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[SCEV] Strengthen SCEVExpander::isHighCostExpansion.

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Summary:

Teach `isHighCostExpansion` to consider divisions by power-of-two
constants as cheap and add a test case.  This change is needed for a new
user of `isHighCostExpansion` that will be added in a subsequent change.

Depends on D8995.

Reviewers: atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8993

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234845 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das
2015-04-14 03:20:32 +00:00
parent dee1e82714
commit 17e08f50b9
2 changed files with 46 additions and 6 deletions
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23
lib/Analysis/ScalarEvolutionExpander.cpp
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@@ -1810,12 +1810,23 @@ bool SCEVExpander::isHighCostExpansionHelper(
if (!Processed.insert(S).second)
return false;
// If the backedge-taken count is a UDiv, it's very likely a UDiv that
// ScalarEvolution's HowFarToZero or HowManyLessThans produced to compute a
// precise expression, rather than a UDiv from the user's code. If we can't
// find a UDiv in the code with some simple searching, assume the former and
// forego rewriting the loop.
if (isa<SCEVUDivExpr>(S)) {
if (auto *UDivExpr = dyn_cast<SCEVUDivExpr>(S)) {
// If the divisor is a power of two and the SCEV type fits in a native
// integer, consider the divison 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 (SC->getValue()->getValue().isPowerOf2()) {
const DataLayout &DL =
L->getHeader()->getParent()->getParent()->getDataLayout();
unsigned Width = cast<IntegerType>(UDivExpr->getType())->getBitWidth();
return DL.isIllegalInteger(Width);
}
// UDivExpr is very likely a UDiv that ScalarEvolution's HowFarToZero or
// HowManyLessThans produced to compute a precise expression, rather than a
// UDiv from the user's code. If we can't find a UDiv in the code with some
// simple searching, assume the former consider UDivExpr expensive to
// compute.
BasicBlock *ExitingBB = L->getExitingBlock();
if (!ExitingBB)
return true;