On 11/26/20 9:15 AM, Richard Sandiford wrote:
> Jeff Law <l...@redhat.com> writes:
>> On 11/13/20 1:14 AM, Richard Sandiford via Gcc-patches wrote:
>>> This patch adds a pointer_mux<T1, T2> class that provides similar
>>> functionality to:
>>>
>>> union { T1 *a; T2 *b; };
>>> ...
>>> bool is_b_rather_than_a;
>>>
>>> except that the is_b_rather_than_a tag is stored in the low bit
>>> of the pointer. See the comments in the patch for a comparison
>>> between the two approaches and why this one can be more efficient.
>>>
>>> I've tried to microoptimise the class a fair bit, since a later
>>> patch uses it extensively in order to keep the sizes of data
>>> structures down.
>>>
>>> gcc/
>>> * mux-utils.h: New file.
>> Do we have any potentially bootstrappable targets where we can't
>> guarantee pointer alignment of at least 16 bits? I see what look like
>> suitable asserts, and presumably if we trigger them, then we're going to
>> need to rethink this and fall back to a separate bit?
> Like you say, the asserts should trap that if it happens. I think the
> fix would be to increase the alignment of the data types involved (which
> we can now do in a standard way -- probably wouldn't have been possible
> before the switch to C++11, at least not without using extensions).
Yea, I keep forgetting about some of the facilities C++ can provide us now.
Seems like we've got a good backup plan. So this is OK by me.
jeff
