	O
sections	O
of	O
the	O
object	B-Language
code	I-Language
with	O
more	O
efficient	O
code	O
.	O
</s>
<s>
The	O
replacement	O
code	O
might	O
replace	O
a	O
linear	O
table	B-Data_Structure
lookup	I-Data_Structure
with	O
a	O
binary	O
search	O
for	O
example	O
or	O
sometimes	O
simply	O
replace	O
a	O
relatively	O
"	O
slow	O
"	O
instruction	O
with	O
a	O
known	O
faster	O
one	O
that	O
was	O
otherwise	O
functionally	O
equivalent	O
within	O
its	O
context	O
.	O
</s>
<s>
For	O
example	O
,	O
on	O
the	O
IBM	B-Device
System/360	I-Device
hardware	O
the	O
CLI	O
instruction	O
was	O
,	O
depending	O
on	O
the	O
particular	O
model	O
,	O
between	O
twice	O
and	O
5	O
times	O
as	O
fast	O
as	O
a	O
CLC	O
instruction	O
for	O
single	O
byte	O
comparisons	O
.	O
</s>
<s>
Modern	O
compilers	B-Language
typically	O
provide	O
optimization	O
options	O
to	O
allow	O
programmers	O
to	O
choose	O
whether	O
or	O
not	O
to	O
execute	O
an	O
optimization	O
pass	O
.	O
</s>
<s>
When	O
a	O
compiler	B-Language
is	O
given	O
a	O
syntactically	O
incorrect	O
program	B-Application
,	O
a	O
good	O
,	O
clear	O
error	O
message	O
is	O
helpful	O
.	O
</s>
<s>
From	O
the	O
perspective	O
of	O
the	O
compiler	B-Language
writer	O
,	O
it	O
is	O
often	O
difficult	O
to	O
achieve	O
.	O
</s>
<s>
The	O
WATFIV	B-Language
Fortran	B-Application
compiler	B-Language
was	O
developed	O
at	O
the	O
University	O
of	O
Waterloo	O
,	O
Canada	O
in	O
the	O
late	O
1960s	O
.	O
</s>
<s>
It	O
was	O
designed	O
to	O
give	O
better	O
error	O
messages	O
than	O
IBM	O
's	O
Fortran	B-Application
compilers	B-Language
of	O
the	O
time	O
.	O
</s>
<s>
In	O
addition	O
,	O
WATFIV	B-Language
was	O
far	O
more	O
usable	O
,	O
because	O
it	O
combined	O
compiling	B-Language
,	O
linking	B-Application
and	O
execution	O
into	O
one	O
step	O
,	O
whereas	O
IBM	O
's	O
compilers	B-Language
had	O
three	O
separate	O
components	O
to	O
run	O
.	O
</s>
<s>
PL/C	B-Language
was	O
a	O
computer	O
programming	O
language	O
developed	O
at	O
Cornell	O
University	O
in	O
the	O
early	O
1970s	O
.	O
</s>
<s>
While	O
PL/C	B-Language
was	O
a	O
subset	O
of	O
IBM	O
's	O
PL/I	B-Language
language	O
,	O
it	O
was	O
designed	O
with	O
the	O
specific	O
goal	O
of	O
being	O
used	O
for	O
teaching	O
programming	O
.	O
</s>
<s>
The	O
two	O
researchers	O
and	O
academic	O
teachers	O
who	O
designed	O
PL/C	B-Language
were	O
Richard	O
W	O
.	O
Conway	O
and	O
Thomas	O
R	O
.	O
Wilcox	O
.	O
</s>
<s>
They	O
submitted	O
the	O
famous	O
article	O
"	O
Design	O
and	O
implementation	O
of	O
a	O
diagnostic	O
compiler	B-Language
for	O
PL/I	B-Language
"	O
published	O
in	O
the	O
Communications	O
of	O
ACM	O
in	O
March	O
1973	O
.	O
</s>
<s>
PL/C	B-Language
eliminated	O
some	O
of	O
the	O
more	O
complex	O
features	O
of	O
PL/I	B-Language
,	O
and	O
added	O
extensive	O
debugging	O
and	O
error	O
recovery	O
facilities	O
.	O
</s>
<s>
The	O
PL/C	B-Language
compiler	B-Language
had	O
the	O
unusual	O
capability	O
of	O
never	O
failing	O
to	O
compile	B-Language
any	O
program	B-Application
,	O
through	O
the	O
use	O
