[LoopUnroll] Allow unrolling if the unrolled size does not exceed loop size.
Summary:
In the following cases, unrolling can be beneficial, even when
optimizing for code size:
1) very low trip counts
2) potential to constant fold most instructions after fully unrolling.
We can unroll in those cases, by setting the unrolling threshold to the
loop size. This might highlight some cost modeling issues and fixing
them will have a positive impact in general.
Reviewers: vsk, efriedma, dmgreen, paquette
Reviewed By: paquette
Differential Revision: https://reviews.llvm.org/D60265
llvm-svn: 358586
diff --git a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
index c113e4d..53015d2 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -207,6 +207,7 @@
if (OptForSize) {
UP.Threshold = UP.OptSizeThreshold;
UP.PartialThreshold = UP.PartialOptSizeThreshold;
+ UP.MaxPercentThresholdBoost = 100;
}
// Apply any user values specified by cl::opt
@@ -993,6 +994,7 @@
if (OnlyWhenForced && !(TM & TM_Enable))
return LoopUnrollResult::Unmodified;
+ bool OptForSize = L->getHeader()->getParent()->hasOptSize();
unsigned NumInlineCandidates;
bool NotDuplicatable;
bool Convergent;
@@ -1000,8 +1002,11 @@
L, SE, TTI, BFI, PSI, OptLevel, ProvidedThreshold, ProvidedCount,
ProvidedAllowPartial, ProvidedRuntime, ProvidedUpperBound,
ProvidedAllowPeeling);
- // Exit early if unrolling is disabled.
- if (UP.Threshold == 0 && (!UP.Partial || UP.PartialThreshold == 0))
+
+ // Exit early if unrolling is disabled. For OptForSize, we pick the loop size
+ // as threshold later on.
+ if (UP.Threshold == 0 && (!UP.Partial || UP.PartialThreshold == 0) &&
+ !OptForSize)
return LoopUnrollResult::Unmodified;
SmallPtrSet<const Value *, 32> EphValues;
@@ -1016,6 +1021,12 @@
<< " instructions.\n");
return LoopUnrollResult::Unmodified;
}
+
+ // When optimizing for size, use LoopSize as threshold, to (fully) unroll
+ // loops, if it does not increase code size.
+ if (OptForSize)
+ UP.Threshold = std::max(UP.Threshold, LoopSize);
+
if (NumInlineCandidates != 0) {
LLVM_DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
return LoopUnrollResult::Unmodified;