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[SCEV] isHighCostExpansionHelper(): use correct TTI hooks

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Summary:
Cost modelling strikes again.
In PR44668 <https://bugs.llvm.org/show_bug.cgi?id=44668> patch series,
i've made the same mistake of always using generic `getOperationCost()`
that i missed in reviewing D73480/D74495 which was later fixed
in 62dd44d76da9aa596fb199bda8b1e8768bb41033.

We should be using more specific hooks instead - `getCastInstrCost()`,
`getArithmeticInstrCost()`, `getCmpSelInstrCost()`.

Evidently, this does not have an effect on the existing testcases,
with unchanged default cost budget. But if it *does* have an effect
on some target, we'll have to segregate tests that use this function
per-target, much like we already do with other TTI-aware transform tests.

There's also an issue that @samparker has brought up in post-commit-review:
>>! In D73501#1905171, @samparker wrote:
> Hi,
> Did you get performance numbers for these patches? We track the performance
> of our (Arm) open source DSP library and the cost model fixes were generally
> a notable improvement, so many thanks for that! But the final patch
> for rewriting exit values has generally been bad, especially considering
> the gains from the modelling improvements. I need to look into it further,
> but on my current test case I'm seeing +30% increase in stack accesses
> with a similar decrease in performance.
> I'm just wondering if you observed any negative effects yourself?

I don't know if this addresses that, or we need D66450 for that.

Reviewers: samparker, spatel, mkazantsev, reames, wmi

Reviewed By: reames

Subscribers: kristof.beyls, hiraditya, danielkiss, llvm-commits, samparker

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75908
This commit is contained in:
Roman Lebedev 2020-03-12 11:33:38 +03:00
parent a43ec896ff
commit f6c10d3f93
1 changed files with 12 additions and 9 deletions
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21
lib/Analysis/ScalarEvolutionExpander.cpp
View File

@ -2173,7 +2173,7 @@ bool SCEVExpander::isHighCostExpansionHelper(
}
const SCEV *Op = CastExpr->getOperand();
BudgetRemaining -=
TTI.getOperationCost(Opcode, S->getType(), Op->getType());
TTI.getCastInstrCost(Opcode, S->getType(), Op->getType());
return isHighCostExpansionHelper(Op, L, At, BudgetRemaining, TTI,
Processed);
}
@ -2184,7 +2184,7 @@ bool SCEVExpander::isHighCostExpansionHelper(
if (auto *SC = dyn_cast<SCEVConstant>(UDivExpr->getRHS())) {
if (SC->getAPInt().isPowerOf2()) {
BudgetRemaining -=
TTI.getOperationCost(Instruction::LShr, S->getType());
TTI.getArithmeticInstrCost(Instruction::LShr, S->getType());
// Note that we don't count the cost of RHS, because it is a constant,
// and we consider those to be free. But if that changes, we would need
// to log2() it first before calling isHighCostExpansionHelper().
@ -2206,7 +2206,8 @@ bool SCEVExpander::isHighCostExpansionHelper(
return false; // Consider it to be free.
// Need to count the cost of this UDiv.
BudgetRemaining -= TTI.getOperationCost(Instruction::UDiv, S->getType());
BudgetRemaining -=
TTI.getArithmeticInstrCost(Instruction::UDiv, S->getType());
return isHighCostExpansionHelper(UDivExpr->getLHS(), L, At, BudgetRemaining,
TTI, Processed) ||
isHighCostExpansionHelper(UDivExpr->getRHS(), L, At, BudgetRemaining,
@ -2219,8 +2220,8 @@ bool SCEVExpander::isHighCostExpansionHelper(
assert(NAry->getNumOperands() >= 2 &&
"Polynomial should be at least linear");
int AddCost = TTI.getOperationCost(Instruction::Add, OpType);
int MulCost = TTI.getOperationCost(Instruction::Mul, OpType);
int AddCost = TTI.getArithmeticInstrCost(Instruction::Add, OpType);
int MulCost = TTI.getArithmeticInstrCost(Instruction::Mul, OpType);
// In this polynominal, we may have some zero operands, and we shouldn't
// really charge for those. So how many non-zero coeffients are there?
@ -2277,20 +2278,22 @@ bool SCEVExpander::isHighCostExpansionHelper(
int PairCost;
switch (S->getSCEVType()) {
case scAddExpr:
PairCost = TTI.getOperationCost(Instruction::Add, OpType);
PairCost = TTI.getArithmeticInstrCost(Instruction::Add, OpType);
break;
case scMulExpr:
// TODO: this is a very pessimistic cost modelling for Mul,
// because of Bin Pow algorithm actually used by the expander,
// see SCEVExpander::visitMulExpr(), ExpandOpBinPowN().
PairCost = TTI.getOperationCost(Instruction::Mul, OpType);
PairCost = TTI.getArithmeticInstrCost(Instruction::Mul, OpType);
break;
case scSMaxExpr:
case scUMaxExpr:
case scSMinExpr:
case scUMinExpr:
PairCost = TTI.getOperationCost(Instruction::ICmp, OpType) +
TTI.getOperationCost(Instruction::Select, OpType);
PairCost = TTI.getCmpSelInstrCost(Instruction::ICmp, OpType,
CmpInst::makeCmpResultType(OpType)) +
TTI.getCmpSelInstrCost(Instruction::Select, OpType,
CmpInst::makeCmpResultType(OpType));
break;
default:
llvm_unreachable("There are no other variants here.");