DR#236 analysis

[Geoff Keating]

On 03/12/2004, at 12:55 PM, Ian Lance Taylor wrote:

> Geoffrey Keating <geoffk@geoffk.org> writes:
>
>>>> For malloc-ed memory, I strongly support the
>>>> idea that the first store should determine its type and the type 
>>>> can't
>>>> change after that.
>>>
>>> If I understand that correctly, that would break many existing
>>> programs, including BFD.
>>
>> Could you give an example?  I'm having trouble of thinking of any
>> portable code for which this would be useful and not easily avoided.
>
> It is possible that I misunderstood your proposal.
>
> In BFD, we have structures like this in allocated memory:
>
> struct elf_link_hash_entry
> {
>
>   ...
>
>   union
>   {
>     /* If this is a weak defined symbol from a dynamic object, this
>        field points to a defined symbol with the same value, if there 
> is
>        one.  Otherwise it is NULL.  */
>     struct elf_link_hash_entry *weakdef;
>
>     /* Hash value of the name computed using the ELF hash function.
>        Used part way through size_dynamic_sections, after we've 
> finished
>        with weakdefs.  */
>     unsigned long elf_hash_value;
>   } u;
>
>   ...
>
> };
>
> Here u.weakdef is used in the first pass of the link, through
> adjust_dynamic_symbol.  u.elf_hash_value is used starting at
> size_dynamic_sections.
>
> This is a simple hack to decrease linker memory usage.  It is portable
> and, I believe, fully standards compliant.  It is a case in which
> malloc'ed memory changes type after the first store.  Certainly it can
> be avoided, but that is not the point.  I think the code is correct
> and reasonable, and I think gcc should not break it.
>
> As I say, I may have misunderstood your suggestion.

I see.  Yes, that's not what I meant.  In this case, the type of the 
malloc-ed memory is 'struct elf_link_hash_entry', and that's what's not 
allowed to change; what I want prohibited is code like:

   void * mem = malloc (max (sizeof (int), sizeof (float));
   *(int *)mem = 1;
   *(float *)mem = 2.0;

where the top-level type of the memory changes.
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