llvm dev - Apr 2014 - [LLVMdev] "Rotate Loop" pass kills "llvm.vectorizer.enable" metadata

Hi.

I faced that "llvm.vectorizer.enable" metadata used by Loop Vectorizer
and
set by OpenMP pragmas are killed after the "Rotate Loop" pass. The
branch
containing the metadata is replaced with another one, but without any
metadata.
So "Loop Vectorizer" doesn't force vectorization of this loop.

I can fix it of course by propagating the metadata to the new branch from
the old one, but there's no guarantee that some other pass will not kill at
again.

May be we need more reliable way for setting metadata for such cases?

- Zinovy.
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----- Original Message -----
> From: "Zinovy Nis" <zinovy.nis at gmail.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Monday, April 7, 2014 8:13:34 AM
> Subject: [LLVMdev] "Rotate Loop" pass kills
"llvm.vectorizer.enable" metadata
> 
> 
> 
> Hi.
> 
> 
> I faced that " llvm.vectorizer.enable" metadata used by Loop
> Vectorizer and set by OpenMP pragmas are killed after the "Rotate
> Loop" pass. The branch containing the metadata is replaced with
> another one, but without any metadata.
> So "Loop Vectorizer" doesn't force vectorization of this
loop.
> 
> 
> I can fix it of course by propagating the metadata to the new branch
> from the old one, but there's no guarantee that some other pass will
> not kill at again.
> 
> 
> May be we need more reliable way for setting metadata for such cases?
No, I think that updating the metadata in the pass is the right solution. The
idea behind metadata is that passes that affect instructions, but don't
explicitly understand the metadata, should drop the metadata. That is what
insures semantic correctness.

That having been said, in this case, are you pointing out that, generally,
passes that replace the loop backedge should not drop the loop id metadata?
Generally, I agree, but I'm not sure there is a lot of code-level
generalization here. That having been said, if there is some way that the
LoopInfo can take care of this automatically in a way that will always be
correct, that sounds good too.

 -Hal
> 
> 
> - Zinovy.
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory