[llvm] r313046 - [LV] Clamp the VF to the trip count
Anna Thomas via llvm-commits
llvm-commits at lists.llvm.org
Tue Sep 12 09:32:45 PDT 2017
Author: annat
Date: Tue Sep 12 09:32:45 2017
New Revision: 313046
URL: http://llvm.org/viewvc/llvm-project?rev=313046&view=rev
Log:
[LV] Clamp the VF to the trip count
Summary:
When the MaxVectorSize > ConstantTripCount, we should just clamp the
vectorization factor to be the ConstantTripCount.
This vectorizes loops where the TinyTripCountThreshold >= TripCount < MaxVF.
Earlier we were finding the maximum vector width, which could be greater than
the trip count itself. The Loop vectorizer does all the work for generating a
vectorizable loop, but in the end we would always choose the scalar loop (since
the VF > trip count). This allows us to choose the VF keeping in mind the trip
count if available.
This is a fix on top of rL312472.
Reviewers: Ayal, zvi, hfinkel, dneilson
Reviewed by: Ayal
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D37702
Modified:
llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
llvm/trunk/test/Transforms/LoopVectorize/X86/vector_max_bandwidth.ll
Modified: llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp?rev=313046&r1=313045&r2=313046&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp (original)
+++ llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp Tue Sep 12 09:32:45 2017
@@ -1960,7 +1960,7 @@ public:
private:
/// \return An upper bound for the vectorization factor, larger than zero.
/// One is returned if vectorization should best be avoided due to cost.
- unsigned computeFeasibleMaxVF(bool OptForSize, unsigned ConstTripCount = 0);
+ unsigned computeFeasibleMaxVF(bool OptForSize, unsigned ConstTripCount);
/// The vectorization cost is a combination of the cost itself and a boolean
/// indicating whether any of the contributing operations will actually
@@ -6161,8 +6161,9 @@ Optional<unsigned> LoopVectorizationCost
return None;
}
+ unsigned TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
if (!OptForSize) // Remaining checks deal with scalar loop when OptForSize.
- return computeFeasibleMaxVF(OptForSize);
+ return computeFeasibleMaxVF(OptForSize, TC);
if (Legal->getRuntimePointerChecking()->Need) {
ORE->emit(createMissedAnalysis("CantVersionLoopWithOptForSize")
@@ -6175,7 +6176,6 @@ Optional<unsigned> LoopVectorizationCost
}
// If we optimize the program for size, avoid creating the tail loop.
- unsigned TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
DEBUG(dbgs() << "LV: Found trip count: " << TC << '\n');
// If we don't know the precise trip count, don't try to vectorize.
@@ -6236,15 +6236,20 @@ LoopVectorizationCostModel::computeFeasi
DEBUG(dbgs() << "LV: The Widest register is: " << WidestRegister
<< " bits.\n");
+ assert(MaxVectorSize <= 64 && "Did not expect to pack so many elements"
+ " into one vector!");
if (MaxVectorSize == 0) {
DEBUG(dbgs() << "LV: The target has no vector registers.\n");
MaxVectorSize = 1;
} else if (ConstTripCount && ConstTripCount < MaxVectorSize &&
- isPowerOf2_32(ConstTripCount))
+ isPowerOf2_32(ConstTripCount)) {
+ // We need to clamp the VF to be the ConstTripCount. There is no point in
+ // choosing a higher viable VF as done in the loop below.
+ DEBUG(dbgs() << "LV: Clamping the MaxVF to the constant trip count: "
+ << ConstTripCount << "\n");
MaxVectorSize = ConstTripCount;
-
- assert(MaxVectorSize <= 64 && "Did not expect to pack so many elements"
- " into one vector!");
+ return MaxVectorSize;
+ }
unsigned MaxVF = MaxVectorSize;
if (MaximizeBandwidth && !OptForSize) {
Modified: llvm/trunk/test/Transforms/LoopVectorize/X86/vector_max_bandwidth.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopVectorize/X86/vector_max_bandwidth.ll?rev=313046&r1=313045&r2=313046&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopVectorize/X86/vector_max_bandwidth.ll (original)
+++ llvm/trunk/test/Transforms/LoopVectorize/X86/vector_max_bandwidth.ll Tue Sep 12 09:32:45 2017
@@ -46,3 +46,29 @@ for.body:
%exitcond = icmp eq i64 %indvars.iv.next, 1000
br i1 %exitcond, label %for.cond.cleanup, label %for.body
}
+
+; We should not choose a VF larger than the constant TC.
+; VF chosen should be atmost 16 (not the max possible vector width = 32 for AVX2)
+define void @not_too_small_tc(i8* noalias nocapture %A, i8* noalias nocapture readonly %B) {
+; CHECK-LABEL: not_too_small_tc
+; CHECK-AVX2: LV: Selecting VF: 16.
+entry:
+ br label %for.body
+
+for.body:
+ %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+ %arrayidx = getelementptr inbounds i8, i8* %B, i64 %indvars.iv
+ %l1 = load i8, i8* %arrayidx, align 4, !llvm.mem.parallel_loop_access !3
+ %arrayidx2 = getelementptr inbounds i8, i8* %A, i64 %indvars.iv
+ %l2 = load i8, i8* %arrayidx2, align 4, !llvm.mem.parallel_loop_access !3
+ %add = add i8 %l1, %l2
+ store i8 %add, i8* %arrayidx2, align 4, !llvm.mem.parallel_loop_access !3
+ %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+ %exitcond = icmp eq i64 %indvars.iv.next, 16
+ br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !4
+
+for.end:
+ ret void
+}
+!3 = !{!3}
+!4 = !{!4}
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