[llvm] 2a2a37d - [IVUsers] Check for preheader instead of loop simplify form

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2021-10-11T23:13:13+02:00
New Revision: 2a2a37d97207ea7017f1ad9d1312e462c0de9e79

URL: https://github.com/llvm/llvm-project/commit/2a2a37d97207ea7017f1ad9d1312e462c0de9e79
DIFF: https://github.com/llvm/llvm-project/commit/2a2a37d97207ea7017f1ad9d1312e462c0de9e79.diff

LOG: [IVUsers] Check for preheader instead of loop simplify form

IVUsers currently makes sure that all loops dominating a user are
in loop simplify form, because SCEVExpander needs a preheader to
insert into. However, loop simplify form requires much more than
that. In particular, it requires dedicated exits, which means that
exits need to be found and walked. For large functions with many
nested loops, this can result in pathological compile-time explosion.

Fix this by only checking the property we're actually interested in,
which is incidentally cheap to check.

Differential Revision: https://reviews.llvm.org/D111493

Added: 
    

Modified: 
    llvm/lib/Analysis/IVUsers.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/IVUsers.cpp b/llvm/lib/Analysis/IVUsers.cpp
index db6cff720642..58d6245e300a 100644
--- a/llvm/lib/Analysis/IVUsers.cpp
+++ b/llvm/lib/Analysis/IVUsers.cpp
@@ -90,11 +90,10 @@ static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
   return false;
 }
 
-/// Return true if all loop headers that dominate this block are in simplified
-/// form.
-static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
-                                 const LoopInfo *LI,
-                                 SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
+/// Return true if all loop headers that dominate this block have a preheader.
+static bool isPreheaderLoopNest(BasicBlock *BB, const DominatorTree *DT,
+                                const LoopInfo *LI,
+                                SmallPtrSetImpl<Loop*> &PreheaderLoopNests) {
   Loop *NearestLoop = nullptr;
   for (DomTreeNode *Rung = DT->getNode(BB);
        Rung; Rung = Rung->getIDom()) {
@@ -102,10 +101,10 @@ static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
     Loop *DomLoop = LI->getLoopFor(DomBB);
     if (DomLoop && DomLoop->getHeader() == DomBB) {
       // If we have already checked this loop nest, stop checking.
-      if (SimpleLoopNests.count(DomLoop))
+      if (PreheaderLoopNests.count(DomLoop))
         break;
       // If the domtree walk reaches a loop with no preheader, return false.
-      if (!DomLoop->isLoopSimplifyForm())
+      if (!DomLoop->getLoopPreheader())
         return false;
       // If we have not already checked this loop nest, remember the loop
       // header nearest to BB. The nearest loop may not contain BB.
@@ -114,7 +113,7 @@ static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
     }
   }
   if (NearestLoop)
-    SimpleLoopNests.insert(NearestLoop);
+    PreheaderLoopNests.insert(NearestLoop);
   return true;
 }
 
@@ -166,7 +165,7 @@ static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
 /// return true.  Otherwise, return false.
 bool IVUsers::AddUsersImpl(Instruction *I,
-                           SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
+                           SmallPtrSetImpl<Loop*> &PreheaderLoopNests) {
   const DataLayout &DL = I->getModule()->getDataLayout();
 
   // Add this IV user to the Processed set before returning false to ensure that
@@ -222,7 +221,7 @@ bool IVUsers::AddUsersImpl(Instruction *I,
       unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
       UseBB = PHI->getIncomingBlock(ValNo);
     }
-    if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
+    if (!isPreheaderLoopNest(UseBB, DT, LI, PreheaderLoopNests))
       return false;
 
     // Descend recursively, but not into PHI nodes outside the current loop.
@@ -234,12 +233,13 @@ bool IVUsers::AddUsersImpl(Instruction *I,
     bool AddUserToIVUsers = false;
     if (LI->getLoopFor(User->getParent()) != L) {
       if (isa<PHINode>(User) || Processed.count(User) ||
-          !AddUsersImpl(User, SimpleLoopNests)) {
+          !AddUsersImpl(User, PreheaderLoopNests)) {
         LLVM_DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
                           << "   OF SCEV: " << *ISE << '\n');
         AddUserToIVUsers = true;
       }
-    } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
+    } else if (Processed.count(User) ||
+               !AddUsersImpl(User, PreheaderLoopNests)) {
       LLVM_DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
                         << "   OF SCEV: " << *ISE << '\n');
       AddUserToIVUsers = true;
@@ -289,12 +289,12 @@ bool IVUsers::AddUsersImpl(Instruction *I,
 }
 
 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
-  // SCEVExpander can only handle users that are dominated by simplified loop
-  // entries. Keep track of all loops that are only dominated by other simple
+  // SCEVExpander can only handle users that are dominated by loops with
+  // preheaders. Keep track of all loops that are only dominated by preheader
   // loops so we don't traverse the domtree for each user.
-  SmallPtrSet<Loop*,16> SimpleLoopNests;
+  SmallPtrSet<Loop*,16> PreheaderLoopNests;
 
-  return AddUsersImpl(I, SimpleLoopNests);
+  return AddUsersImpl(I, PreheaderLoopNests);
 }
 
 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {