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[LoopUnrolling] Re-prioritize Peeling and Partial unrolling

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Summary:
In current implementation the loop peeling happens after trip-count based partial unrolling and may
sometimes not happen at all due to it (for example, if trip count is known, but UP.Partial = false). This
is generally bad, the more than there are some situations where peeling is profitable even if the partial
unrolling is disabled.

This patch is a NFC which reorders peeling and partial unrolling application and prepares the code for
implementation of the said optimizations.

Patch by Max Kazantsev!

Reviewers: sanjoy, anna, reames, apilipenko, igor-laevsky, mkuper

Reviewed By: mkuper

Subscribers: mkuper, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D30243

llvm-svn: 296897
This commit is contained in:
Sanjoy Das 2017-03-03 18:19:10 +00:00
parent 1ba7dfb74b
commit f07ac23c21
3 changed files with 18 additions and 11 deletions
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3
include/llvm/Transforms/Utils/UnrollLoop.h
View File

@ -53,7 +53,8 @@ bool UnrollRuntimeLoopRemainder(Loop *L, unsigned Count,
bool PreserveLCSSA);
void computePeelCount(Loop *L, unsigned LoopSize,
TargetTransformInfo::UnrollingPreferences &UP);
TargetTransformInfo::UnrollingPreferences &UP,
unsigned &TripCount);
bool peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI, ScalarEvolution *SE,
DominatorTree *DT, AssumptionCache *AC, bool PreserveLCSSA);

18
lib/Transforms/Scalar/LoopUnrollPass.cpp
View File

@ -784,7 +784,15 @@ static bool computeUnrollCount(
}
}
// 4rd priority is partial unrolling.
// 4th priority is loop peeling
computePeelCount(L, LoopSize, UP, TripCount);
if (UP.PeelCount) {
UP.Runtime = false;
UP.Count = 1;
return ExplicitUnroll;
}
// 5th priority is partial unrolling.
// Try partial unroll only when TripCount could be staticaly calculated.
if (TripCount) {
UP.Partial |= ExplicitUnroll;
@ -847,14 +855,6 @@ static bool computeUnrollCount(
<< "Unable to fully unroll loop as directed by unroll(full) pragma "
"because loop has a runtime trip count.");
// 5th priority is loop peeling
computePeelCount(L, LoopSize, UP);
if (UP.PeelCount) {
UP.Runtime = false;
UP.Count = 1;
return ExplicitUnroll;
}
// 6th priority is runtime unrolling.
// Don't unroll a runtime trip count loop when it is disabled.
if (HasRuntimeUnrollDisablePragma(L)) {

8
lib/Transforms/Utils/LoopUnrollPeel.cpp
View File

@ -61,7 +61,8 @@ static bool canPeel(Loop *L) {
// Return the number of iterations we want to peel off.
void llvm::computePeelCount(Loop *L, unsigned LoopSize,
TargetTransformInfo::UnrollingPreferences &UP) {
TargetTransformInfo::UnrollingPreferences &UP,
unsigned &TripCount) {
UP.PeelCount = 0;
if (!canPeel(L))
return;
@ -70,6 +71,11 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
if (!L->empty())
return;
// Bail if we know the statically calculated trip count.
// In this case we rather prefer partial unrolling.
if (TripCount)
return;
// If the user provided a peel count, use that.
bool UserPeelCount = UnrollForcePeelCount.getNumOccurrences() > 0;
if (UserPeelCount) {