[llvm] 31764ea - [LCSSA] Extract a utility for deciding if a new use requires a new lcssa phi [NFC]

[Philip Reames via llvm-commits]

Author: Philip Reames
Date: 2021-03-17T12:14:01-07:00
New Revision: 31764ea29503b2c0b33a14f77e9a0687693b3c63

URL: https://github.com/llvm/llvm-project/commit/31764ea29503b2c0b33a14f77e9a0687693b3c63
DIFF: https://github.com/llvm/llvm-project/commit/31764ea29503b2c0b33a14f77e9a0687693b3c63.diff

LOG: [LCSSA] Extract a utility for deciding if a new use requires a new lcssa phi [NFC]

(Triggered by a review comment on D98728, but otherwise unrelated.)

Added: 
    

Modified: 
    llvm/include/llvm/Analysis/LoopInfo.h
    llvm/lib/Analysis/LoopInfo.cpp
    llvm/lib/Transforms/Scalar/LICM.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Analysis/LoopInfo.h b/llvm/include/llvm/Analysis/LoopInfo.h
index a5717bae12c3..c2c68004a9f9 100644
--- a/llvm/include/llvm/Analysis/LoopInfo.h
+++ b/llvm/include/llvm/Analysis/LoopInfo.h
@@ -1199,6 +1199,14 @@ class LoopInfo : public LoopInfoBase<BasicBlock, Loop> {
 
     return true;
   }
+
+  // Return true if a new use of V added in ExitBB would require an LCSSA PHI
+  // to be inserted at the begining of the block.  Note that V is assumed to
+  // dominate ExitBB, and ExitBB must be the exit block of some loop.  The
+  // IR is assumed to be in LCSSA form before the planned insertion.
+  bool wouldBeOutOfLoopUseRequiringLCSSA(const Value *V,
+                                         const BasicBlock *ExitBB) const;
+
 };
 
 // Allow clients to walk the list of nested loops...

diff  --git a/llvm/lib/Analysis/LoopInfo.cpp b/llvm/lib/Analysis/LoopInfo.cpp
index 5d3621935b8a..93a00215e34a 100644
--- a/llvm/lib/Analysis/LoopInfo.cpp
+++ b/llvm/lib/Analysis/LoopInfo.cpp
@@ -924,6 +924,31 @@ void LoopInfo::erase(Loop *Unloop) {
   }
 }
 
+bool
+LoopInfo::wouldBeOutOfLoopUseRequiringLCSSA(const Value *V,
+                                            const BasicBlock *ExitBB) const {
+  if (V->getType()->isTokenTy())
+    // We can't form PHIs of token type, so the definition of LCSSA excludes
+    // values of that type.
+    return false;
+
+  const Instruction *I = dyn_cast<Instruction>(V);
+  if (!I)
+    return false;
+  const Loop *L = getLoopFor(I->getParent());
+  if (!L)
+    return false;
+  if (L->contains(ExitBB))
+    // Could be an exit bb of a subloop and contained in defining loop
+    return false;
+
+  // We found a (new) out-of-loop use location, for a value defined in-loop.
+  // (Note that because of LCSSA, we don't have to account for values defined
+  // in sibling loops.  Such values will have LCSSA phis of their own in the
+  // common parent loop.)
+  return true;
+}
+
 AnalysisKey LoopAnalysis::Key;
 
 LoopInfo LoopAnalysis::run(Function &F, FunctionAnalysisManager &AM) {

diff  --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index da054c67fe8d..777e89e37e80 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -1510,16 +1510,15 @@ static Instruction *cloneInstructionInExitBlock(
   // LCSSA. That will eliminate creating PHI nodes just to nuke them when
   // sinking bottom-up.
   for (Use &Op : New->operands())
-    if (Instruction *OInst = dyn_cast<Instruction>(Op))
-      if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
-        if (!OLoop->contains(&PN) && !Op->getType()->isTokenTy()) {
-          PHINode *OpPN =
-              PHINode::Create(OInst->getType(), PN.getNumIncomingValues(),
-                              OInst->getName() + ".lcssa", &ExitBlock.front());
-          for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
-            OpPN->addIncoming(OInst, PN.getIncomingBlock(i));
-          Op = OpPN;
-        }
+    if (LI->wouldBeOutOfLoopUseRequiringLCSSA(Op.get(), PN.getParent())) {
+      auto *OInst = cast<Instruction>(Op.get());
+      PHINode *OpPN =
+        PHINode::Create(OInst->getType(), PN.getNumIncomingValues(),
+                        OInst->getName() + ".lcssa", &ExitBlock.front());
+      for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
+        OpPN->addIncoming(OInst, PN.getIncomingBlock(i));
+      Op = OpPN;
+    }
   return New;
 }
 
@@ -1860,18 +1859,17 @@ class LoopPromoter : public LoadAndStorePromoter {
   // (if legal) if doing so would add an out-of-loop use to an instruction
   // defined in-loop.
   Value *maybeInsertLCSSAPHI(Value *V, BasicBlock *BB) const {
-    if (Instruction *I = dyn_cast<Instruction>(V))
-      if (Loop *L = LI.getLoopFor(I->getParent()))
-        if (!L->contains(BB) && !I->getType()->isTokenTy()) {
-          // We need to create an LCSSA PHI node for the incoming value and
-          // store that.
-          PHINode *PN = PHINode::Create(I->getType(), PredCache.size(BB),
-                                        I->getName() + ".lcssa", &BB->front());
-          for (BasicBlock *Pred : PredCache.get(BB))
-            PN->addIncoming(I, Pred);
-          return PN;
-        }
-    return V;
+    if (!LI.wouldBeOutOfLoopUseRequiringLCSSA(V, BB))
+      return V;
+
+    Instruction *I = cast<Instruction>(V);
+    // We need to create an LCSSA PHI node for the incoming value and
+    // store that.
+    PHINode *PN = PHINode::Create(I->getType(), PredCache.size(BB),
+                                  I->getName() + ".lcssa", &BB->front());
+    for (BasicBlock *Pred : PredCache.get(BB))
+      PN->addIncoming(I, Pred);
+    return PN;
   }
 
 public: