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* Use the new PHINode::hasConstantValue method to simplify some code

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* Teach this code to move allocas out of the loop when tail call eliminating
  a call marked 'tail'.  This implements TailCallElim/move_alloca_for_tail_call.ll
* Do not perform this transformation if a call is marked 'tail' and if there
  are allocas that we cannot move out of the loop in #2.  Doing so would increase
  the stack usage of the function.  This implements fixes
  PR615 and TailCallElim/dont-tce-tail-marked-call.ll.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22690 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2005-08-07 04:27:41 +00:00
parent 8cc70cbea4
commit ce869ee05b
1 changed files with 66 additions and 26 deletions
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92
lib/Transforms/Scalar/TailRecursionElimination.cpp
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@ -51,6 +51,7 @@
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
@ -68,7 +69,9 @@ namespace {
private:
bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
std::vector<PHINode*> &ArgumentPHIs);
bool &TailCallsAreMarkedTail,
std::vector<PHINode*> &ArgumentPHIs,
bool CannotTailCallElimCallsMarkedTail);
bool CanMoveAboveCall(Instruction *I, CallInst *CI);
Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
};
@ -93,12 +96,21 @@ static bool AllocaMightEscapeToCalls(AllocaInst *AI) {
/// FunctionContainsAllocas - 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) {
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))
if (AllocaMightEscapeToCalls(AI))
return true;
return false;
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()->front() ||
!isa<ConstantInt>(AI->getArraySize()))
CannotTCETailMarkedCall = true;
}
return RetVal;
}
bool TailCallElim::runOnFunction(Function &F) {
@ -107,21 +119,34 @@ bool TailCallElim::runOnFunction(Function &F) {
if (F.getFunctionType()->isVarArg()) return false;
BasicBlock *OldEntry = 0;
bool TailCallsAreMarkedTail = false;
std::vector<PHINode*> ArgumentPHIs;
bool MadeChange = false;
bool FunctionContainsEscapingAllocas = false;
// CannotTCETailMarkedCall - If true, we cannot perform TCE 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
// 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)
FunctionContainsEscapingAllocas = CheckForEscapingAllocas(BB);
if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
break;
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
MadeChange |= ProcessReturningBlock(Ret, OldEntry, ArgumentPHIs);
FunctionContainsEscapingAllocas =
CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
}
// Second pass, change any tail calls to loops.
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs,CannotTCETailMarkedCall);
// If we eliminated any tail recursions, it's possible that we inserted some
// silly PHI nodes which just merge an initial value (the incoming operand)
// with themselves. Check to see if we did and clean up our mess if so. This
@ -131,23 +156,11 @@ bool TailCallElim::runOnFunction(Function &F) {
unsigned NumIncoming = ArgumentPHIs[0]->getNumIncomingValues();
for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
PHINode *PN = ArgumentPHIs[i];
Value *V = 0;
for (unsigned op = 0, e = NumIncoming; op != e; ++op) {
Value *Op = PN->getIncomingValue(op);
if (Op != PN) {
if (V == 0) {
V = Op; // First value seen?
} else if (V != Op) {
V = 0;
break;
}
}
}
// If the PHI Node is a dynamic constant, replace it with the value it is.
if (V) {
PN->replaceAllUsesWith(V);
PN->getParent()->getInstList().erase(PN);
if (Value *PNV = PN->hasConstantValue()) {
PN->replaceAllUsesWith(PNV);
PN->eraseFromParent();
}
}
}
@ -268,7 +281,9 @@ Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
}
bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
std::vector<PHINode*> &ArgumentPHIs) {
bool &TailCallsAreMarkedTail,
std::vector<PHINode*> &ArgumentPHIs,
bool CannotTailCallElimCallsMarkedTail) {
BasicBlock *BB = Ret->getParent();
Function *F = BB->getParent();
@ -289,6 +304,11 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
--BBI;
}
// If this call is marked as a tail call, and if there are dynamic allocas in
// the function, we cannot perform this optimization.
if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
return false;
// If we are introducing accumulator recursion to eliminate associative
// operations after the call instruction, this variable contains the initial
// value for the accumulator. If this value is set, we actually perform
@ -334,6 +354,17 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
BasicBlock *NewEntry = new BasicBlock(OldName, F, OldEntry);
new BranchInst(OldEntry, NewEntry);
// If this tail call is marked 'tail' and if there are any allocas in the
// entry block, move them up to the new entry block.
TailCallsAreMarkedTail = CI->isTailCall();
if (TailCallsAreMarkedTail)
// Move all fixed sized allocas from OldEntry to NewEntry.
for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
NEBI = NewEntry->begin(); OEBI != E; )
if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
if (isa<ConstantInt>(AI->getArraySize()))
NewEntry->getInstList().splice(NEBI, OldEntry->getInstList(), AI);
// Now that we have created a new block, which jumps to the entry
// block, insert a PHI node for each argument of the function.
// For now, we initialize each PHI to only have the real arguments
@ -348,6 +379,15 @@ bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
}
}
// If this function has self recursive calls in the tail position where some
// are marked tail and some are not, only transform one flavor or another. We
// have to choose whether we move allocas in the entry block to the new entry
// block or not, so we can't make a good choice for both. NOTE: We could do
// slightly better here in the case that the function has no entry block
// allocas.
if (TailCallsAreMarkedTail && !CI->isTailCall())
return false;
// Ok, now that we know we have a pseudo-entry block WITH all of the
// required PHI nodes, add entries into the PHI node for the actual
// parameters passed into the tail-recursive call.