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Enhance the sinking code to handle diamond patterns. Patch by 

Carlo Alberto Ferraris.

llvm-svn: 157736
This commit is contained in:
Duncan Sands 2012-05-31 08:09:49 +00:00
parent b76d5c965d
commit 339bb61e32
2 changed files with 95 additions and 75 deletions
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144
llvm/lib/Transforms/Scalar/Sink.cpp
View File

@ -27,6 +27,7 @@
using namespace llvm;
STATISTIC(NumSunk, "Number of instructions sunk");
STATISTIC(NumSinkIter, "Number of sinking iterations");
namespace {
class Sinking : public FunctionPass {
@ -55,6 +56,7 @@ namespace {
bool ProcessBlock(BasicBlock &BB);
bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores);
bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const;
};
} // end anonymous namespace
@ -98,20 +100,19 @@ bool Sinking::runOnFunction(Function &F) {
LI = &getAnalysis<LoopInfo>();
AA = &getAnalysis<AliasAnalysis>();
bool EverMadeChange = false;
bool MadeChange, EverMadeChange = false;
while (1) {
bool MadeChange = false;
do {
MadeChange = false;
DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
// Process all basic blocks.
for (Function::iterator I = F.begin(), E = F.end();
I != E; ++I)
MadeChange |= ProcessBlock(*I);
// If this iteration over the code changed anything, keep iterating.
if (!MadeChange) break;
EverMadeChange = true;
}
EverMadeChange |= MadeChange;
NumSinkIter++;
} while (MadeChange);
return EverMadeChange;
}
@ -174,6 +175,43 @@ static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
return true;
}
/// IsAcceptableTarget - Return true if it is possible to sink the instruction
/// in the specified basic block.
bool Sinking::IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const {
assert(Inst && "Instruction to be sunk is null");
assert(SuccToSinkTo && "Candidate sink target is null");
// It is not possible to sink an instruction into its own block. This can
// happen with loops.
if (Inst->getParent() == SuccToSinkTo)
return false;
// If the block has multiple predecessors, this would introduce computation
// on different code paths. We could split the critical edge, but for now we
// just punt.
// FIXME: Split critical edges if not backedges.
if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
// We cannot sink a load across a critical edge - there may be stores in
// other code paths.
if (!isSafeToSpeculativelyExecute(Inst))
return false;
// We don't want to sink across a critical edge if we don't dominate the
// successor. We could be introducing calculations to new code paths.
if (!DT->dominates(Inst->getParent(), SuccToSinkTo))
return false;
// Don't sink instructions into a loop.
Loop *succ = LI->getLoopFor(SuccToSinkTo), *cur = LI->getLoopFor(Inst->getParent());
if (succ != 0 && succ != cur)
return false;
}
// Finally, check that all the uses of the instruction are actually
// dominated by the candidate
return AllUsesDominatedByBlock(Inst, SuccToSinkTo);
}
/// SinkInstruction - Determine whether it is safe to sink the specified machine
/// instruction out of its current block into a successor.
bool Sinking::SinkInstruction(Instruction *Inst,
@ -190,85 +228,41 @@ bool Sinking::SinkInstruction(Instruction *Inst,
// "x = y + z" down if it kills y and z would increase the live ranges of y
// and z and only shrink the live range of x.
// Loop over all the operands of the specified instruction. If there is
// anything we can't handle, bail out.
BasicBlock *ParentBlock = Inst->getParent();
// SuccToSinkTo - This is the successor to sink this instruction to, once we
// decide.
BasicBlock *SuccToSinkTo = 0;
// FIXME: This picks a successor to sink into based on having one
