[LoopPeel][NFC] Exit early if there is no room for peeling

[fhahn]

llvm/lib/Transforms/Utils/LoopPeel.cpp

389

I think it's not quite clear what 'no room' means here. the condition here checks 2 things: 1) the size after peeling one iteration exceeds the threshold and 2) that there is at least on iteration to peel.

It would be good to include both in the comment here.

[ikudrin]

• Update the comment.
• Remove checking for !UnrollPeelMaxCount because a similar condition is anyway checked in line 397.

llvm/lib/Transforms/Utils/LoopPeel.cpp

if (UserPeelCount) {
     PP.PeelProfiledIterations = true;
     return;
   }
 
   // Skip peeling if it's disabled.
   if (!PP.AllowPeeling)
     return;
 
+  // Skip peeling if there is no room for it.
+  if (2 * LoopSize > Threshold || !UnrollPeelMaxCount)
+    return;
+
   unsigned AlreadyPeeled = 0;
   if (auto Peeled = getOptionalIntLoopAttribute(L, PeeledCountMetaData))
     AlreadyPeeled = *Peeled;
   // Stop if we already peeled off the maximum number of iterations.
   if (AlreadyPeeled >= UnrollPeelMaxCount)
     return;
 
   // Here we try to get rid of Phis which become invariants after 1, 2, ..., N
   // iterations of the loop. For this we compute the number for iterations after
   // which every Phi is guaranteed to become an invariant, and try to peel the
   // maximum number of iterations among these values, thus turning all those
   // Phis into invariants.
-  // First, check that we can peel at least one iteration.
-  if (2 * LoopSize <= Threshold && UnrollPeelMaxCount > 0) {
-    // Store the pre-calculated values here.
-    SmallDenseMap<PHINode *, Optional<unsigned> > IterationsToInvariance;
-    // Now go through all Phis to calculate their the number of iterations they
-    // need to become invariants.
-    // Start the max computation with the PP.PeelCount value set by the target
-    // in TTI.getPeelingPreferences or by the flag -unroll-peel-count.
-    unsigned DesiredPeelCount = TargetPeelCount;
-    BasicBlock *BackEdge = L->getLoopLatch();
-    assert(BackEdge && "Loop is not in simplified form?");
-    for (auto BI = L->getHeader()->begin(); isa<PHINode>(&*BI); ++BI) {
-      PHINode *Phi = cast<PHINode>(&*BI);
-      auto ToInvariance = calculateIterationsToInvariance(
-          Phi, L, BackEdge, IterationsToInvariance);
-      if (ToInvariance)
-        DesiredPeelCount = std::max(DesiredPeelCount, *ToInvariance);
-    }
+
+  SmallDenseMap<PHINode *, Optional<unsigned>> IterationsToInvariance;
+    auto ToInvariance = calculateIterationsToInvariance(Phi, L, BackEdge,
+                                                        IterationsToInvariance);
 
-    // Pay respect to limitations implied by loop size and the max peel count.
-    unsigned MaxPeelCount = UnrollPeelMaxCount;
-    MaxPeelCount = std::min(MaxPeelCount, Threshold / LoopSize - 1);
 
-    DesiredPeelCount = std::max(DesiredPeelCount,
-                                countToEliminateCompares(*L, MaxPeelCount, SE));
 
-    if (DesiredPeelCount == 0)
-      DesiredPeelCount = peelToTurnInvariantLoadsDerefencebale(*L, DT);
 
-    if (DesiredPeelCount > 0) {
-      DesiredPeelCount = std::min(DesiredPeelCount, MaxPeelCount);
-      // Consider max peel count limitation.
-      assert(DesiredPeelCount > 0 && "Wrong loop size estimation?");
-      if (DesiredPeelCount + AlreadyPeeled <= UnrollPeelMaxCount) {
-        LLVM_DEBUG(dbgs() << "Peel " << DesiredPeelCount
-                          << " iteration(s) to turn"
-                          << " some Phis into invariants.\n");
-        PP.PeelCount = DesiredPeelCount;
-        PP.PeelProfiledIterations = false;
-        return;
-      }
-    }
-  }
 
   // Bail if we know the statically calculated trip count.
   // In this case we rather prefer partial unrolling.
   if (TripCount)
     return;
 
   // Do not apply profile base peeling if it is disabled.
   if (!PP.PeelProfiledIterations)