[llvm] r179026 - Generate PPC early conditional returns

[Hal Finkel]

----- Original Message -----
> From: "Evan Cheng" <evan.cheng at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Chris Lattner" <clattner at apple.com>, llvm-commits at cs.uiuc.edu
> Sent: Tuesday, April 9, 2013 10:53:28 AM
> Subject: Re: [llvm] r179026 - Generate PPC early conditional returns
> 
> 
> 
> Sent from my iPad
> 
> On Apr 8, 2013, at 10:09 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > ----- Original Message -----
> >> From: "Chris Lattner" <clattner at apple.com>
> >> To: "Hal Finkel" <hfinkel at anl.gov>
> >> Cc: llvm-commits at cs.uiuc.edu
> >> Sent: Monday, April 8, 2013 8:04:17 PM
> >> Subject: Re: [llvm] r179026 - Generate PPC early conditional
> >> returns
> >> 
> >> 
> >> On Apr 8, 2013, at 9:24 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> >> 
> >>> Author: hfinkel
> >>> Date: Mon Apr  8 11:24:03 2013
> >>> New Revision: 179026
> >>> 
> >>> URL: http://llvm.org/viewvc/llvm-project?rev=179026&view=rev
> >>> Log:
> >>> Generate PPC early conditional returns
> >>> 
> >>> PowerPC has a conditional branch to the link register (return)
> >>> instruction: BCLR.
> >>> This should be used any time when we'd otherwise have a
> >>> conditional
> >>> branch to a
> >>> return. This adds a small pass, PPCEarlyReturn, which runs just
> >>> prior to the
> >>> branch selection pass (and, importantly, after block placement)
> >>> to
> >>> generate
> >>> these conditional returns when possible. It will also eliminate
> >>> unconditional
> >>> branches to returns (these happen rarely; most of the time these
> >>> have already
> >>> been tail duplicated by the time PPCEarlyReturn is invoked). This
> >>> is a nice
> >>> optimization for small functions that do not maintain a stack
> >>> frame.
> >> 
> >> Hi Hal,
> >> 
> >> Should this just be a special case of general predication/if
> >> conversion?
> > 
> > It seems like it should be, but I'm not sure. The regular if
> > conversion pass is generally added in addPreSched2, which is after
> > epilogue generation, and so it could work. When I looked through
> > the source to IfConversion.cpp, it did not seem like it would have
> > supported this transformation. IfConversion seems to be looking
> > for blocks to predicate in between the conditional branch and the
> > common successor, and not specifically concerned with tail merging
> > a predicated common successor into the common predecessor;
> > something which may only make sense if the common sucessor is a
> > return). This may really be more of a peephole-style optimization
> > than a predication optimization. Evan?
> > 
> 
> It may require some work to get it right for this case but the if
> converter does generate conditional returns.

I committed some initial support in r179134. Thanks for the suggestion!

At present, if conversion alone does not find all of the cases that PPCEarlyReturn finds (I included a simple test case in the commit), but of course find others, so the largest win currently is to keep both.

Next is:

 - If conversion sometimes seems not to remove blocks from the successors list when they no longer are successors, causing some dead return-only blocks in the resulting code. I'll see if I can fix this.

 - If conversion currently assumes a boolean ability to predicate; for PPC, this is not true. Some branches can take all kinds of predicates, but some can only take certain kinds. Also, some branches can take multiple predicates (one from each class). Maybe isPredicable could be extended to take a Pred array?

Thanks again,
Hal

> 
> Evan
> 
> > -Hal
> > 
> >> 
> >> -Chris
> >> 
>