[llvm] r212443 - CodeGen: it turns out that NAND is not the same thing as BIC. At all.

[Duncan Sands]

Hi Tim,

On 07/07/14 11:06, Tim Northover wrote:
> Author: tnorthover
> Date: Mon Jul  7 04:06:35 2014
> New Revision: 212443
>
> URL: http://llvm.org/viewvc/llvm-project?rev=212443&view=rev
> Log:
> CodeGen: it turns out that NAND is not the same thing as BIC. At all.
>
> We've been performing the wrong operation on ARM for "atomicrmw nand" for
> years, since "a NAND b" is "~(a & b)" rather than ARM's very tempting "a & ~b".
> This bled over into the generic expansion pass.
>
> So I assume no-one has ever actually tried to do an atomic nand in the real
> world. Oh well.

this may have been a feature, not a mistake: it might simply have been trying to 
be compatible with GCC which also did a & ~b for atomic nand for years (it was 
even documented to be this way).  The semantics were changed to ~(a & b) in GCC 
4.4.  See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37908

Ciao, Duncan.