From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marc Espie <espie@quatramaran.ens.fr>
To: egcs@egcs.cygnus.com
Subject: Re: type based aliasing again
Date: Thu, 09 Sep 1999 07:12:00 -0000
Message-id: <199909091415.QAA30861@quatramaran.ens.fr>
References: <199909090211.TAA03202@kankakee.wrs.com>
X-SW-Source: 1999-09/msg00354.html

In article < 199909090211.TAA03202@kankakee.wrs.com > you write:
>Is it less confusing to a user to have the compiler predictably
>generate wrong code, or for it to almost always generate right code,
>even for bad code, except in really obscure and hard to understand
>cases?

As far as aliasing goes, what would do wonders would be to have a
thorough section of the documentation explaining what goes on, in
understandable english, not standards legalese.

A few examples of code that works, and code that WILL break would help
immensely as well... Specifically, issues with `poor man inheritance in C',
where one casts structure pointers about to more generic classes, and other
instances of concrete code (the kind of which you find in kernels) would
be useful.

... and I won't volunteer for that one, as I am still confused about what
kind of casts are valid and which kinds are not.

For instance, I've been reviewing some m4 source, which uses a stack
declared like this:

union type
	{
	char *string;
	int position;
	} stack[MAX_SIZE];

and then proceeds to cast stack to (char **) and pass this as argv
to a macro processing function.

Is this code actually valid under ANSI aliasing rules ?

(I know that it needs sizeof(char *) == sizeof(union type), but that's
another point)

From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marc Espie <espie@quatramaran.ens.fr>
To: egcs@egcs.cygnus.com
Subject: Re: type based aliasing again
Date: Thu, 30 Sep 1999 18:02:00 -0000
Message-ID: <199909091415.QAA30861@quatramaran.ens.fr>
References: <199909090211.TAA03202@kankakee.wrs.com>
X-SW-Source: 1999-09n/msg00354.html
Message-ID: <19990930180200.5KevM0UPoRPxHJB6yozI9MmhVZDD5TP626uKOv-wz9c@z>

In article < 199909090211.TAA03202@kankakee.wrs.com > you write:
>Is it less confusing to a user to have the compiler predictably
>generate wrong code, or for it to almost always generate right code,
>even for bad code, except in really obscure and hard to understand
>cases?

As far as aliasing goes, what would do wonders would be to have a
thorough section of the documentation explaining what goes on, in
understandable english, not standards legalese.

A few examples of code that works, and code that WILL break would help
immensely as well... Specifically, issues with `poor man inheritance in C',
where one casts structure pointers about to more generic classes, and other
instances of concrete code (the kind of which you find in kernels) would
be useful.

... and I won't volunteer for that one, as I am still confused about what
kind of casts are valid and which kinds are not.

For instance, I've been reviewing some m4 source, which uses a stack
declared like this:

union type
	{
	char *string;
	int position;
	} stack[MAX_SIZE];

and then proceeds to cast stack to (char **) and pass this as argv
to a macro processing function.

Is this code actually valid under ANSI aliasing rules ?

(I know that it needs sizeof(char *) == sizeof(union type), but that's
another point)