Strict aliasing and chains revisited

Till Straumann strauman at
Thu Oct 21 15:42:09 UTC 2010

  On 10/21/2010 12:51 AM, Sebastian Huber wrote:
> On 10/21/2010 01:49 AM, Till Straumann wrote:
>>   I didn't think about this carefully but I guess a clean solution
>> would be sacrificing the space for 1 pointer and make the
>> Chain_Control a proper struct comprised of 2 Chain_Nodes.
> The chain is a pivot data structure in RTEMS and some operations are not that
> efficient if you sacrifice the node overlap.  Also chain controls are used in
> great numbers (one ready chain per task priority).  We should keep that structure.
>> A hacked solution would be making Chain_Control a (hopefully properly
>> packed)
>> union Chain_Control {
>>     struct {
>>        Chain_Node node;
>>        Chain_Node *fill;
>>     } head;
>>     struct {
>>        Chain_Node  *fill;
>>        Chain_Node  node;
>>     } tail;
>> } Chain_Control;
> Yes, this is an alternative to the casts.  I have no preference here.
I don't think the casts remove the alias problem. The fact
that it doesn't seem to happen doesn't guarantee that the
code is correct (gcc behavior changes).

I would feel more comfortable with a proper declaration,
i.e., avoiding type punning.
> Till, what do you think about the change in watchdoginsert.c?
If there are compiler memory  barriers in place then
there should be no problem. You mentioned the barrier
in ISR_Flash() -- is there also a barrier before entry
to the routine? This might not be necessary - I'm travelling
and don't have time + access to the code for proper
analysis, sorry - but we should double-check.

- T.
> [...]

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