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

[Hal Finkel]

----- Original Message -----
> From: "Cameron McInally" <cameron.mcinally at nyu.edu>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Duncan Sands" <duncan.sands at gmail.com>, llvm-commits at cs.uiuc.edu, "Tim Northover" <t.p.northover at gmail.com>
> Sent: Monday, July 7, 2014 8:58:24 AM
> Subject: Re: [llvm] r212443 - CodeGen: it turns out that NAND is not the same thing as BIC. At all.
> 
> On Mon, Jul 7, 2014 at 9:44 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> > ----- Original Message -----
> >> From: "Duncan Sands" <duncan.sands at gmail.com>
> >> To: llvm-commits at cs.uiuc.edu
> >> Sent: Monday, July 7, 2014 4:35:37 AM
> >> Subject: Re: [llvm] r212443 - CodeGen: it turns out that NAND is
> >> not the same thing as BIC. At all.
> >>
> >> 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
> >
> > Is this a problem, then, because we claim compatibility with gcc
> > 4.2.1? I wonder if this breaks anything in the wild.
> 
> If it helps... I intended to change this in 2011, but dropped the
> ball
> on non-X86 backends.
> 
> https://groups.google.com/forum/#!topic/llvm-commit/RLFo4DHaYR8

Okay, so ARM and PPC have now been updated. Does X86 still need to be updated? If so, we better do that now ;) -- Also, the original thread mentioned a warning, and we should probably do that too. We need updates to the documentation, and also in the release notes.

 -Hal

> 
> -Cameron
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory