[cfe-dev] blocks and lambdas

[John McCall]

On Feb 9, 2011, at 1:55 AM, David Chisnall wrote:

> On 9 Feb 2011, at 00:29, John McCall wrote:
> 
>> On Feb 8, 2011, at 3:56 PM, David Chisnall wrote:
>>> Blocks are a cross-platform solution, they're just not well supported by other compilers yet[1], but they're quite useable on a variety of platforms.
>>> 
>>> [1] Actually, they're not well supported by clang yet either - we allocate too many structures for __block variables.
>> 
>> Can you clarify what you mean here?  I'm not sure how to allocate fewer structures for __block variables, given how a single variable can be shared between multiple block objects.
> 
> A __block variable can be shared between multiple blocks, but it often isn't.  We really only need to allocate one block byref structure for each set of variable / block pairs, i.e. if two variables are only referenced by the same (set of) block(s) and no others, then they can be combined.  There is a note in the spec to this effect.  I looked at implementing it a while ago, but the effort-reward ratio was too high unless I actually find some code where the byref allocation time is a bottleneck.  The simplest (and most common) case, where a group of __block variables is only referenced by a single block would be quite easy to improve though.

Ah yes, that's a good point.  It would also be good to demote __block variables that aren't actually captured.  Both of these would be easier if the variables listed the blocks that capture them.

John.