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Support pointer comparisons against constants, when looking at the inline-cost 

savings from a pointer argument becoming an alloca. Sometimes callees will even
compare a pointer to null and then branch to an otherwise unreachable block!
Detect these cases and compute the number of saved instructions, instead of
bailing out and reporting no savings.

llvm-svn: 148941
This commit is contained in:
Nick Lewycky 2012-01-25 08:27:40 +00:00
parent 25442b8e72
commit 70d50ee8fb
2 changed files with 94 additions and 1 deletions
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54
llvm/lib/Analysis/InlineCost.cpp
View File

@ -222,6 +222,11 @@ unsigned CodeMetrics::CountCodeReductionForAlloca(Value *V) {
if (!V->getType()->isPointerTy()) return 0; // Not a pointer
unsigned Reduction = 0;
// Looking at ICmpInsts will never abort the analysis and return zero, and
// analyzing them is expensive, so save them for last so that we don't do
// extra work that we end up throwing out.
SmallVector<ICmpInst *, 4> ICmpInsts;
SmallVector<Value *, 4> Worklist;
Worklist.push_back(V);
do {
@ -275,6 +280,10 @@ unsigned CodeMetrics::CountCodeReductionForAlloca(Value *V) {
Reduction += InlineConstants::InstrCost;
break;
}
} else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
if (!isa<Constant>(ICI->getOperand(1)))
return 0;
ICmpInsts.push_back(ICI);
} else {
// If there is some other strange instruction, we're not going to be
// able to do much if we inline this.
@ -283,6 +292,51 @@ unsigned CodeMetrics::CountCodeReductionForAlloca(Value *V) {
}
} while (!Worklist.empty());
while (!ICmpInsts.empty()) {
ICmpInst *ICI = ICmpInsts.pop_back_val();
// An icmp pred (alloca, C) becomes true if the predicate is true when
// equal and false otherwise.
bool Result = ICI->isTrueWhenEqual();
SmallVector<Instruction *, 4> Worklist;
Worklist.push_back(ICI);
do {
Instruction *U = Worklist.pop_back_val();
Reduction += InlineConstants::InstrCost;
for (Value::use_iterator UI = U->use_begin(), UE = U->use_end();
UI != UE; ++UI) {
Instruction *I = dyn_cast<Instruction>(*UI);
if (!I || I->mayHaveSideEffects()) continue;
if (I->getNumOperands() == 1)
Worklist.push_back(I);
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
// If BO produces the same value as U, then the other operand is
// irrelevant and we can put it into the Worklist to continue
// deleting dead instructions. If BO produces the same value as the
// other operand, we can delete BO but that's it.
if (Result == true) {
if (BO->getOpcode() == Instruction::Or)
Worklist.push_back(I);
if (BO->getOpcode() == Instruction::And)
Reduction += InlineConstants::InstrCost;
} else {
if (BO->getOpcode() == Instruction::Or ||
BO->getOpcode() == Instruction::Xor)
Reduction += InlineConstants::InstrCost;
if (BO->getOpcode() == Instruction::And)
Worklist.push_back(I);
}
}
if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
BasicBlock *BB = BI->getSuccessor(Result ? 0 : 1);
if (BB->getSinglePredecessor())
Reduction += InlineConstants::InstrCost * BB->size();
}
}
} while (!Worklist.empty());
}
return Reduction;
}

39
llvm/test/Transforms/Inline/alloca-bonus.ll
View File

@ -42,3 +42,42 @@ define void @inner2(i32 *%ptr) {
call void @llvm.lifetime.start(i64 0, i8* %E)
ret void
}
define void @outer3() {
; CHECK: @outer3
; CHECK-NOT: call void @inner3
%ptr = alloca i32
call void @inner3(i32* %ptr, i1 undef)
ret void
}
define void @inner3(i32 *%ptr, i1 %x) {
%A = icmp eq i32* %ptr, null
%B = and i1 %x, %A
br i1 %A, label %bb.true, label %bb.false
bb.true:
; This block musn't be counted in the inline cost.
%t1 = load i32* %ptr
%t2 = add i32 %t1, 1
%t3 = add i32 %t2, 1
%t4 = add i32 %t3, 1
%t5 = add i32 %t4, 1
%t6 = add i32 %t5, 1
%t7 = add i32 %t6, 1
%t8 = add i32 %t7, 1
%t9 = add i32 %t8, 1
%t10 = add i32 %t9, 1
%t11 = add i32 %t10, 1
%t12 = add i32 %t11, 1
%t13 = add i32 %t12, 1
%t14 = add i32 %t13, 1
%t15 = add i32 %t14, 1
%t16 = add i32 %t15, 1
%t17 = add i32 %t16, 1
%t18 = add i32 %t17, 1
%t19 = add i32 %t18, 1
%t20 = add i32 %t19, 1
ret void
bb.false:
ret void
}