TUHS

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Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

by Paul Ruizendaal

Doug, “In fact Dennis's compiler did not use AID instructions for that purpose.” Whilst local variables are indeed accessed as an offset from the base pointer, I’m not sure that the above statement is correct. In the V6 compiler (-sp) was certainly used to push arguments to the stack, and when the register allocation overflowed, the interim results were pushed to the stack as well with (-sp). See c10.c, the case CALL in rcexpr(), the function comarg() and sptab (which is right at the end of table.s) Links: https://www.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/c/c10.c https://www.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/c/table.s For interim result pushing/popping I refer to the FS and SS macro’s. Dennis discusses these in his “A tour of the C compiler” paper. https://www.jslite.net/cgi-bin/9995/doc/tip/doc/old-ctour.pdf Of course this is all implementational detail, not a core design aspect - as Richard Miller showed in his port to the Interdata architecture (including a port of the Ritchie C compiler). Maybe the sentence should have read: "In fact Dennis's compiler did not rely on having AID instructions for that purpose." Paul @Warren: at moments like these I really like having the line highlight feature that we discussed before Summer.

4 years, 8 months

Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

by jnc＠mercury.lcs.mit.edu

> From: John Cowan > That's always true on the PDP-11 and Vax ... because the processor > architecture (which has pre-increment and post-decrement instructions, > but not their counterparts) After Doug's message, I carefull re-read this, and I'm not sure it's correct? The PDP-11 has pre-decrement and post-increment, not the other way around (as above) - unless I'm misunderstanding what you meant by those terms? That's why: *p++ = 0; turns (if p is in R2) into CLR (R2)+ R2 is used, and then incremented after it has been used. Noel

4 years, 8 months

Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

by Doug McIlroy

> That's always true on the PDP-11 and Vax, no matter what the OS, > because the processor architecture (which has pre-increment and > post-decrement instructions, but not their counterparts) makes > anything but a downward-growing stack unmanageable. I hadn't heard this urban legend before. A stack is certainly manageable without auto-increment/decrement (AID) instructions. In fact Dennis's compiler did not use AID instructions for that purpose. AID instructions are nice for a LIFO stack, in which a stacked item disappears as soon as it's used--as do intermediate results in expression evaluation. But when the stack contains local variables that are accessed multiple times, the accesses are offset from the stack pointer. If AID instructions set the pointer, then the offset of a particular local varies throughout the code. The compiler can cope with that (I once wrote a compiler that did so), but a debugger will have a devil of a time doing a backtrace when the offset of the return address in each stack frame depends on the location counter. AID instructions are fine for sequentially accessing arrays, and in principle Ken's ++ and -- operators can exploit them. Yet idiomatic C typically wants post-increment and pre-decrement instructions--the opposite of what DEC offered. Examples: char a[N], b[N]; char *ap = a; char *bp = b; int i; for(i=0; i<N; i++) *ap++ = *bp++; int a[N], b[N]; int i = N; while(--i >= 0) a[i] = b[i]; Doug

4 years, 8 months

Memory management in Dennis Ritchie's C Compiler

by Dibyendu Majumdar

Hi, I have a project to revive the C compiler from V7/V10. I wanted to check if anyone here knows about the memory management in the compiler (c0 only for now). I am trying to migrate the memory management to malloc/free but I am struggling to understand exactly how memory is being managed. Thanks and Regards Dibyendu

4 years, 8 months

Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

by Steve Simon

I am fairly sure the interdata port from Wollongong used the v6 c compiler, and this lived on in the “official” v7 port from perkin elmer, it still used the v6 compiler. i remember the pain of the global namespace for structure members. -Steve

4 years, 8 months

Re: [TUHS] bare m4 (was BTL summmer employees)

by Doug McIlroy

> >> Even high-school employees could make lasting contributions. I am > >> indebted to Steve for a technique he conceived during his first summer > >> assignment: using macro definitions as if they were units of associative > >> memory. This view of macros stimulated previously undreamed-of uses. > > > Can you give some examples of what this looked like? > See attached for an answer to Arnold's question Doug

4 years, 9 months

BTL pranks [was AT&T Research]

by Doug McIlroy

> Did the non-Unix people also pull pranks like the watertower? One of my favorites was by John Kelly, a Texas original, who refused the department-head perk of a rug so he could stamp his cigarettes out on the vinyl floor. John came from Visual and Acoustics Research, where digital signal processing pressed the frontiers of computing. Among his publications was the completely synthetic recording of "Daisy, Daisy" released circa 1963. Kelly electrified the computer center with a blockbuster prank a year or two before that. As was typical of many machine rooms, a loudspeaker hooked to the low-order bit of the accumulator played gentle white noise in the background. The noise would turn into a shriek when the computer got into a tight loop, calling the operators to put the program out of its misery. Out of the blue one day, the loudspeaker called for help more articulately: "Help, I'm caught in a loop. Help, I'm caught in a loop. ..." it intoned in a slow Texas drawl. News of the talking computer spread instantly and folks croweded into the machine room to marvel before the operators freed the poor prisoner. Doug

4 years, 9 months

Style command source available?

by Richard Salz

Is the style command (and perhaps other DWB programs) legally available anywhere? Replies to me will be summarized for the list. :) tnx.

4 years, 9 months

Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

by norman＠oclsc.org

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

4 years, 9 months

Re: [TUHS] Memory management in Dennis Ritchie's C Compiler

by jnc＠mercury.lcs.mit.edu

> From: Dibyendu Majumdar > the C compiler from V7/V10. I wanted to check if anyone here knows > about the memory management in the compiler (c0 only for now). I am > trying to migrate the memory management to malloc/free but I am > struggling to understand exactly how memory is being managed. Well, I don't know much about the V7 compiler; the V6 one, which I have looked at, doesn't (except for the optimizer, C2) use allocated memory at all. The V7 compiler seems to use sbrk() (the system call to manage the location of the end of a process' data space), and manage the additional data space 'manually'; it does not seem to use a true generic heap. See gblock() in c01.c: https://minnie.tuhs.org//cgi-bin/utree.pl?file=V7/usr/src/cmd/c/c01.c which seems to use two static variables (curbase and coremax) to manage that additional memory: p = curbase; if ((curbase =+ n) >= coremax) { if (sbrk(1024) == -1) { error("Out of space"); exit(1); } coremax =+ 1024; } return(p); My guess is that there's no 'free' at all; each C source file is processed by a new invocation of C0, and the old 'heap' is thrown away when the process exits when it gets to the EOF. Noel

4 years, 9 months

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