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SimplifyCFG: Add CostRemaining parameter to DominatesMergePoint

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130527 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Peter Collingbourne 2011-04-29 18:47:31 +00:00
parent 8a70192b51
commit f15907fead
2 changed files with 39 additions and 9 deletions
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47
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -201,11 +201,20 @@ static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
/// which works well enough for us.
///
/// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
/// see if V (which must be an instruction) is cheap to compute and is
/// non-trapping. If both are true, the instruction is inserted into the set
/// and true is returned.
/// see if V (which must be an instruction) and its recursive operands
/// that do not dominate BB have a combined cost lower than CostRemaining and
/// are non-trapping. If both are true, the instruction is inserted into the
/// set and true is returned.
///
/// The cost for most non-trapping instructions is defined as 1 except for
/// Select whose cost is 2.
///
/// After this function returns, CostRemaining is decreased by the cost of
/// V plus its non-dominating operands. If that cost is greater than
/// CostRemaining, false is returned and CostRemaining is undefined.
static bool DominatesMergePoint(Value *V, BasicBlock *BB,
SmallPtrSet<Instruction*, 4> *AggressiveInsts) {
SmallPtrSet<Instruction*, 4> *AggressiveInsts,
unsigned &CostRemaining) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I) {
// Non-instructions all dominate instructions, but not all constantexprs
@ -232,12 +241,17 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB,
// instructions in the 'if region'.
if (AggressiveInsts == 0) return false;
// If we have seen this instruction before, don't count it again.
if (AggressiveInsts->count(I)) return true;
// Okay, it looks like the instruction IS in the "condition". Check to
// see if it's a cheap instruction to unconditionally compute, and if it
// only uses stuff defined outside of the condition. If so, hoist it out.
if (!I->isSafeToSpeculativelyExecute())
return false;
unsigned Cost = 0;
switch (I->getOpcode()) {
default: return false; // Cannot hoist this out safely.
case Instruction::Load:
@ -246,11 +260,13 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB,
// predecessor.
if (PBB->getFirstNonPHIOrDbg() != I)
return false;
Cost = 1;
break;
case Instruction::GetElementPtr:
// GEPs are cheap if all indices are constant.
if (!cast<GetElementPtrInst>(I)->hasAllConstantIndices())
return false;
Cost = 1;
break;
case Instruction::Add:
case Instruction::Sub:
@ -264,13 +280,23 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB,
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
Cost = 1;
break; // These are all cheap and non-trapping instructions.
case Instruction::Select:
Cost = 2;
break;
}
// Okay, we can only really hoist these out if their operands are not
// defined in the conditional region.
if (Cost > CostRemaining)
return false;
CostRemaining -= Cost;
// Okay, we can only really hoist these out if their operands do
// not take us over the cost threshold.
for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i)
if (!DominatesMergePoint(*i, BB, 0))
if (!DominatesMergePoint(*i, BB, AggressiveInsts, CostRemaining))
return false;
// Okay, it's safe to do this! Remember this instruction.
AggressiveInsts->insert(I);
@ -1220,6 +1246,7 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) {
// instructions. While we are at it, keep track of the instructions
// that need to be moved to the dominating block.
SmallPtrSet<Instruction*, 4> AggressiveInsts;
unsigned MaxCostVal0 = 1, MaxCostVal1 = 1;
for (BasicBlock::iterator II = BB->begin(); isa<PHINode>(II);) {
PHINode *PN = cast<PHINode>(II++);
@ -1229,8 +1256,10 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) {
continue;
}
if (!DominatesMergePoint(PN->getIncomingValue(0), BB, &AggressiveInsts) ||
!DominatesMergePoint(PN->getIncomingValue(1), BB, &AggressiveInsts))
if (!DominatesMergePoint(PN->getIncomingValue(0), BB, &AggressiveInsts,
MaxCostVal0) ||
!DominatesMergePoint(PN->getIncomingValue(1), BB, &AggressiveInsts,
MaxCostVal1))
return false;
}

1
test/Transforms/SimplifyCFG/PhiBlockMerge.ll
View File

@ -6,6 +6,7 @@
define i32 @test(i1 %a, i1 %b) {
; CHECK: br i1 %a
br i1 %a, label %M, label %O
; CHECK: O:
O: ; preds = %0
; CHECK: select i1 %b, i32 0, i32 1
; CHECK-NOT: phi