[cfe-commits] Alignment of return from __builtin_alloca

[Chad Rosier]

On Jun 7, 2012, at 5:22 PM, Jakob Stoklund Olesen wrote:

> 
> On Jun 7, 2012, at 4:33 PM, Chandler Carruth <chandlerc at google.com> wrote:
> 
>> However, it does seem like this should be no more than an 'and' instruction in the worst case (we don't have known alignment of the stack).
> 
> Realigning the stack requires the function to have a frame pointer, so frame pointer elimination is no longer possible. If you  have dynamically sized allocas, a reserved base pointer register is also necessary.
> 
> Reserving 2 out of 7 registers on x86 can have quite the performance impact.
> 
> Base pointer support is quite new, BTW.

Very new: http://llvm.org/viewvc/llvm-project?rev=158087&view=rev  

> Previously, we simply refused to compile functions that required stack realignment and dynamic allocas.

Actually, clang was silently miscompiling the code. :/

 Chad

> 
> /jakob
> 
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