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Add a separate unrolling threshold when the current function is being optimized for size.

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The threshold value of 50 is arbitrary, and I chose it simply by analogy to the inlining thresholds, where
the baseline unrolling threshold is slightly smaller than the baseline inlining threshold.  This could
undoubtedly use some tuning.

llvm-svn: 113306
This commit is contained in:
Owen Anderson 2010-09-07 23:15:30 +00:00
parent f1418ef333
commit 38493be0ce
1 changed files with 20 additions and 4 deletions
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24
lib/Transforms/Scalar/LoopUnrollPass.cpp
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@ -49,6 +49,12 @@ namespace {
/// that the loop unroll should be performed regardless of how much
/// code expansion would result.
static const unsigned NoThreshold = UINT_MAX;
// Threshold to use when optsize is specified (and there is no
// explicit -unroll-threshold).
static const unsigned OptSizeUnrollThreshold = 50;
unsigned CurrentThreshold;
bool runOnLoop(Loop *L, LPPassManager &LPM);
@ -88,12 +94,22 @@ static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls) {
}
bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
LoopInfo *LI = &getAnalysis<LoopInfo>();
BasicBlock *Header = L->getHeader();
DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
<< "] Loop %" << Header->getName() << "\n");
(void)Header;
// Determine the current unrolling threshold. While this is normally set
// from UnrollThreshold, it is overridden to a smaller value if the current
// function is marked as optimize-for-size, and the unroll threshold was
// not user specified.
CurrentThreshold = UnrollThreshold;
if (Header->getParent()->hasFnAttr(Attribute::OptimizeForSize) &&
UnrollThreshold.getNumOccurrences() == 0)
CurrentThreshold = OptSizeUnrollThreshold;
// Find trip count
unsigned TripCount = L->getSmallConstantTripCount();
@ -111,7 +127,7 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
}
// Enforce the threshold.
if (UnrollThreshold != NoThreshold) {
if (CurrentThreshold != NoThreshold) {
unsigned NumCalls;
unsigned LoopSize = ApproximateLoopSize(L, NumCalls);
DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
@ -120,16 +136,16 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
return false;
}
uint64_t Size = (uint64_t)LoopSize*Count;
if (TripCount != 1 && Size > UnrollThreshold) {
if (TripCount != 1 && Size > CurrentThreshold) {
DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
<< " because size: " << Size << ">" << UnrollThreshold << "\n");
<< " because size: " << Size << ">" << CurrentThreshold << "\n");
if (!UnrollAllowPartial) {
DEBUG(dbgs() << " will not try to unroll partially because "
<< "-unroll-allow-partial not given\n");
return false;
}
// Reduce unroll count to be modulo of TripCount for partial unrolling
Count = UnrollThreshold / LoopSize;
Count = CurrentThreshold / LoopSize;
while (Count != 0 && TripCount%Count != 0) {
Count--;
}