[LoopUnroll] Eliminate PreserveOnlyFirst parameter [nfc]

[reames]

Summary

This is a first step towards simplifying the transform interface to be less error prone. The basic idea is that querying SCEV is cheap (since it's cached) and we can just check for properties related to branch folding in the transform method instead of relying on the heuristic part to pass everything in correctly.

[nikic]

LGTM

llvm/include/llvm/Transforms/Utils/UnrollLoop.h

 
 struct UnrollLoopOptions {
   unsigned Count;
   unsigned TripCount;
   bool Force;
   bool AllowRuntime;
   bool AllowExpensiveTripCount;
   bool PreserveCondBr;
-  bool PreserveOnlyFirst;
   unsigned TripMultiple;
   unsigned PeelCount;
   bool UnrollRemainder;
   bool ForgetAllSCEV;
 };
 
 LoopUnrollResult UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
                             ScalarEvolution *SE, DominatorTree *DT,

llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp

static LoopUnrollResult tryToUnrollLoop(
   // Save loop properties before it is transformed.
   MDNode *OrigLoopID = L->getLoopID();
 
   // Unroll the loop.
   Loop *RemainderLoop = nullptr;
   LoopUnrollResult UnrollResult = UnrollLoop(
       L,
       {UP.Count, TripCount, UP.Force, UP.Runtime, UP.AllowExpensiveTripCount,
-       UseUpperBound, MaxOrZero, TripMultiple, PP.PeelCount, UP.UnrollRemainder,
+       UseUpperBound, TripMultiple, PP.PeelCount, UP.UnrollRemainder,
        ForgetAllSCEV},
       LI, &SE, &DT, &AC, &TTI, &ORE, PreserveLCSSA, &RemainderLoop);
   if (UnrollResult == LoopUnrollResult::Unmodified)
     return LoopUnrollResult::Unmodified;
 
   if (RemainderLoop) {
     Optional<MDNode *> RemainderLoopID =
         makeFollowupLoopID(OrigLoopID, {LLVMLoopUnrollFollowupAll,

llvm/lib/Transforms/Utils/LoopUnroll.cpp

 /// is relaxed from the general definition of trip count which is the number of
 /// times the loop header executes. Note that UnrollLoop assumes that the loop
 /// counter test is in LatchBlock in order to remove unnecesssary instances of
 /// the test.  If control can exit the loop from the LatchBlock's terminator
 /// prior to TripCount iterations, flag PreserveCondBr needs to be set.
 ///
 /// PreserveCondBr indicates whether the conditional branch of the LatchBlock
 /// needs to be preserved.  It is needed when we use trip count upper bound to
-/// fully unroll the loop. If PreserveOnlyFirst is also set then only the first
-/// conditional branch needs to be preserved.
+/// fully unroll the loop.
 ///
 /// Similarly, TripMultiple divides the number of times that the LatchBlock may
 /// execute without exiting the loop.
 ///
 /// If AllowRuntime is true then UnrollLoop will consider unrolling loops that
 /// have a runtime (i.e. not compile time constant) trip count.  Unrolling these
 /// loops require a unroll "prologue" that runs "RuntimeTripCount % Count"
 /// iterations before branching into the unrolled loop.  UnrollLoop will not
LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
   // it in-place after the transformation, or entirely rebuild LCSSA. TODO: For
   // now we just recompute LCSSA for the outer loop, but it should be possible
   // to fix it in-place.
   bool NeedToFixLCSSA =
       PreserveLCSSA && CompletelyUnroll &&
       any_of(ExitBlocks,
              [](const BasicBlock *BB) { return isa<PHINode>(BB->begin()); });
 
+  const unsigned MaxTripCount = SE->getSmallConstantMaxTripCount(L);
+  const bool MaxOrZero = SE->isBackedgeTakenCountMaxOrZero(L);
+
+  const bool PreserveOnlyFirst = ULO.Count == MaxTripCount && MaxOrZero;
+
   // The current loop unroll pass can unroll loops that have
   // (1) single latch; and
   // (2a) latch is unconditional; or
   // (2b) latch is conditional and is an exiting block
   // FIXME: The implementation can be extended to work with more complicated
   // cases, e.g. loops with multiple latches.
   BranchInst *LatchBI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
 
auto SetDest = [&](BasicBlock *Src, bool WillExit, bool ExitOnTrue) {
       BranchInst::Create(Dest, Term);
       Term->eraseFromParent();
 
       DTU.applyUpdates({{DominatorTree::Delete, Src, DeadSucc}});
     };
 
     auto WillExit = [&](unsigned i, unsigned j) -> Optional<bool> {
       if (CompletelyUnroll) {
-        if (ULO.PreserveCondBr && j && !(ULO.PreserveOnlyFirst && i != 0))
+        if (ULO.PreserveCondBr && j && !(PreserveOnlyFirst && i != 0))
           return None;
         return j == 0;
       }
 
       if (RuntimeTripCount && j != 0)
         return false;
 
       if (j != BreakoutTrip &&

llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

#endif
   auto UnrollResult = LoopUnrollResult::Unmodified;
   if (remainderLoop && UnrollRemainder) {
     LLVM_DEBUG(dbgs() << "Unrolling remainder loop\n");
     UnrollResult =
         UnrollLoop(remainderLoop,
                    {/*Count*/ Count - 1, /*TripCount*/ Count - 1,
                     /*Force*/ false, /*AllowRuntime*/ false,
                     /*AllowExpensiveTripCount*/ false, /*PreserveCondBr*/ true,
-                    /*PreserveOnlyFirst*/ false, /*TripMultiple*/ 1,
+                    /*TripMultiple*/ 1,
                     /*PeelCount*/ 0, /*UnrollRemainder*/ false, ForgetAllSCEV},
                    LI, SE, DT, AC, TTI, /*ORE*/ nullptr, PreserveLCSSA);
   }
 
   if (ResultLoop && UnrollResult != LoopUnrollResult::FullyUnrolled)
     *ResultLoop = remainderLoop;
   NumRuntimeUnrolled++;
   return true;
 }