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Restore LoopUnswitch's block-oriented threshold. LoopUnswitch now checks both

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the estimated code size and the number of blocks when deciding whether to
do a non-trivial unswitch. This protects it from some very undesirable
worst-case behavior on large numbers of loop-unswitchable conditions, such
as in the testcase in PR5259.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84661 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-10-20 20:06:09 +00:00
parent 480c529e02
commit cd53f64367
1 changed files with 26 additions and 35 deletions
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61
lib/Transforms/Scalar/LoopUnswitch.cpp
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@ -138,7 +138,6 @@ namespace {
void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
unsigned getLoopUnswitchCost(Value *LIC);
void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
BasicBlock *ExitBlock);
void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
@ -400,25 +399,6 @@ bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
return true;
}
/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
/// we choose to unswitch current loop on the specified value.
///
unsigned LoopUnswitch::getLoopUnswitchCost(Value *LIC) {
// If the condition is trivial, always unswitch. There is no code growth for
// this case.
if (IsTrivialUnswitchCondition(LIC))
return 0;
// FIXME: This is overly conservative because it does not take into
// consideration code simplification opportunities.
CodeMetrics Metrics;
for (Loop::block_iterator I = currentLoop->block_begin(),
E = currentLoop->block_end();
I != E; ++I)
Metrics.analyzeBasicBlock(*I);
return Metrics.NumInsts;
}
/// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
/// LoopCond == Val to simplify the loop. If we decide that this is profitable,
/// unswitch the loop, reprocess the pieces, then return true.
@ -427,24 +407,35 @@ bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val){
initLoopData();
Function *F = loopHeader->getParent();
// If the condition is trivial, always unswitch. There is no code growth for
// this case.
if (!IsTrivialUnswitchCondition(LoopCond)) {
// Check to see if it would be profitable to unswitch current loop.
// Check to see if it would be profitable to unswitch current loop.
unsigned Cost = getLoopUnswitchCost(LoopCond);
// Do not do non-trivial unswitch while optimizing for size.
if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
return false;
// Do not do non-trivial unswitch while optimizing for size.
if (Cost && OptimizeForSize)
return false;
if (Cost && !F->isDeclaration() && F->hasFnAttr(Attribute::OptimizeForSize))
return false;
// FIXME: This is overly conservative because it does not take into
// consideration code simplification opportunities and code that can
// be shared by the resultant unswitched loops.
CodeMetrics Metrics;
for (Loop::block_iterator I = currentLoop->block_begin(),
E = currentLoop->block_end();
I != E; ++I)
Metrics.analyzeBasicBlock(*I);
if (Cost > Threshold) {
// FIXME: this should estimate growth by the amount of code shared by the
// resultant unswitched loops.
//
DEBUG(errs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << ", cost too high: "
<< currentLoop->getBlocks().size() << "\n");
return false;
// Limit the number of instructions to avoid causing significant code
// expansion, and the number of basic blocks, to avoid loops with
// large numbers of branches which cause loop unswitching to go crazy.
// This is a very ad-hoc heuristic.
if (Metrics.NumInsts > Threshold ||
Metrics.NumBlocks * 5 > Threshold) {
DEBUG(errs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << ", cost too high: "
<< currentLoop->getBlocks().size() << "\n");
return false;
}
}
Constant *CondVal;