Hello,
I'm trying to understand a quirk in 32-bit x86 code generation conventions:
on Linux, when returning a structure from a function by value:
struct S {
int i, j;
};
struct S f(int x)
{
return (struct S){x, sizeof(void*)};
}
the caller reserves space for the returned structure and passes a pointer
to that space as an 'invisible' extra argument, which is prepended to
the argument list; the callee returns this pointer (although the caller
knows it anyway); it's as if the function is transformed to
struct S *f(struct S *ret, int x)
{
ret->i = x;
ret->j = sizeof(void*);
return ret;
}
with one essential difference: the function 'f' itself is responsible for
popping the extra argument off the stack (the 'x' argument is popped by
its caller).
This necessitates using the return-with-immediate ('ret 4') instruction
instead of the normal 'ret'; this is the only instance where this variant of
the 'ret' instruction is used in code generation for C programs on Linux
normally.
I wonder how this exception came to be.
Early versions of GCC (e.g. gcc-1.27) did not implement this convention, i.e.
the caller was responsible for popping the invisible pointer similar to the
normal arguments. The "callee-pops" variant was implemented later for
"compatibility with other compilers", and the option that controls this is
called -fpcc-struct-return, which also disables returning small structures in
registers (in the example above GCC would return the value in EDX:EAX register
pair).
Operating systems differ on following this convention. For example, FreeBSD
and OpenBSD do not, and neither does Windows.
Looking at 386BSD tree in unix-history-repo, I see that it used GCC, not PCC.
Where can I look at the history of x86 code generation development in PCC?
Do I understand correctly that i386 code generation in PCC evolved in parallel
with GCC, and at some point GCC was convinced to adopt the (less efficient)
PCC calling convention by default?
Did PCC prefer to do this stack adjustment for the invisible pointer on the
callee side for some other platform, and the behavior was merely carried over
to the i386 port?
Thank you.
Alexander
> The Documenter's Workbench is sort of the unsung hero
> of Unix. It is why Unix exists, Unix was done to write patents and
> troff and the Documenter's Workbench was all about that.
My response along the following lines seems to have gone astray.
The prime reason for Unix was the desire of Ken, Dennis, and Joe
Ossanna to have a pleasant environment for software development.
The fig leaf that got the nod from Multics-burned management was
that an early use would be to develop a "stand-alone" word-processing
system for use in typing pools and secretarial offices. Perhaps they
had in mind "dedicated", as distinct from "stand-alone"; that's
what eventuated in various cases, most notably in the legal/patent
department and in the AT&T CEO's office.
Both those systems were targets of opportunity, not foreseen from the
start. When Unix was up and running on the PDP-11, Joe got wind of
the legal department having installed a commercial word processor.
He went to pitch Unix as an alternative and clinched a trial by
promising to make roff able to number lines by tomorrow in order to
fulfill a patent-office requirement that the commercial system did
not support.
Modems were installed so legal-department secretaries could try the
Research machine. They liked it and Joe's superb customer service.
Soon the legal department got a system of their own. Joe went on to
create nroff and troff. Document preparation became a widespread use
of Unix, but no stand-alone word-processing system was ever undertaken.
Doug
Decades ago (mid 1993) I produced a CD-ROM with free software for
Novell's UnixWare entitled "Applications for UnixWare". This was at
Novell's request, and they distributed the CDs at trade shows.
There's nothing very special in the CD, but I recently received a
request for it, so I put it up with a minimal description at
http://www.lemis.com/grog/Software/Applications-for-UnixWare.php
Feel free to copy it (with attribution).
Greg
--
Sent from my desktop computer.
Finger grog(a)lemis.com for PGP public key.
See complete headers for address and phone numbers.
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reports problems, please read http://lemis.com/broken-MUA.php
All, I've just placed this document at
https://www.tuhs.org/Archive/Documentation/TechReports/Baker_Struct/bsbstru…
Many thanks to Doug for passing it on!
Cheers, Warren
----- Forwarded message from Douglas McIlroy -----
Warren,
Someone asked if Brenda Baker's original technical memorandum about
struct is available. I scanned my copy and passed it on, secure in my
belief that the Labs won't care since it is essentially the same as
her journal publication.
