[llvm] r212443 - CodeGen: it turns out that NAND is not the same thing as BIC. At all.
duncan.sands at gmail.com
Mon Jul 7 02:35:37 PDT 2014
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
> 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
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