[LLVMdev] mem2reg optimization

Nicolas Capens nicolas at capens.net
Thu Oct 2 10:17:38 PDT 2008

Hi all,

I haven't had a chance yet to do some accurate profiling, but does anyone
have some more insight in the use list order? *bump*

It seems very valuable to me (from a performance standpoint) to keep the use
list in (reverse) order. But if this can't be guaranteed it might still be
interesting to have some fast variants of passes that do make some
assumptions (like my patch for mem2reg). I believe I can optimize more
passes by making such assumption about the order of the use lists, so any
and all information about that is welcome.

Thanks a lot,


-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Nicolas Capens
Sent: Saturday, 27 September, 2008 00:48
To: LLVM Developers Mailing List
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] mem2reg optimization

Hi Andrew,

I can't see any reason not to run mem2reg as the first pass (except  
maybe run a CFG simplify first, which doesn't alter the use list as  
far as I know). Also, I doubt that any pass can actually make a valid  
change in the order of loads and stores. And finally, why would any  
pass want to change the order of the use list in the first place?

Only passes that create new stores and/or loads and don't insert them  
into the use list in the same reverse order could cause problems. But  
as far as I know that's only during codegen (e.g. specific  
instructions that need an operand to be in memory, and spills and  
loads added during register allocation).

Anyway, my experience with LLVM is quite limited so I might be  
overlooking something important. So if Dave's solution works in all  
cases and has comparable performance then that's obviously better.  
I'll try to compare performance early next week and keep you posted.

Kind regards,


On 26 Sep 2008, at 17:56, "Andrew Lenharth" <andrewl at lenharth.org>  

> On Fri, Sep 26, 2008 at 10:41 AM, David Greene <dag at cray.com> wrote:
>> On Thursday 25 September 2008 13:15, David Greene wrote:
>>> I like your approach of using the use lists but I'm not sure the  
>>> ordering
>>> is guaranteed.  If it is, your approach is superior.
>> I got my patch updated to work with TOT.  Here it is.  Comments  
>> welcome.
>> I still prefer Nicolas' solution if his use list ordering  
>> assumption is valid.
> Isn't the ordering an artifact of the creation of the users?  If you
> run mem2reg after other optimizations, why would you still have any
> ordering guarantees?
> Andrew
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