of	O
extensive	O
automatic	O
correction	O
of	O
many	O
syntax	O
errors	O
and	O
by	O
converting	O
any	O
remaining	O
syntax	O
errors	O
to	O
output	O
statements	O
.	O
</s>
<s>
Just-in-time	O
(	O
JIT	O
)	O
compilation	B-Language
is	O
the	O
generation	O
of	O
executable	B-Application
code	I-Application
on-the-fly	O
or	O
as	O
close	O
as	O
possible	O
to	O
its	O
actual	O
execution	O
,	O
to	O
take	O
advantage	O
of	O
runtime	O
metrics	O
or	O
other	O
performance-enhancing	O
options	O
.	O
</s>
<s>
Most	O
modern	O
compilers	B-Language
have	O
a	O
lexer	B-Application
and	O
parser	B-Language
that	O
produce	O
an	O
intermediate	O
representation	O
of	O
the	O
program	B-Application
.	O
</s>
<s>
The	O
intermediate	O
representation	O
is	O
a	O
simple	O
sequence	O
of	O
operations	O
which	O
can	O
be	O
used	O
by	O
an	O
optimizer	O
and	O
a	O
code	B-Application
generator	I-Application
which	O
produces	O
instructions	O
in	O
the	O
machine	B-Language
language	I-Language
of	O
the	O
target	O
processor	O
.	O
</s>
<s>
Because	O
the	O
code	B-Application
generator	I-Application
uses	O
an	O
intermediate	O
representation	O
,	O
the	O
same	O
code	B-Application
generator	I-Application
can	O
be	O
used	O
for	O
many	O
different	O
high-level	B-Language
languages	I-Language
.	O
</s>
<s>
Three-address	B-Application
code	I-Application
,	O
also	O
known	O
as	O
a	O
quadruple	O
or	O
quad	O
is	O
a	O
common	O
form	O
,	O
where	O
there	O
is	O
an	O
operator	O
,	O
two	O
operands	O
,	O
and	O
a	O
result	O
.	O
</s>
<s>
Two-address	O
code	O
or	O
triples	O
have	O
a	O
stack	O
to	O
which	O
results	O
are	O
written	O
,	O
in	O
contrast	O
to	O
the	O
explicit	O
variables	O
of	O
three-address	B-Application
code	I-Application
.	O
</s>
<s>
SSA	O
simplifies	O
optimization	O
and	O
code	B-Application
generation	I-Application
.	O
</s>
<s>
A	O
code	B-Application
generator	I-Application
generates	O
machine	B-Language
language	I-Language
instructions	O
for	O
the	O
target	O
processor	O
.	O
</s>
<s>
Sethi	B-Algorithm
–	I-Algorithm
Ullman	I-Algorithm
algorithm	I-Algorithm
or	O
Sethi	O
–	O
Ullman	O
numbering	O
is	O
a	O
method	O
to	O
minimise	O
the	O
number	O
of	O
registers	B-General_Concept
needed	O
to	O
hold	O
variables	O
.	O
</s>
<s>
GNU	B-Application
Compiler	I-Application
Collection	I-Application
,	O
formerly	O
the	O
GNU	B-Application
C	I-Application
Compiler	I-Application
.	O
</s>
<s>
Originally	O
authored	O
by	O
Richard	O
Stallman	O
in	O
1987	O
,	O
GCC	B-Application
is	O
a	O
major	O
modern	O
compiler	B-Language
which	O
is	O
used	O
to	O
compile	B-Language
many	O
free	B-Application
software	I-Application
projects	O
,	O
notably	O
Linux	B-Application
.	O
</s>
<s>
Turbo	B-Application
Pascal	I-Application
,	O
created	O
by	O
Anders	O
Hejlsberg	O
,	O
first	O
released	O
in	O
1983	O
.	O
</s>
<s>
WATFOR	B-Language
,	O
created	O
at	O
the	O
University	O
of	O
Waterloo	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
popular	O
educational	O
compilers	B-Language
,	O
although	O
now	O
largely	O
obsolete	O
.	O
</s>