// successor that dominates all the uses. However, there are cases where
// sinking can happen but where the sink point isn't a successor. For
// example:
// x = computation
// if () {} else {}
// use x
// the instruction could be sunk over the whole diamond for the
// if/then/else (or loop, etc), allowing it to be sunk into other blocks
// after that.
// Instructions can only be sunk if all their uses are in blocks
// dominated by one of the successors.
// Look at all the successors and decide which one
// we should sink to.
for (succ_iterator SI = succ_begin(ParentBlock),
E = succ_end(ParentBlock); SI != E; ++SI) {
if (AllUsesDominatedByBlock(Inst, *SI)) {
SuccToSinkTo = *SI;
break;
}
// Look at all the postdominators and see if we can sink it in one.
DomTreeNode *DTN = DT->getNode(Inst->getParent());
for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
I != E && SuccToSinkTo == 0; ++I) {
BasicBlock *Candidate = (*I)->getBlock();
if ((*I)->getIDom()->getBlock() == Inst->getParent() &&
IsAcceptableTarget(Inst, Candidate))
SuccToSinkTo = Candidate;
}
// If no suitable postdominator was found, look at all the successors and
// decide which one we should sink to, if any.
for (succ_iterator I = succ_begin(Inst->getParent()),
E = succ_end(Inst->getParent()); I != E && SuccToSinkTo == 0; ++I) {
if (IsAcceptableTarget(Inst, *I))
SuccToSinkTo = *I;
}
// If we couldn't find a block to sink to, ignore this instruction.
if (SuccToSinkTo == 0)
return false;
// It is not possible to sink an instruction into its own block. This can
// happen with loops.
if (Inst->getParent() == SuccToSinkTo)
return false;
DEBUG(dbgs() << "Sink instr " << *Inst);
DEBUG(dbgs() << "to block ";
WriteAsOperand(dbgs(), SuccToSinkTo, false));
// If the block has multiple predecessors, this would introduce computation on
// a path that it doesn't already exist. We could split the critical edge,
// but for now we just punt.
// FIXME: Split critical edges if not backedges.
if (SuccToSinkTo->getUniquePredecessor() != ParentBlock) {
// We cannot sink a load across a critical edge - there may be stores in
// other code paths.
if (!isSafeToSpeculativelyExecute(Inst)) {
DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
return false;
}
// We don't want to sink across a critical edge if we don't dominate the
// successor. We could be introducing calculations to new code paths.
if (!DT->dominates(ParentBlock, SuccToSinkTo)) {
DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
return false;
}
// Don't sink instructions into a loop.
if (LI->isLoopHeader(SuccToSinkTo)) {
DEBUG(dbgs() << " *** PUNTING: Loop header found\n");
return false;
}
// Otherwise we are OK with sinking along a critical edge.
DEBUG(dbgs() << "Sinking along critical edge.\n");
}
// Determine where to insert into. Skip phi nodes.
BasicBlock::iterator InsertPos = SuccToSinkTo->begin();
while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos))
++InsertPos;
DEBUG(dbgs() << "Sink" << *Inst << " (";
WriteAsOperand(dbgs(), Inst->getParent(), false);
dbgs() << " -> ";
WriteAsOperand(dbgs(), SuccToSinkTo, false);
dbgs() << ")\n");
// Move the instruction.
Inst->moveBefore(InsertPos);
Inst->moveBefore(SuccToSinkTo->getFirstInsertionPt());
return true;
}

26
llvm/test/Transforms/Sink/basic.ll
View File

@ -36,3 +36,29 @@ true:
false:
ret i32 0
}
; Sink to the nearest post-dominator
; CHECK: @diamond
; CHECK: X:
; CHECK-NEXT: phi
; CHECK-NEXT: mul nsw
; CHECK-NEXT: sub
define i32 @diamond(i32 %a, i32 %b, i32 %c) {
%1 = mul nsw i32 %c, %b
%2 = icmp sgt i32 %a, 0
br i1 %2, label %B0, label %B1
B0: ; preds = %0
br label %X
B1: ; preds = %0
br label %X
X: ; preds = %5, %3
%.01 = phi i32 [ %c, %B0 ], [ %a, %B1 ]
%R = sub i32 %1, %.01
ret i32 %R
}