For the memo to be genuinely available, I'd like to send it to TUHS.
With that in mind I redacted information about corporate organization
and distribution channels. However the memo still bears the AT&T logo
and the words "not for publication".
Doug
----- End forwarded message -----
Thanks to Doug and Warren for getting Brenda Baker's memo on struct
online at
https://www.tuhs.org/Archive/Documentation/TechReports/Baker_Struct/bsbstru…
She later published a formal article about that work in
An Algorithm for Structuring Flowgraphs
Journal of the Association for Computing Machinery 24(1) 98--120 January 1977
https://doi.org/10.1145/321992.321999
-------------------------------------------------------------------------------
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- University of Utah FAX: +1 801 581 4148 -
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-------------------------------------------------------------------------------
Last week there was a bit of discussion about the different shells
that would eventually lead to srb writing the shell that took his name
and the command syntax and semantics that most modern shells use
today. Some of you may remember, VMS had a command interpreter
called DCL (Digital Command language), part of an attempt to make
command syntax uniform across DEC's different operating systems
(TOPS-20 also used DCL). As DEC started to recognize the value of the
Unix marketplace, a project was born in DEC's Commercial Languages and
Tools group to bring the Unix Bourne shell to VMS and to sell it as a
product they called DEC Shell.
I had been part of that effort and one of the issues we had to solve
is providing formal UNIX pipe semantics. They of course needed to
somehow implement UNIX style process pipelines. VMS from the
beginning has had an interprocess communications pseudo-device called
the mailbox that can be written to and read from via the usual I/O
mechanism (the QIO system service). A large problem with them is that
it is not possible to detect the "broken pipe" condition with a
mailbox and that feature deficiency made them unsuitable for use with
DEC Shell. So the team had me write a new device driver, based
closely on the mailbox driver, but that could detect broken pipes
lines UNIX-style.
Shortly after I finished the VMS pipe driver, the team at DECwest had
started work on the MICA project, which was Dave Culter's proposed OS
unification. Dave's team had developed a machine architecture called
PRISM (Proposed RISC Machine) to be the VAX follow-on. For forward
compatibility purposes, PRISM would have to support both Ultrix and
VMS. Dave and team had already written a microkernel-based,
lightweight OS for VAX called VAXeln that was intended for real-time
applications. His new idea was to have a MACH-like microkernel OS
which he called MICA and then to put three user mode personality
modules on top of that:
P.VMS, implementing the VMS system services and ABI
P.Ultrix, implementing the Unix system calls and ABI
P.TBD, a new OS API and ABI intended to supersede VMS
So I wrote the attached "why pipes" memo to explain to Cutler's team
why it was important to implement pipes natively in P.TBD if they
wanted that OS to be a viable follow-on to VMS and Ultrix.
In the end, Dick Sites's 64-bit RISC machine architecture proposal,
which was called Alpha, won out over PRISM. Cutler and a bunch of his
DECwest engineering team went off to Microsoft. Dave's idea of a
microkernel-based OS with multiple personalities of course saw the
light of day originally as NT OS/2, but because of the idea of
multiple personalities, when Microsoft and IBM divorced Dave was able
to quickly pivot to the now infamous Win32 personality, as what would
be called Windows NT. It was also easy for Softway Systems to later
complete the NT POSIX layer for their Interix product, which now a few
generations later is called WSL by Microsoft.
-Paul W.
> You made a comment that MINI-UNIX wasn't available outside of Bell...
No, I didn't say that. What I _actually_ said was: "don't think they were in
wide use outside Bell".
Noel
PS: Can I appeal to people to please take a few seconds of their time and
trim messages they are replying to? Thank you.
> From: Andrew Hume
> the actual configuration of Lions; PDP 11/40 was
> 128 Kbytes of core memory
> ...
