Teach TailRecursionElimination to consider 'nocapture' when deciding whether

calls can be marked tail.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166405 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 67 insertions, 56 deletions

diff --git a/lib/Transforms/Scalar/TailRecursionElimination.cpp b/lib/Transforms/Scalar/TailRecursionElimination.cpp
index 6557d630a9..453f87a296 100644
--- a/lib/Transforms/Scalar/TailRecursionElimination.cpp
+++ b/lib/Transforms/Scalar/TailRecursionElimination.cpp
@@ -69,6 +69,7 @@
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Support/ValueHandle.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/STLExtras.h"
 using namespace llvm;
@@ -117,34 +118,45 @@ FunctionPass *llvm::createTailCallEliminationPass() {
   return new TailCallElim();
 }
 
-/// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by
-/// callees of this function.  We only do very simple analysis right now, this
-/// could be expanded in the future to use mod/ref information for particular
-/// call sites if desired.
-static bool AllocaMightEscapeToCalls(AllocaInst *AI) {
-  // FIXME: do simple 'address taken' analysis.
-  return true;
+/// CanTRE - Scan the specified basic block for alloca instructions.
+/// If it contains any that are variable-sized or not in the entry block,
+/// returns false.
+static bool CanTRE(AllocaInst *AI) {
+  // Because of PR962, we don't TRE allocas outside the entry block.
+
+  // If this alloca is in the body of the function, or if it is a variable
+  // sized allocation, we cannot tail call eliminate calls marked 'tail'
+  // with this mechanism.
+  BasicBlock *BB = AI->getParent();
+  return BB == &BB->getParent()->getEntryBlock() &&
+         isa<ConstantInt>(AI->getArraySize());
 }
 
-/// CheckForEscapingAllocas - Scan the specified basic block for alloca
-/// instructions.  If it contains any that might be accessed by calls, return
-/// true.
-static bool CheckForEscapingAllocas(BasicBlock *BB,
-                                    bool &CannotTCETailMarkedCall) {
-  bool RetVal = false;
-  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
-    if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
-      RetVal |= AllocaMightEscapeToCalls(AI);
-
-      // If this alloca is in the body of the function, or if it is a variable
-      // sized allocation, we cannot tail call eliminate calls marked 'tail'
-      // with this mechanism.
-      if (BB != &BB->getParent()->getEntryBlock() ||
-          !isa<ConstantInt>(AI->getArraySize()))
-        CannotTCETailMarkedCall = true;
-    }
-  return RetVal;
-}
+struct AllocaCaptureTracker : public CaptureTracker {
+  AllocaCaptureTracker() : Captured(false) {}
+
+  void tooManyUses() { Captured = true; }
+
+  bool shouldExplore(Use *U) {
+    Value *V = U->get();
+    if (isa<CallInst>(V) || isa<InvokeInst>(V))
+      UsesAlloca.push_back(V);
+
+    return true;
+  }
+
+  bool captured(Use *U) {
+    if (isa<ReturnInst>(U->getUser()))
+      return false;
+
+    Captured = true;
+    return true;
+  }
+
+  SmallVector<WeakVH, 64> UsesAlloca;
+
+  bool Captured;
+};
 
 bool TailCallElim::runOnFunction(Function &F) {
   // If this function is a varargs function, we won't be able to PHI the args
@@ -157,38 +169,34 @@ bool TailCallElim::runOnFunction(Function &F) {
   bool MadeChange = false;
   bool FunctionContainsEscapingAllocas = false;
 
-  // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
+  // CanTRETailMarkedCall - If false, we cannot perform TRE on tail calls
   // marked with the 'tail' attribute, because doing so would cause the stack
-  // size to increase (real TCE would deallocate variable sized allocas, TCE
+  // size to increase (real TRE would deallocate variable sized allocas, TRE
   // doesn't).
-  bool CannotTCETailMarkedCall = false;
-
-  // Loop over the function, looking for any returning blocks, and keeping track
-  // of whether this function has any non-trivially used allocas.
-  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
-    if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
-      break;
-
-    FunctionContainsEscapingAllocas |=
-      CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
+  bool CanTRETailMarkedCall = true;
+
+  // Find calls that can be marked tail.
+  AllocaCaptureTracker ACT;
+  for (Function::iterator BB = F.begin(), EE = F.end(); BB != EE; ++BB) {
+    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+      if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
+        CanTRETailMarkedCall &= CanTRE(AI);
+        PointerMayBeCaptured(I, &ACT);
+        if (ACT.Captured)
+          return false;
+      }
+    }
   }
 
-  /// FIXME: The code generator produces really bad code when an 'escaping
-  /// alloca' is changed from being a static alloca to being a dynamic alloca.
-  /// Until this is resolved, disable this transformation if that would ever
-  /// happen.  This bug is PR962.
-  if (FunctionContainsEscapingAllocas)
-    return false;
-
-  // Second pass, change any tail calls to loops.
+  // Second pass, change any tail recursive calls to loops.
   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
     if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
       bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
-                                          ArgumentPHIs,CannotTCETailMarkedCall);
+                                          ArgumentPHIs, !CanTRETailMarkedCall);
       if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
         Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
                                           TailCallsAreMarkedTail, ArgumentPHIs,
-                                          CannotTCETailMarkedCall);
+                                          !CanTRETailMarkedCall);
       MadeChange |= Change;
     }
   }
@@ -210,21 +218,24 @@ bool TailCallElim::runOnFunction(Function &F) {
     }
   }
 
-  // Finally, if this function contains no non-escaping allocas, or calls
-  // setjmp, mark all calls in the function as eligible for tail calls
-  //(there is no stack memory for them to access).
+  // Finally, if this function contains no non-escaping allocas and doesn't
+  // call setjmp, mark all calls in the function as eligible for tail calls
+  // (there is no stack memory for them to access).
+  std::sort(ACT.UsesAlloca.begin(), ACT.UsesAlloca.end());
+
   if (!FunctionContainsEscapingAllocas && !F.callsFunctionThatReturnsTwice())
     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
       for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
-        if (CallInst *CI = dyn_cast<CallInst>(I)) {
-          CI->setTailCall();
-          MadeChange = true;
-        }
+        if (CallInst *CI = dyn_cast<CallInst>(I))
+          if (!std::binary_search(ACT.UsesAlloca.begin(), ACT.UsesAlloca.end(),
+                                  CI)) {
+            CI->setTailCall();
+            MadeChange = true;
+          }
 
   return MadeChange;
 }
 
-
 /// CanMoveAboveCall - Return true if it is safe to move the specified
 /// instruction from after the call to before the call, assuming that all
 /// instructions between the call and this instruction are movable.