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[SCEV] SCEVExpander::isHighCostExpansionHelper(): cost-model UDiv by power-of-two as LShr 

Summary:
Like with casts, we need to subtract the cost of `lshr` instruction
from budget, and recurse into LHS operand.
Seems "pretty obviously correct" to me?

To be noted, there is a number of other shortcuts we //could// cost-model:
* `... + (-1 * ...)` -> `... - ...` <-  likely very frequent case
* `x - (rem x, power-of-2)`, which is currently `(x udiv power-of-2) * power-of-2` -> `x & -log2(power-of-2)`
* `rem x, power-of-2`, which is currently `x - ((x udiv power-of-2) * power-of-2)` -> `x & log2(power-of-2)-1`
* `... * power-of-2` -> `... << log2(power-of-2)` <- likely not very beneficial

Reviewers: reames, mkazantsev, wmi, sanjoy

Reviewed By: mkazantsev

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73718
This commit is contained in:
Roman Lebedev 2020-02-25 21:51:22 +03:00
parent f90973f486
commit b8793f0dab
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GPG Key ID: 083C3EBB4A1689E0
1 changed files with 10 additions and 12 deletions
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22
llvm/lib/Analysis/ScalarEvolutionExpander.cpp
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@ -2180,20 +2180,18 @@ bool SCEVExpander::isHighCostExpansionHelper(
if (auto *UDivExpr = dyn_cast<SCEVUDivExpr>(S)) {
// If the divisor is a power of two and the SCEV type fits in a native
// integer (and the LHS not expensive), consider the division cheap
// irrespective of whether it occurs in the user code since it can be
// lowered into a right shift.
if (auto *SC = dyn_cast<SCEVConstant>(UDivExpr->getRHS()))
// If the divisor is a power of two count this as a logical right-shift.
if (auto *SC = dyn_cast<SCEVConstant>(UDivExpr->getRHS())) {
if (SC->getAPInt().isPowerOf2()) {
if (isHighCostExpansionHelper(UDivExpr->getLHS(), L, At,
BudgetRemaining, TTI, Processed))
return true;
const DataLayout &DL =
L->getHeader()->getParent()->getParent()->getDataLayout();
unsigned Width = cast<IntegerType>(UDivExpr->getType())->getBitWidth();
return DL.isIllegalInteger(Width);
BudgetRemaining -=
TTI.getOperationCost(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().
return isHighCostExpansionHelper(UDivExpr->getLHS(), L, At,
BudgetRemaining, TTI, Processed);
}
}
// UDivExpr is very likely a UDiv that ScalarEvolution's HowFarToZero or
// HowManyLessThans produced to compute a precise expression, rather than a