llvm dev - Oct 2012 - [LLVMdev] Tail Duplication Questions

I'm reading up on LLVM's implementation of tail duplication and the
description is confusing:

http://llvm.org/docs/Passes.html

  -tailduplicate: Tail Duplication

  This pass performs a limited form of tail duplication, intended to
  simplify CFGs by removing some unconditional branches. This pass is
  necessary to straighten out loops created by the C front-end, but also
  is capable of making other code nicer. After this pass is run, the CFG
  simplify pass should be run to clean up the mess.

First, lib/CodeGen/TailDuplication.cpp says the pass name is
-tailduplication, not -tailduplicate.  Am I looking at the right pass?

  INITIALIZE_PASS(TailDuplicatePass, "tailduplication", "Tail
Duplication",
                  false, false)

Second, SimplifyCFG is a FunctionPass, not a MachineFunctionPass.  What
cleanup pass is this description really referring to?

Thanks!

                         -David

On 10/31/2012 10:17 AM, dag at cray.com wrote:
>
> Second, SimplifyCFG is a FunctionPass, not a MachineFunctionPass.  What
> cleanup pass is this description really referring to?
There is a tail duplication that runs on bitcode.  I think this is the 
one that the document refers to.

-Krzysztof


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation

We used to have an LLVM-IR pass did tail duplication
(http://llvm.org/viewvc/llvm-project/llvm/tags/RELEASE_29/final/lib/Transforms/Scalar/TailDuplication.cpp)
It was not used and went away with 3.0. The documentation is out of
date.

commit f940a1a869b4fe6f857e7fd8aeb97e7b7e9b390e
Author: Rafael Espindola <rafael.espindola at gmail.com>
Date:   Tue Aug 30 23:03:45 2011 +0000

    Remove the old tail duplication pass. It is not used and is unable to update
    ssa, so it has to be run really early in the pipeline. Any replacement
    should probably use the SSAUpdater.

    git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk at 138841
91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/docs/ReleaseNotes.html b/docs/ReleaseNotes.html
index 6188ffd4..4feb907 100644
--- a/docs/ReleaseNotes.html
+++ b/docs/ReleaseNotes.html
@@ -583,6 +583,8 @@ it run faster:</p>
 <ul>
   <li>The <code>LowerSetJmp</code> pass wasn't used
effectively by any
       target and has been removed.</li>
+  <li>The old <code>TailDup</code> pass was not used in the
standard pipeline
+      and was unable to update ssa form, so it has been removed.
   <li>The syntax of volatile loads and stores in IR has been changed to
       "<code>load
volatile</code>"/"<code>store volatile</code>".
The old
       syntax ("<code>volatile
load</code>"/"<code>volatile store</code>")


On Wed, Oct 31, 2012 at 10:17 AM,  <dag at cray.com>
wrote:
> I'm reading up on LLVM's implementation of tail duplication and the
> description is confusing:
>
> http://llvm.org/docs/Passes.html
>
>   -tailduplicate: Tail Duplication
>
>   This pass performs a limited form of tail duplication, intended to
>   simplify CFGs by removing some unconditional branches. This pass is
>   necessary to straighten out loops created by the C front-end, but also
>   is capable of making other code nicer. After this pass is run, the CFG
>   simplify pass should be run to clean up the mess.
>
> First, lib/CodeGen/TailDuplication.cpp says the pass name is
> -tailduplication, not -tailduplicate.  Am I looking at the right pass?
>
>   INITIALIZE_PASS(TailDuplicatePass, "tailduplication",
"Tail Duplication",
>                   false, false)
>
> Second, SimplifyCFG is a FunctionPass, not a MachineFunctionPass.  What
> cleanup pass is this description really referring to?
>
> Thanks!
>
>                          -David
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Arnold Schwaighofer <arnold.schwaighofer at gmail.com> writes:
> We used to have an LLVM-IR pass did tail duplication
>
(http://llvm.org/viewvc/llvm-project/llvm/tags/RELEASE_29/final/lib/Transforms/Scalar/TailDuplication.cpp)
> It was not used and went away with 3.0. The documentation is out of
> date.
Ah.  So is the MachineFunction version expected to work correctly?

                               -David