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SLPVectorizer: limit the number of alias checks to reduce the runtime.

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In case of blocks with many memory-accessing instructions, alias checking can take lot of time
(because calculating the memory dependencies has quadratic complexity).
I chose a limit which resulted in no changes when running the benchmarks.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226439 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Erik Eckstein 2015-01-19 09:33:38 +00:00
parent e6e4c786b1
commit 04075c619b
1 changed files with 16 additions and 1 deletions
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17
lib/Transforms/Vectorize/SLPVectorizer.cpp
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@ -75,6 +75,10 @@ static const unsigned MinVecRegSize = 128;
static const unsigned RecursionMaxDepth = 12;
// Limit the number of alias checks. The limit is chosen so that
// it has no negative effect on the llvm benchmarks.
static const unsigned AliasedCheckLimit = 10;
/// \returns the parent basic block if all of the instructions in \p VL
/// are in the same block or null otherwise.
static BasicBlock *getSameBlock(ArrayRef<Value *> VL) {
@ -2754,11 +2758,22 @@ void BoUpSLP::BlockScheduling::calculateDependencies(ScheduleData *SD,
Instruction *SrcInst = BundleMember->Inst;
AliasAnalysis::Location SrcLoc = getLocation(SrcInst, SLP->AA);
bool SrcMayWrite = BundleMember->Inst->mayWriteToMemory();
unsigned numAliased = 0;
while (DepDest) {
assert(isInSchedulingRegion(DepDest));
if (SrcMayWrite || DepDest->Inst->mayWriteToMemory()) {
if (SLP->isAliased(SrcLoc, SrcInst, DepDest->Inst)) {
// Limit the number of alias checks, becaus SLP->isAliased() is
// the expensive part in the following loop.
if (numAliased >= AliasedCheckLimit
|| SLP->isAliased(SrcLoc, SrcInst, DepDest->Inst)) {
// We increment the counter only if the locations are aliased
// (instead of counting all alias checks). This gives a better
// balance between reduced runtime accurate dependencies.
numAliased++;
DepDest->MemoryDependencies.push_back(BundleMember);
BundleMember->Dependencies++;
ScheduleData *DestBundle = DepDest->FirstInBundle;