[PATCH] D103584: [LoopUnroll] Eliminate PreserveOnlyFirst parameter [nfc]

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added a reviewer: nikic.
Herald added subscribers: javed.absar, zzheng, bollu, hiraditya, mcrosier.
reames requested review of this revision.
Herald added a project: LLVM.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D103584

Files:
  llvm/include/llvm/Transforms/Utils/UnrollLoop.h
  llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
  llvm/lib/Transforms/Utils/LoopUnroll.cpp
  llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp


Index: llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
===================================================================
--- llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -987,7 +987,7 @@
                    {/*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);
   }
Index: llvm/lib/Transforms/Utils/LoopUnroll.cpp
===================================================================
--- llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -256,8 +256,7 @@
 ///
 /// 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.
@@ -381,6 +380,11 @@
       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
@@ -755,7 +759,7 @@
 
     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;
       }
Index: llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -1166,7 +1166,7 @@
   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)
Index: llvm/include/llvm/Transforms/Utils/UnrollLoop.h
===================================================================
--- llvm/include/llvm/Transforms/Utils/UnrollLoop.h
+++ llvm/include/llvm/Transforms/Utils/UnrollLoop.h
@@ -71,7 +71,6 @@
   bool AllowRuntime;
   bool AllowExpensiveTripCount;
   bool PreserveCondBr;
-  bool PreserveOnlyFirst;
   unsigned TripMultiple;
   unsigned PeelCount;
   bool UnrollRemainder;


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