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From: norman@oclsc.org (Norman Wilson)
To: tuhs@tuhs.org
Subject: Re: [TUHS] Memory management in Dennis Ritchie's C Compiler
Date: Mon, 17 Aug 2020 18:32:29 -0400 (EDT)	[thread overview]
Message-ID: <20200817223229.5401A4422E@lignose.oclsc.org> (raw)

Dan Cross:

  I'll confess I haven't looked _that_ closely, but I rather imagine that the
  V10 compiler is a descendant of PCC rather than Dennis's V6/V7 PDP-11
  compiler.

====

Correct.  From 8/e to the end of official Research UNIX,
cc was pcc2 with a few research-specific hacks.

As Dan says, lcc was there too, but not used a lot.
I'm not sure which version of lcc it was; probably it
was already out-of-date.

In my private half-backed 10/e descendant system, which
runs only on MicroVAXes in my basement, cc is an lcc
descendant instead.  I took the lcc on which the book
was based and re-ported it to the VAX to get an ISO-
compliant C compiler, and made small changes to libc
and /usr/include to afford ISO-C compliance there too.

The hardest but most-interesting part was optimizing.
lcc does a lot of optimization work by itself, and
initially I'd hoped to dispense with a separate c2
pass entirely, but that turns out not to be feasible
on machines like the VAX or the PDP-11: internally
lcc separates something like
	c = *p++;
into two operations
	c = *p;
	p++;
and makes two distinct calls to the code generator.
To sew them back together from
	cvtbl	(p),c
	incl	p
to
	cvtbl	(p)+,c
requires external help; lcc just can't see that
what it thinks of as two distinct expressions
can be combined.

It's more than 15 years since I last looked at any
of this stuff, but I vaguely remember that lcc has
its own interesting (but ISO/POSIX-compatible)
memory-allocation setup.  It allows several nested
contexts' worth of allocation, freeing an inner
context when there's no longer any need for it.
For example, once the compiler has finished with
a function and has no further need for its local
symbols, it frees the associated memory.

See the lcc book for details.  Read the book anyway;
it's the one case I know of in which the authors
followed strict Literate Programming rules and made
a big success of it.  Not only is the compiler well-
documented, but the result is a wonderful tour
through the construction and design decisions of a
large program that does real work.

Norman Wilson
Toronto ON

             reply	other threads:[~2020-08-17 22:34 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-17 22:32 Norman Wilson [this message]
2020-08-17 22:55 ` Bakul Shah
2020-08-17 23:12 ` Chet Ramey
2020-08-18  6:33 ` arnold
  -- strict thread matches above, loose matches on Subject: below --
2020-08-27  8:37 Paul Ruizendaal
2020-08-26 15:59 Noel Chiappa
2020-08-26 16:09 ` Warner Losh
2020-08-26 19:13   ` Clem Cole
2020-08-26 16:14 ` Jim Geist
2020-08-26 17:31 ` John Cowan
2020-08-26 13:24 Doug McIlroy
2020-08-26 15:41 ` John Cowan
2020-08-25  9:36 Steve Simon
2020-08-17 19:51 Noel Chiappa
2020-08-17 19:27 Noel Chiappa
2020-08-17 19:30 ` Larry McVoy
2020-08-17 19:44   ` Dan Halbert
2020-08-17 19:50   ` Paul Winalski
2020-08-17 22:05     ` Arthur Krewat
2020-08-18  0:52 ` Rob Gingell
2020-08-17  2:02 Noel Chiappa
2020-08-17 18:02 ` Paul Winalski
2020-08-17 18:13   ` Jim Geist
2020-08-17 18:48     ` Paul Winalski
2020-08-17 19:08       ` Jim Geist
2020-08-17 19:28         ` Paul Winalski
2020-08-17 19:35           ` Jim Geist
2020-08-17 19:41             ` Richard Salz
2020-08-17 23:40               ` Steffen Nurpmeso
2020-08-15 21:50 Dibyendu Majumdar
2020-08-16 15:20 ` arnold
2020-08-16 15:27   ` Jim Geist
2020-08-17 16:13 ` Dan Cross
2020-08-17 20:16   ` Dibyendu Majumdar
2020-08-17 20:34     ` Paul Winalski
2020-08-17 20:43       ` Dibyendu Majumdar
2020-08-17 21:05         ` Warner Losh
2020-08-17 22:47         ` Dave Horsfall
2020-08-17 23:06           ` Nemo Nusquam
2020-08-17 21:29     ` Dibyendu Majumdar
2020-08-24 15:58     ` Dan Cross
2020-08-24 17:08       ` John Cowan
2020-08-24 17:15         ` Clem Cole
2020-08-24 17:24           ` Clem Cole
2020-08-24 17:20         ` Dan Cross
2020-08-25 23:06           ` Arthur Krewat

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