Re: What is the meaning of an expression?

From	dave_thompson_2@comcast.net
Newsgroups	comp.compilers
Subject	Re: What is the meaning of an expression?
Date	2022-01-30 22:51 -0500
Organization	A noiseless patient Spider
Message-ID	<22-01-121@comp.compilers> (permalink)
References	(4 earlier) <22-01-067@comp.compilers> <22-01-068@comp.compilers> <22-01-069@comp.compilers> <22-01-070@comp.compilers> <22-01-076@comp.compilers>

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On Thu, 20 Jan 2022 13:02:34 +0100, Hans-Peter Diettrich
<DrDiettrich1@netscape.net> wrote:
...
> ARAIR K&R C defined the value of a function call to be the value
> contained in the accumulator after return. A decision with horrible
> consequences if you look at compiler and library source code of that
> time. OTOH it derogates the meaning of an expression if at any time one
> can find a value in the defined result register.
...
> [Early C didn't have default return values, but since the compilers also
> didn't do much type checking, I can believe there was a code that
> worked by accident becaues the value of the last expression in a function
> happened to be in the register where the caller looked for the result. ...]

Through K&R1 C didn't have 'void' -- all functions had some return
type, which if not written defaulted to 'int' (as in BCPL and B). The
_compilers_ did typecheck but not most _linkers_ so a mismatch across
separately-compiled files was one way to get this problem; one of the
features of 'lint' was to catch such. (FORTRAN had the same issue, and
there were some tools for it, but I don't recall one as prominent.
COBOL, at least then, didn't have value-returning subprograms,
although it could have mismatch on arguments. I don't recall what
Pascals did -- when they had separate compilation at all -- and never
saw a non-toy algol. C++ overloading effectively required typesafe
linkage, and Ada assumed a 'repository' preventing mismatches.)

Another way is if you 'fall off the end' of a (non-void) function, or
through the first standard (C89/90) you wrote and executed a return
statement with no expression; before void this was widely used for
functions with no useful value. If the callsite doesn't discard the
notional 'value' of a call that does one of these, it is officially
undefined behavior and in practice usually takes whatever value is
lying about in the register used by the calling sequence -- but on
PDP-11 and Interdata at least that wasn't 'the accumulator' because
they had no such thing.

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What is the meaning of an expression? Roger L Costello <costello@mitre.org> - 2022-01-14 12:15 +0000
  Re: What is the meaning of an expression? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2022-01-14 18:20 +0000
  Re: What is the meaning of an expression? Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2022-01-15 00:28 +0100
  Re: What is the meaning of an expression? gah4 <gah4@u.washington.edu> - 2022-01-14 17:58 -0800
  Re: What is the meaning of an expression? George Neuner <gneuner2@comcast.net> - 2022-01-15 02:05 -0500
  Re: What is the meaning of an expression? "matt.ti...@gmail.com" <matt.timmermans@gmail.com> - 2022-01-15 06:21 -0800
    Re: What is the meaning of an expression? Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2022-02-03 12:50 +0000
  Re: What is the meaning of an expression? Jan Ziak <0xe2.0x9a.0x9b@gmail.com> - 2022-01-16 07:44 -0800
    Re: What is the meaning of an expression? Jan Ziak <0xe2.0x9a.0x9b@gmail.com> - 2022-01-17 15:45 -0800
      Re: What is the meaning of an expression? Jan Ziak <0xe2.0x9a.0x9b@gmail.com> - 2022-01-18 10:03 -0800
        Re: What is the meaning of an expression? gah4 <gah4@u.washington.edu> - 2022-01-18 15:18 -0800
          Re: What is the meaning of an expression? Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2022-01-19 11:54 +0100
            Re: What is the meaning of an expression? Jan Ziak <0xe2.0x9a.0x9b@gmail.com> - 2022-01-19 09:17 -0800
              Re: What is the meaning of an expression? gah4 <gah4@u.washington.edu> - 2022-01-19 14:03 -0800
              Re: What is the meaning of an expression? Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2022-01-20 13:02 +0100
                Re: What is the meaning of an expression? dave_thompson_2@comcast.net - 2022-01-30 22:51 -0500
            Re: What is the meaning of an expression? Christopher F Clark <christopher.f.clark@compiler-resources.com> - 2022-01-19 20:13 +0200
              Re: What is the meaning of an expression? Thomas Koenig <tkoenig@netcologne.de> - 2022-01-22 20:46 +0000
  Re: What is the meaning of an expression? Thomas Koenig <tkoenig@netcologne.de> - 2022-01-19 20:51 +0000

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