[LoopUnroll] Move code to exit early. NFC.

[haicheng]

Summary

Just to save some compilation time if possible.

[mzolotukhin]

LGTM, thanks!

Michael

llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp

static bool tryToUnrollLoop(Loop *L, DominatorTree &DT, LoopInfo *LI,
                             bool PreserveLCSSA,
                             Optional<unsigned> ProvidedCount,
                             Optional<unsigned> ProvidedThreshold,
                             Optional<bool> ProvidedAllowPartial,
                             Optional<bool> ProvidedRuntime,
                             Optional<bool> ProvidedUpperBound) {
   DEBUG(dbgs() << "Loop Unroll: F[" << L->getHeader()->getParent()->getName()
                << "] Loop %" << L->getHeader()->getName() << "\n");
-  if (HasUnrollDisablePragma(L)) {
+  if (HasUnrollDisablePragma(L)) 
+    return false;
+  if (!L->isLoopSimplifyForm()) { 
+    DEBUG(
+        dbgs() << "  Not unrolling loop which is not in loop-simplify form.\n");
     return false;
   }
 
   unsigned NumInlineCandidates;
   bool NotDuplicatable;
   bool Convergent;
   TargetTransformInfo::UnrollingPreferences UP = gatherUnrollingPreferences(
       L, TTI, ProvidedThreshold, ProvidedCount, ProvidedAllowPartial,
       ProvidedRuntime, ProvidedUpperBound);
+  // Exit early if unrolling is disabled.
+  if (UP.Threshold == 0 && (!UP.Partial || UP.PartialThreshold == 0))
+    return false;
   unsigned LoopSize = ApproximateLoopSize(
       L, NumInlineCandidates, NotDuplicatable, Convergent, TTI, &AC, UP.BEInsns);
   DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");
   if (NotDuplicatable) {
     DEBUG(dbgs() << "  Not unrolling loop which contains non-duplicatable"
                  << " instructions.\n");
     return false;
   }
   if (NumInlineCandidates != 0) {
     DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
     return false;
   }
-  if (!L->isLoopSimplifyForm()) {
-    DEBUG(
-        dbgs() << "  Not unrolling loop which is not in loop-simplify form.\n");
-    return false;
-  }
 
   // Find trip count and trip multiple if count is not available
   unsigned TripCount = 0;
   unsigned MaxTripCount = 0;
   unsigned TripMultiple = 1;
   // If there are multiple exiting blocks but one of them is the latch, use the
   // latch for the trip count estimation. Otherwise insist on a single exiting
   // block for the trip count estimation.
   BasicBlock *ExitingBlock = L->getLoopLatch();
   if (!ExitingBlock || !L->isLoopExiting(ExitingBlock))
     ExitingBlock = L->getExitingBlock();
   if (ExitingBlock) {
     TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
     TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
   }
 
-  // Exit early if unrolling is disabled.
-  if (UP.Threshold == 0 && (!UP.Partial || UP.PartialThreshold == 0))
-    return false;
-
   // If the loop contains a convergent operation, the prelude we'd add
   // to do the first few instructions before we hit the unrolled loop
   // is unsafe -- it adds a control-flow dependency to the convergent
   // operation.  Therefore restrict remainder loop (try unrollig without).
   //
   // TODO: This is quite conservative.  In practice, convergent_op()
   // is likely to be called unconditionally in the loop.  In this
   // case, the program would be ill-formed (on most architectures)