Re: another C-like language? was Compilers :)

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.compilers
Subject	Re: another C-like language? was Compilers :)
Date	2023-01-09 11:24 -0800
Organization	None to speak of
Message-ID	<23-01-030@comp.compilers> (permalink)
References	(1 earlier) <23-01-002@comp.compilers> <23-01-003@comp.compilers> <23-01-008@comp.compilers> <23-01-020@comp.compilers> <23-01-024@comp.compilers>

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David Brown <david.brown@hesbynett.no> writes:
[...]
>> [In a mutant version of C with nested scopes, I suppose so, but when C compilers
>> ran in 24K bytes, it didn't. -John]
>
> I don't have my copy of K&R handy, or a pre-K&R Unix C manuals, but I
> expect someone will correct me if I'm wrong :-)  As far as I know, the C
> described in "The C Programming Language" in 1978, when 24 KB was still
> a big deal, supported declarations at the start of any compound
> statement block.  That is, nested scopes.  It's possible that pre-K&R C
> compilers were more limited.
> [I actually used that 24K C compiler in about 1975 and I am reasonably sure
> it did not let you put declarations other than in the outer block.  There's
> a 1978 edition of K&R at archive.org and by then it did let you put
> declarations in any block.  It's a little harder than what you say because
> declarations in non-overlapping blocks should overlay each other, e.g.:
>
> foo() {
>  int a:
>  ...
>  {
>     int b[100];
>     somefunc(b);
>  }
>  {
>     float c[100];
>     otherfunc(c);
>  }
> }
>
> you want b and c to use the same storage.  It's not hard, but it's a little
> more than promoting and renaming. -John]

The 1975 C reference manual <https://www.bell-labs.com/usr/dmr/www/cman.pdf>
allows declarations at the top of a function body but not at the top of
a compound statement.  K&R1 (1978) does allow declarations at the top of
a compound statement.

In the example above, you'd certainly *want* b and c to use the same
storage, but the language doesn't require it.  But it's a simple enough
optimization that I'd expect all compilers to do it (or something more
sophisticated).  Also, a compiler could either generate code that
allocates storage for each block on entry to the block, or that
allocates the maximum size on entry to the function.  I haven't bothered
to look into what compilers actually do.  (And it gets more complicated
if you introduce variable-length arrays.)

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for XCOM Labs
void Void(void) { Void(); } /* The recursive call of the void */
[On a PDP-11 or Vax, adjusting the size of local storage was a single
instruction to adjust the stack pointer at the entry to or exit from
each block. In the example above, imagine that between the two blocks
is a call to bloatfunc() which has a large stack frame and you can see
why it would have been worth it.  I will admit that if you had goto
statements, that made it considerably messier. -John]

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