> but note that because ... of addressing weirdness (the top 8KB were
> memory-mapped to I/O registers), Lions' PDP actually had 112KB of main
> memory
I think that '112KB' must be an error; the 8KB for the 'I/O page' (as DEC
eventually named ir, long after the rest of the world had started using the
term :-) were deducted from the _UNIBUS_ address space, meaning a UNIBUS -11
(the 'pure' UNIBUS -11's, i.e. other than the -11/70, -11/44, etc) could have
a maximum of 248KB of main memory (which is on the UNIBUS).
A pure UNIBUS -11 with 128KB of main memory (like Lions') has... 128KB of
main memory. The 'small memory management model' -11's (like the /40, /60,
/23, etc) can use at most 64KB of that _at any moment in time_ for user
processes (i.e. directly accessible by the CPU, in 'user' mode).
(The kernel on such machines is basically retricted to 56KB at any moment in
time, since one 'segment/page' - the terminology changed over time - has to be
dedicated to the I/O page: the memory management control registers are in
that, so once the CPU can no longer 'see' them, it's stuck. Long, potentially
interesting digression about, and ways to semi-work around that, elided,
unless people want to hear it.)
> From: Noel Chiappa
> The -11/40 (as it was at first) that I had at LCS had, to start with,
> I'm pretty sure, 3 MM11-L units .. - i.e. 48KB. I know this sounds
> incredible, and I'm having a hard time believing it myself, wondering
> if my memory is failing with age
It is:
# size /lib/c0
13440+2728+10390=26558 (63676)
('c1' takes 14848+6950+2088=23886, FWIW.) So 'my' -11/40 must have had more
than 48KB.
MINI-UNIX provides, on an -11/05 type machine with the maximum of 56KB of
addressable main memory (if you plugged in 64KB worth, the /05 CPU couldn't
'see' the top 8KB of that), up to 32KB for a user process. So that will
just hold the stock V6 C compiler.
I'm not now sure how much memory my -11 _did_ have initially, but it's not
important.
Noel
I enjoy this dc(1) discussion. I am a daily dc user, and since my fisrt
calculator was an HP-45 (circa 1973) RPN felt right. However I think
dc pre-dates ALL HP calculators, since there was one in the 1st Edition
written in assembly.
I extened my version of dc (way before gnu existed)
based on common buttons I used from HP calculators:
CMD WHAT
# comment, to new line
~ change sign of top of stack (CHS key)
| 1/top of stack (1/x key)
e push 99 digits of e (2.718..) on the stack
@ push 99 digits of pi on the stack (looks like a circle)
r reverse top of stack (x<>y key)
I had been fascinated with pi stemimg from the Star Trek epsiode Wolf
in the Fold where Spock uses it to usa all computing power
"Computer, this is a Class A compulsory directive. Compute to
the last digit the value of pi."
"As we know, the value of pi is a transcendental figure without
resolution. The computer banks will work on this problem to the
exclusion of all else until we order it to stop."
As it was supposed to be "arbitrary precision" here was my tool.
So I wrote Machin formula in dc slowing increasing the scale and printing
the results. In the orginal dc, yes the whole part was arbitrary, but the
decimal part (scale) was limited to 99. Well that became a disappiontment.
(this program is lost to time)
So I decided to rewrite it but increasing pi as a whole numbers instead
of increasing scale (ex. 31415, 314159, 3141592, ... etc)
I still have that program which is simply this --
[sxln_1ll*dsllb*dszli2+dsi5li^*/+dsn4*lelzli239li^*/+dse-dlx!=Q]sQ
1[ddsb5/sn239/se1ddsisllQx4*pclb10*lPx]dsPx
if you run it you'll notice the last 1 to 2 digits are wrong due to precision.
