llvm dev - Apr 2021 - [RFC] [X86] Emit unaligned vector moves on avx machine with option control.

So the application software is unchangeable, right?


-----Original Message-----
From: paul.robinson at sony.com <paul.robinson at sony.com> 
Sent: Monday, April 19, 2021 11:02 PM
To: lebedev.ri at gmail.com
Cc: jyknight at google.com; Liu, Chen3 <chen3.liu at intel.com>; Luo,
Yuanke <yuanke.luo at intel.com>; 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.
> > Hi James,
> >
> > It's apparent from your reply that you misunderstand one thing:
> > The mine has *already* exploded.
> 
> > I still don't know exactly what Intel is facing, but at Sony we
have
> > games already shipped that CANNOT BE FIXED because they are embedded 
> > in DVD.  It is literally physically impossible to fix the buggy 
> > software, and we have a moral contract with users that their games 
> > will continue to run on all future releases of the console.
> Are they JIT'ed? If not, i'm not really sure how this change to the
> X86 backend would retroactively "fix" already-compiled code.
No; the actual problem is that buggy game code uses a type that is tagged as
32-byte aligned but allocates data that is 16-byte aligned.
The problem is when the (immutable) game calls (updated) system software that
expects 32-byte alignment, and doesn't get it.

The backend change allows our system software not to trap on the misaligned data
that the caller gives to it.
--paulr

> So the application software is unchangeable, right?
Exactly right.  The application software works fine within itself.

The system software, which we update roughly twice a year, also 
accepted the misaligned data, until Clang was modified to emit the 
aligned (trapping) opcodes.  We had to fix that so the system 
software would continue to allow the (buggy but unchangeable)
application software to continue to work.

Yes, it is indeed the case that we can update the system software 
but not the game software.  I think it would be a distraction to
spell out the scenarios but please accept that it is the case.
--paulr