Re: Scope of identifiers

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Scope of identifiers
Date	2017-06-14 09:19 -0700
Organization	None to speak of
Message-ID	<lnd1a6ee4d.fsf@kst-u.example.com> (permalink)
References	(5 earlier) <lnwp8hez7v.fsf@kst-u.example.com> <eb8fd32a-b3d5-4767-8a78-3d85ade2fc92@googlegroups.com> <ln60fzg0la.fsf@kst-u.example.com> <e3512b0c-6925-4ef8-ae8b-f142f2de4fc3@googlegroups.com> <e60a5ee2-13c8-4326-b9e6-9055bdd69b25@googlegroups.com>

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supercat@casperkitty.com writes:
> On Tuesday, June 13, 2017 at 11:02:59 PM UTC-5, David Kleinecke wrote:
>> In other words in a primitive pre-C there was no initialization
>> and the declarations could be thought of as simultaneous and all
>> the scopes as the same. This simplicity is lost when initialization
>> was introduced.
>
> Under C89, a single-pass implementation given:
>
>    int x = expressionYieldingLotsOfCode;
>    int y;
>
> could generate code:
>
>    bump stack pointer to leave space for x
>    evaluate expresstionYieldingLotsOfCode
>    bump stack pointer to leave space for y
>
> The compiler wouldn't need to know about y, or much space it would take,
> when generating code for the big expression.  Under C99, however, a
> compiler has to handle:
>
>   someLabel:
>    arbitraryStatement;
>    int x = expressionYieldingLotsOfCode;
>    int y;
>
> If jumping before a declaration would end a variable's lifetime, an
> implementation could simply precede the jump with code that would reset
> the stack pointer to the value it had the first time it ran the
> arbitrary statement.  Under C99 rules, however, space would need to
> remain reserved for y the second time the code runs.

Under C90, as under C99, the lifetime of an object with automatic
storage duration begins on entry to the enclosing block.
A C90 compiler could defer allocation until control reaches the
declaration, but that's an optimization, not part of the visible
semantics.  A C99 compiler can perform exactly the same optimization
if there are no labels preceding the declaration.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Scope of identifiers Steve Keller <keller@no.invalid> - 2017-06-12 18:08 +0200
  Re: Scope of identifiers "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 12:22 -0400
    Re: Scope of identifiers Keith Thompson <kst-u@mib.org> - 2017-06-12 09:39 -0700
  Re: Scope of identifiers Keith Thompson <kst-u@mib.org> - 2017-06-12 09:38 -0700
    Re: Scope of identifiers "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 14:41 -0400
      Re: Scope of identifiers Keith Thompson <kst-u@mib.org> - 2017-06-12 12:39 -0700
        Re: Scope of identifiers "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 15:53 -0400
          Re: Scope of identifiers Keith Thompson <kst-u@mib.org> - 2017-06-12 13:19 -0700
            Re: Scope of identifiers David Kleinecke <dkleinecke@gmail.com> - 2017-06-13 11:01 -0700
              Re: Scope of identifiers jameskuyper@verizon.net - 2017-06-13 11:25 -0700
              Re: Scope of identifiers Keith Thompson <kst-u@mib.org> - 2017-06-13 12:16 -0700
                Re: Scope of identifiers David Kleinecke <dkleinecke@gmail.com> - 2017-06-13 21:02 -0700
                Re: Scope of identifiers Ian Collins <ian-news@hotmail.com> - 2017-06-14 21:53 +1200
                Re: Scope of identifiers supercat@casperkitty.com - 2017-06-14 08:10 -0700
                Re: Scope of identifiers Keith Thompson <kst-u@mib.org> - 2017-06-14 09:19 -0700
                Re: Scope of identifiers supercat@casperkitty.com - 2017-06-14 10:14 -0700
                Re: Scope of identifiers Keith Thompson <kst-u@mib.org> - 2017-06-14 09:12 -0700
  Re: Scope of identifiers Robert Wessel <robertwessel2@yahoo.com> - 2017-06-12 13:02 -0500

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