The next problem became small memory. I still have thes saved output before
it crashed at 1024 digits. No idea what specs of the machine it was run
on anymore its really old --
3141592653589793238462643383279502884197169399375105820974944592307816\
4062862089986280348253421170679821480865132823066470938446095505822317\
2535940812848111745028410270193852110555964462294895493038196442881097\
5665933446128475648233786783165271201909145648566923460348610454326648\
2133936072602491412737245870066063155881748815209209628292540917153643\
6789259036001133053054882046652138414695194151160943305727036575959195\
3092186117381932611793105118548074462379962749567351885752724891227938\
1830119491298336733624406566430860213949463952247371907021798609437027\
7053921717629317675238467481846766940513200056812714526356082778577134\
2757789609173637178721468440901224953430146549585371050792279689258923\
5420199561121290219608640344181598136297747713099605187072113499999983\
7297804995105973173281609631859502445945534690830264252230825334468503\
5261931188171010003137838752886587533208381420617177669147303598253490\
4287554687311595628638823537875937519577818577805321712268066130019278\
76611195909216420198938095257201065485862972
out of space: salloc
all 8587356 rel 8587326 headmor 1
nbytes -28318
stk 71154 rd 125364 wt 125367 beg 125364 last 125367
83 11 0
30 IOT trap - core dumped
But I was much happier with that.
On a side note: programming dc is hard. There was no comment character.
And it's a pain to read, and it's a pain to debug.
When I discovered the Chudnovsky algorithm for pi, of course I implemented it
in dc --
[0ksslk3^16lkd12+sk*-lm*lhd1+sh3^/smlx_262537412640768000*sxll545140134+dsllm*lxlnk/ls+dls!=P]sP
7sn[6sk1ddshsxsm13591409dsllPx10005v426880*ls/K3-k1/pcln14+snlMx]dsMx
At 99 digits of scale it ran out in 7 rounds, but now with that limitation
removed and large memeories it just goes on and on.....
-Brian
PS: Thanks for the fast OpenBSD version of dc, Otto.
Otto Moerbeek wrote:
> On Thu, Feb 17, 2022 at 01:44:07PM -0800, Bakul Shah wrote:
>
> > On Feb 17, 2022, at 1:18 PM, Dave Horsfall <dave at horsfall.org> wrote:
> > >
> > > On Thu, 17 Feb 2022, Tom Ivar Helbekkmo via TUHS wrote:
> > >
> > >> Watching the prime number generator (from the Wikipedia page on dc)
> > >> running on the 11/23 is much more entertaining than doing it on the
> > >> modern workstation I'm typing this on:
> > >>
> > >> 2p3p[dl!d2+s!%0=@l!l^!<#]s#[s/0ds^]s@[p]s&[ddvs^3s!l#x0<&2+l.x]ds.x
> > >
> > > Wow... About 10s on my old MacBook Pro, and I gave up on my ancient
> > > FreeBSD box.
> >
> > That may be because FreeBSD continues computing primes while the MacOS
> > dc gives up after a while!
> >
> > freebsd (ryzen 2700 3.2Ghz): # note: I interrupted dc after a while
> > $ command time dc <<< '2p3p[dl!d2+s!%0=@l!l^!<#]s#[s/0ds^]s@[p]s&[ddvs^3s!l#x0<&2+l.x]ds.x' > xxx
> > ^C 11.93 real 11.79 user 0.13 sys
> > $ wc xxx
> > 47161 47161 319109 xxx
> > $ size `which dc`
> > text data bss dec hex filename
> > 238159 2784 11072 252015 0x3d86f /usr/bin/dc
> >
> > MacOS (m1 pro, prob. 2Ghz)
> > $ command time dc <<< '2p3p[dl!d2+s!%0=@l!l^!<#]s#[s/0ds^]s@[p]s&[ddvs^3s!l#x0<&2+l.x]ds.x' > xxx
> > time: command terminated abnormally
> > 1.00 real 0.98 user 0.01 sys
> > [2] 37135 segmentation fault command time dc <<< > xxx
> > $ wc xxx
> > 7342 7342 42626 xxx
> > $ size `which dc`
> > __TEXT __DATA __OBJC others dec hex
> > 32768 16384 0 4295016448 4295065600 100018000
> >
>
> MacOS uses the GNU implementation which has a long standing issue with
> deep recursion. It even cannot handle the tail recursive calls used
> here and will run out of its stack.
>
> -Otto
Someone on one of these lists seems to be at ok-labs.com; well...
aneurin% host ok-labs.com
;; connection timed out; no servers could be reached
aneurin%
Houston, we have a problem... Could whoever is responsible please
sprinkle some fairy dust somewhere?
Thanks.
-- Dave
