[LLVMdev] Spilled variables using unaligned moves

[Evan Cheng]

On Jul 15, 2008, at 3:21 AM, Nicolas Capens wrote:

> Hi Evan,
>
> Could you maybe point me to the source files where this issue might  
> originate? I’d like to learn more about LLVM’s innards but so far  
> I’ve just scraped the surface and I don’t know where what phase of  
> instruction selection / register allocation / stack layout / etc.  
> happens.

It's in X86InstrInfo.cpp. Please see my reply to Anton's email for  
details.

>
> If I understand correctly this issue might be fixed by moving stack  
> pointer alignment before register allocation? Is this something that  
> might be reasonably straightforward or are there complicated  
> dependencies involved?

This is already being taken care of.

Evan

>
> Thanks again,
>
> Nicolas
>
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev- 
> bounces at cs.uiuc.edu] On Behalf Of Evan Cheng
> Sent: Monday, 14 July, 2008 20:35
> To: LLVM Developers Mailing List
> Cc: anton at korobeynikov.info
> Subject: Re: [LLVMdev] Spilled variables using unaligned moves
>
> This is on Windows / Cygwin? I think the dynamic stack pointer re- 
> alignment doesn't happen until post- register allocation.
>
> Assuming there aren't other instructions between the prologue and  
> the first movups that mess up esp (there shouldn't), this is indeed  
> a bug. Please file a bug and attach a bc file. Thanks.
>
> Evan
>
> On Jul 14, 2008, at 7:43 AM, Nicolas Capens wrote:
>
>
> Hi all,
>
> It looks like vector spills don’t use aligned moves even though the  
> stack is aligned. This seems like an optimization opportunity.
>
> The attached replacement of fibonacci.cpp generates x86 code like  
> this:
>
> 03A70010  push        ebp
> 03A70011  mov         ebp,esp
> 03A70013  and         esp,0FFFFFFF0h
> 03A70019  sub         esp,1A0h
> ...
> 03A7006C  movups      xmmword ptr [esp+180h],xmm7
> ...
> 03A70229  mulps       xmm1,xmmword ptr [esp+180h]
> ...
> 03A70682  movups      xmm0,xmmword ptr [esp+180h]
>
> Note how stores and loads use unaligned moves while it could use  
> aligned moves. It’s also interesting that the multiply does  
> correctly assume the stack to be 16-byte aligned.
>
> Is there something I’m doing wrong (again), or is this already known?
>
> Thanks a lot,
>
> Nicolas Capens
>
> <fibonacci.cpp>
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