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Re: autoconf bug

From: Paul Eggert
Subject: Re: autoconf bug
Date: 14 Jul 2003 13:27:23 -0700
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3

Joerg Schilling <address@hidden> writes:

> Why would you call this a GCC bug?
> GCC just detects that 'dummy' is not (except for an address) used and does
> not reserve stack space for this object.

The C Standard requires that different objects must have different
addresses.  It doesn't matter whther an object's contents are never
accessed; if the object exists, it must have a unique address.

The standard does allow a few objects to be coelesced and to share
addresses.  These include string literals and (in C99) compound
literals with const-qualified types.  But objects must be distinct
unless otherwise specified.

> >But I'm puzzled as to why this matters, if you're using GCC.
> >GCC has builtin alloca, so shouldn't this issue be moot for GCC?
> ????

With AC_FUNC_ALLOCA, if 'configure' detects that the compiler-supplied
alloca works, then it shouldn't even test for any of this stuff.  It
should say something like this:

checking for working alloca.h... yes
checking for alloca... yes

and then go on to the next subject.

> I was just working on a portable software signal system that needs to
> know the direction in which the stack grows... so I was looking for a
> usable test mechanism.

OK, but that part of the Autoconf code isn't general; it is intended
to be used only for configuring the LIBOBJ replacement for alloca.

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