[PATCH] IRBuilder: Allow retrieval of the inserted instructions

[Arnold Schwaighofer]

On 02/05/15, Hal Finkel   wrote:
> ----- Original Message -----
> > From: "Adam Nemet" <anemet at apple.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: reviews+D7421+public+86df0334cf13545b at reviews.llvm.org, "llvm-commits" <llvm-commits at cs.uiuc.edu>, "Nadav
> > Rotem(nrotem at apple.com)" <nrotem at apple.com>, "Arnold Schwaighofer" <aschwaighofer at apple.com>, chandlerc at gmail.com
> > Sent: Thursday, February 5, 2015 2:19:24 AM
> > Subject: Re: [PATCH] IRBuilder: Allow retrieval of the inserted instructions
> > 
> > On Feb 4, 2015, at 10:15 PM, Hal Finkel < hfinkel at anl.gov > wrote:
> > 
> > 
> > ----- Original Message -----
> > 
> > 
> > From: "Adam Nemet" < anemet at apple.com >
> > To: "Hal Finkel" < hfinkel at anl.gov >
> > Cc: reviews+D7421+public+86df0334cf13545b at reviews.llvm.org ,
> > "llvm-commits" < llvm-commits at cs.uiuc.edu >, "Nadav Rotem <
> > nrotem at apple.com > ( nrotem at apple.com )" < nrotem at apple.com >,
> > "Arnold Schwaighofer" < aschwaighofer at apple.com >,
> > chandlerc at gmail.com
> > Sent: Wednesday, February 4, 2015 10:04:54 PM
> > Subject: Re: [PATCH] IRBuilder: Allow retrieval of the inserted
> > instructions
> > 
> > On Feb 4, 2015, at 6:14 PM, Hal Finkel < hfinkel at anl.gov > wrote:
> > 
> > ----- Original Message -----
> > 
> > From: "Adam Nemet" < anemet at apple.com >
> > To: anemet at apple.com , hfinkel at anl.gov , nrotem at apple.com ,
> > aschwaighofer at apple.com , chandlerc at gmail.com
> > Cc: llvm-commits at cs.uiuc.edu
> > Sent: Wednesday, February 4, 2015 5:58:45 PM
> > Subject: [PATCH] IRBuilder: Allow retrieval of the inserted
> > instructions
> > 
> > Hi hfinkel, nadav, aschwaighofer, chandlerc,
> > 
> > This is an RFC.
> > 
> > When the Loop Vectorizer builds the instruction sequence for checks
> > it tries
> > to determine the first instruction that was emitted in the current
> > block.
> > This is then used to split the block.
> > 
> > Silly question: Why don't we split the block first?
> > 
> > 
> > 
> > Because we may end up not needing any checks in which case we don’t
> > split. The decision of whether checks are necessary is local to both
> > addRuntimeCheck and addStrideCheck.
> > Okay, but an alternative design would be to always split early, and
> > then let SimplifyCFG put them back together again should we not
> > actually end up with any checks, right? I'm not necessarily
> > advocating for this approach, and think that the marker code will be
> > cleaner than the existing getFirstInst code, but splitting the block
> > eagerly will also create a trivial way of marking that point, and I
> > want to understand if there are any downsides to that approach
> > before we invent something new.
> > 
> > 
> > 
> > I am not aware of any other downsides but I haven’t written this code
> > of course. Hopefully Nadav/Arnold will chime in if there is
> > something to add here.



We run SimplifyCFG after the loop vectorizer and rely on it to clean up other parts of the loop skeleton that we create. I would not expect that introducing the extra blocks would do harm.