[Beignet] [PATCH] GBE: We need use exiting block here.

[Song, Ruiling]

LLVM 3.6 requires the second parameter to getSmallConstantTripCount() need to be exiting block.
See code below.
unsigned ScalarEvolution::getSmallConstantTripCount(Loop *L,
                                                    BasicBlock *ExitingBlock) {
  assert(ExitingBlock && "Must pass a non-null exiting block!");
  assert(L->isLoopExiting(ExitingBlock) &&
         "Exiting block must actually branch out of the loop!");

I checked LoopUnrollPass in llvm. The logic below maybe better than my modification:
  BasicBlock *ExitingBlock = L->getLoopLatch();
  if (!ExitingBlock || !L->isLoopExiting(ExitingBlock))
    ExitingBlock = L->getExitingBlock();
  if (ExitingBlock) {
    TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
    TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
  }

> -----Original Message-----
> From: Zhigang Gong [mailto:zhigang.gong at linux.intel.com]
> Sent: Wednesday, February 11, 2015 4:15 PM
> To: Song, Ruiling
> Cc: beignet at lists.freedesktop.org
> Subject: Re: [Beignet] [PATCH] GBE: We need use exiting block here.
> 
> What the assertion does it hit with LLVM 3.6?
> 
> The original purpose to use latch block is:
> 
>   // Find "latch trip count". UnrollLoop assumes that control cannot exit
>   // via the loop latch on any iteration prior to TripCount. The loop may exit
>   // early via an earlier branch.
> 
> So if you use an exiting block here, you change the original logic, and as you
> may know, one loop may have many exiting block, and some exiting block
> may exit very early, so you may get a wrong currTripCount for the current
> loop if you change to use an exiting block here.
> 
> On Wed, Feb 11, 2015 at 04:38:33PM +0800, Ruiling Song wrote:
> > According to the API explanation, we should use exiting block instead
> > of latch block. llvm 3.6 place an assert on this.
> >
> > Signed-off-by: Ruiling Song <ruiling.song at intel.com>
> > ---
> >  backend/src/llvm/llvm_unroll.cpp |   12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/backend/src/llvm/llvm_unroll.cpp
> > b/backend/src/llvm/llvm_unroll.cpp
> > index f0ad4b4..38bfa3e 100644
> > --- a/backend/src/llvm/llvm_unroll.cpp
> > +++ b/backend/src/llvm/llvm_unroll.cpp
> > @@ -177,19 +177,19 @@ namespace gbe {
> >        bool handleParentLoops(Loop *L, LPPassManager &LPM) {
> >          Loop *currL = L;
> >          ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
> > -        BasicBlock *latchBlock = currL->getLoopLatch();
> > +        BasicBlock *ExitBlock = currL->getExitingBlock();
> >          unsigned currTripCount = 0;
> >          bool shouldUnroll = true;
> > -        if (latchBlock)
> > -          currTripCount = SE->getSmallConstantTripCount(L,
> latchBlock);
> > +        if (ExitBlock)
> > +          currTripCount = SE->getSmallConstantTripCount(L,
> > + ExitBlock);
> >
> >          while(currL) {
> >            Loop *parentL = currL->getParentLoop();
> >            unsigned parentTripCount = 0;
> >            if (parentL) {
> > -            BasicBlock *parentLatchBlock = parentL->getLoopLatch();
> > -            if (parentLatchBlock)
> > -              parentTripCount =
> SE->getSmallConstantTripCount(parentL, parentLatchBlock);
> > +            BasicBlock *parentExitBlock = parentL->getExitingBlock();
> > +            if (parentExitBlock)
> > +              parentTripCount =
> > + SE->getSmallConstantTripCount(parentL, parentExitBlock);
> >            }
> >            if ((parentTripCount != 0 && currTripCount /
> parentTripCount > 16) ||
> >                (currTripCount > 32)) {
> > --
> > 1.7.10.4
> >
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