On 2015-03-12 13:55:50 -0600, Martin Sebor wrote:
On 03/12/2015 03:10 AM, Vincent Lefevre wrote:
Well, this depends on the interpretation of effective types in the
case of a union. For instance, when writing
union { char a[16]; int b; } u;
u.b = 1;
you don't set the member only (an
On Wed, Mar 11, 2015 at 05:08:16PM +0100, Vincent Lefevre wrote:
On 2015-03-11 14:27:25 +0100, Robbert Krebbers wrote:
But what about long long on 32 bits machines. For example:
union {
long long a;
struct { char b1; long long b2; } b;
} u;
Will GCC perform similar
On 2015-03-11 17:39:31 +0100, Jakub Jelinek wrote:
On Wed, Mar 11, 2015 at 05:31:01PM +0100, Vincent Lefevre wrote:
(in C only one union member can be active at any time,
we as extension allow type punning through unions etc.)
I disagree that it is an extension. The standard does not
BTW, the following is forbidden (and makes no sense), but is accepted
by GCC without a warning:
int foo (void)
{
union { char a[8]; int b; } u = { .a = { 0 }, .b = 1 };
return u.b;
}
--
Vincent Lefèvre vinc...@vinc17.net - Web: https://www.vinc17.net/
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On 03/12/2015 03:10 AM, Vincent Lefevre wrote:
On 2015-03-12 00:19:55 +0100, Robbert Krebbers wrote:
On 03/11/2015 05:31 PM, Vincent Lefevre wrote:
I disagree that it is an extension. The standard does not say
that one union member can be active at any time.
The interpretation under which
On 2015-03-12 00:19:55 +0100, Robbert Krebbers wrote:
On 03/11/2015 05:31 PM, Vincent Lefevre wrote:
I disagree that it is an extension. The standard does not say
that one union member can be active at any time.
The interpretation under which this is allowed in confirmed by
Note 95 of
On 2015-03-12 01:05:42 +, Joseph Myers wrote:
On Wed, 11 Mar 2015, Vincent Lefevre wrote:
BTW, the following is forbidden (and makes no sense), but is accepted
by GCC without a warning:
int foo (void)
{
union { char a[8]; int b; } u = { .a = { 0 }, .b = 1 };
return u.b;
On 03/11/2015 07:27 AM, Robbert Krebbers wrote:
Dear Joseph,
On 03/10/2015 11:01 PM, Joseph Myers wrote:
and did u.b.b2 = f (u.a); instead of u.b.b2 = u.a;, that would not be
undefined (see 6.8.6.4 and GCC PR 43784).
Thanks for the references, those are useful!
But what about long long on 32
On 2015-03-11 17:11:55 +0100, Jakub Jelinek wrote:
On Wed, Mar 11, 2015 at 05:08:16PM +0100, Vincent Lefevre wrote:
On 2015-03-11 14:27:25 +0100, Robbert Krebbers wrote:
But what about long long on 32 bits machines. For example:
union {
long long a;
struct { char b1; long
On 2015-03-11 14:27:25 +0100, Robbert Krebbers wrote:
But what about long long on 32 bits machines. For example:
union {
long long a;
struct { char b1; long long b2; } b;
} u;
Will GCC perform similar optimizations as for the case of big structs? I
tried to play around with long
On 11 March 2015 at 16:11, Jakub Jelinek wrote:
There is some PR about it in our bugzilla, and the conclusion is that
it is both invalid (in C only one union member can be active at any time,
we as extension allow type punning through unions etc.)
and we really don't want to support it.
I
On Wed, Mar 11, 2015 at 05:31:01PM +0100, Vincent Lefevre wrote:
(in C only one union member can be active at any time,
we as extension allow type punning through unions etc.)
I disagree that it is an extension. The standard does not say
that one union member can be active at any time.
On 03/11/2015 05:31 PM, Vincent Lefevre wrote:
I disagree that it is an extension. The standard does not say
that one union member can be active at any time.
The interpretation under which this is allowed in confirmed by
Note 95 of 6.5.2.3p3.
Effective types disallow to access a union member
Dear Joseph,
On 03/10/2015 11:01 PM, Joseph Myers wrote:
and did u.b.b2 = f (u.a); instead of u.b.b2 = u.a;, that would not be
undefined (see 6.8.6.4 and GCC PR 43784).
Thanks for the references, those are useful!
But what about long long on 32 bits machines. For example:
union {
long long
On Tue, Mar 10, 2015 at 10:11 AM, Robbert Krebbers
mailingli...@robbertkrebbers.nl wrote:
Dear Richard,
On 03/10/2015 09:51 AM, Richard Biener wrote:
struct X { int i; int j; };
int foo (struct X *p, struct X *q)
{
q-j = 1;
p-i = 0;
return q-j;
}
will optimize to return 1.
On 03/10/2015 05:18 PM, Martin Sebor wrote:
I suspect every compiler relies on this requirement in certain
cases otherwise copying would require making use of temporary
storage. Here's an example:
Thanks, this example is indeed not already undefined by effective types,
nor 6.2.6.1p6.
An
On 03/10/2015 05:44 PM, Robbert Krebbers wrote:
On 03/10/2015 05:18 PM, Martin Sebor wrote:
I suspect every compiler relies on this requirement in certain
cases otherwise copying would require making use of temporary
storage. Here's an example:
Thanks, this example is indeed not already
On 03/09/2015 01:26 PM, Robbert Krebbers wrote:
I was wondering whether GCC uses 6.5.16.1p3 of the C11 standard as a
license to perform certain optimizations. If so, could anyone provide me
an example program.
In particular, I am interested about the then the overlap shall be
exact part of
On Tue, 10 Mar 2015, Robbert Krebbers wrote:
On 03/10/2015 05:44 PM, Robbert Krebbers wrote:
On 03/10/2015 05:18 PM, Martin Sebor wrote:
I suspect every compiler relies on this requirement in certain
cases otherwise copying would require making use of temporary
storage. Here's an
On Mon, Mar 9, 2015 at 8:26 PM, Robbert Krebbers
mailingli...@robbertkrebbers.nl wrote:
I was wondering whether GCC uses 6.5.16.1p3 of the C11 standard as a license
to perform certain optimizations. If so, could anyone provide me an example
program.
In particular, I am interested about the
Dear Richard,
On 03/10/2015 09:51 AM, Richard Biener wrote:
struct X { int i; int j; };
int foo (struct X *p, struct X *q)
{
q-j = 1;
p-i = 0;
return q-j;
}
will optimize to return 1. If *p and *q were allowed to overlap
(p-i == q-j)
this would invoke undefined behavior.
Thanks for
I was wondering whether GCC uses 6.5.16.1p3 of the C11 standard as a
license to perform certain optimizations. If so, could anyone provide me
an example program.
In particular, I am interested about the then the overlap shall be
exact part of 6.5.16.1p3:
If the value being stored in an
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