[llvm] r181897 - Support unaligned load/store on more ARM targets

[Derek Schuff]

reverted in r181944.


On Wed, May 15, 2013 at 4:01 PM, Anton Korobeynikov <anton at korobeynikov.info
> wrote:

> Derek,
>
> Please revert. The patch is still under review / discussion.
>
> On Wed, May 15, 2013 at 8:08 PM, Derek Schuff <dschuff at google.com> wrote:
> > Author: dschuff
> > Date: Wed May 15 11:08:30 2013
> > New Revision: 181897
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=181897&view=rev
> > Log:
> > Support unaligned load/store on more ARM targets
> >
> > This patch matches GCC behavior: the code used to only allow unaligned
> > load/store on ARM for v6+ Darwin, it will now allow unaligned load/store
> for
> > v6+ Darwin as well as for v7+ on other targets.
> >
> > The distinction is made because v6 doesn't guarantee support (but LLVM
> assumes
> > that Apple controls hardware+kernel and therefore have conformant v6
> CPUs),
> > whereas v7 does provide this guarantee (and Linux behaves sanely).
> >
> > Overall this should slightly improve performance in most cases because of
> > reduced I$ pressure.
> >
> > Patch by JF Bastien
> >
> > Modified:
> >     llvm/trunk/lib/Target/ARM/ARMSubtarget.cpp
> >
> > Modified: llvm/trunk/lib/Target/ARM/ARMSubtarget.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/ARMSubtarget.cpp?rev=181897&r1=181896&r2=181897&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/lib/Target/ARM/ARMSubtarget.cpp (original)
> > +++ llvm/trunk/lib/Target/ARM/ARMSubtarget.cpp Wed May 15 11:08:30 2013
> > @@ -162,10 +162,23 @@ void ARMSubtarget::resetSubtargetFeature
> >    if (!isThumb() || hasThumb2())
> >      PostRAScheduler = true;
> >
> > -  // v6+ may or may not support unaligned mem access depending on the
> system
> > -  // configuration.
> > -  if (!StrictAlign && hasV6Ops() && isTargetDarwin())
> > -    AllowsUnalignedMem = true;
> > +  if (!StrictAlign) {
> > +    // Assume pre-ARMv6 doesn't support unaligned accesses.
> > +    //
> > +    // ARMv6 may or may not support unaligned accesses depending on the
> > +    // SCTLR.U bit, which is architecture-specific. We assume ARMv6
> > +    // Darwin targets support unaligned accesses, and others don't.
> > +    //
> > +    // ARMv7 always has SCTLR.U set to 1, but it has a new SCTLR.A bit
> > +    // which raises an alignment fault on unaligned accesses. Linux
> > +    // defaults this bit to 0 and handles it as a system-wide (not
> > +    // per-process) setting. It is therefore safe to assume that ARMv7+
> > +    // targets support unaligned accesses.
> > +    //
> > +    // The above behavior is consistent with GCC.
> > +    if (hasV7Ops() || (hasV6Ops() && isTargetDarwin()))
> > +      AllowsUnalignedMem = true;
> > +  }
> >
> >    // NEON f32 ops are non-IEEE 754 compliant. Darwin is ok with it by
> default.
> >    uint64_t Bits = getFeatureBits();
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
>
> --
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130515/0779ed49/attachment.html>