[llvm-dev] [RFC] [X86] Emit unaligned vector moves on avx machine with option control.

[Craig Topper via llvm-dev]

Paul, was your patch attached to a command line option, if so what was the
default, or did you just always use unaligned instructions?

~Craig


On Thu, Apr 15, 2021 at 8:55 AM via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> I’ve debated whether to chime in, and decided it can’t hurt.
>
>
>
> Sony had to do a similar downstream patch for PS4.  Our use-case is pretty
> constrained, though.  There’s only one toolchain, there’s only one target
> chip, so we don’t have any portability considerations to think about.  What
> we do have are games shipping on DVD that can’t be re-released and can’t
> even necessarily be patched, and a strict backward compatibility
> requirement.  So, if there’s a game out there that didn’t happen to follow
> all the alignment requirements, and it worked on console version 1.00, it
> still has to be working on version 100.00.  (FTR, we’re currently on about
> 8.00.)
>
>
>
> I don’t think we ever seriously considered upstreaming our patch.  The
> circumstances where it’s really necessary do exist, but are pretty limited.
>
>
>
> I don’t think arguments of the form “it’s okay because X Y and Z” are
> going to be persuasive.  “We have this situation in the following
> circumstances” might help people understand.
>
> --paulr
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Luo,
> Yuanke via llvm-dev
> *Sent:* Thursday, April 15, 2021 10:07 AM
> *To:* James Y Knight <jyknight at google.com>; Liu, Chen3 <
> chen3.liu at intel.com>
> *Cc:* llvm-dev at lists.llvm.org; Maslov, Sergey V <sergey.v.maslov at intel.com
> >
> *Subject:* Re: [llvm-dev] [RFC] [X86] Emit unaligned vector moves on avx
> machine with option control.
>
>
>
> Yes, replacing aligned move instruction with unaligned move instruction
> doesn’t solve all the issue that happens in optimization pipeline, but it
> doesn’t make things worse. One advantage for unaligned move is that it
> makes the behavior the same no matter the mov instruction is folded or not.
> Do you think it is worth to support this feature if compiler can help users
> avoid changing their complex legacy code?
>
>
>
> Thanks
>
> Yuanke
>
>
>
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