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LoopVectorizer: When we vectorizer and widen loops we process many elements at once. This is a good thing, except for
small loops. On small loops post-loop that handles scalars (and runs slower) can take more time to execute than the rest of the loop. This patch disables widening of loops with a small static trip count. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171798 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -101,7 +101,13 @@ EnableIfConversion("enable-if-conversion", cl::init(true), cl::Hidden,
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cl::desc("Enable if-conversion during vectorization."));
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cl::desc("Enable if-conversion during vectorization."));
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/// We don't vectorize loops with a known constant trip count below this number.
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/// We don't vectorize loops with a known constant trip count below this number.
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static const unsigned TinyTripCountThreshold = 16;
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static const unsigned TinyTripCountVectorThreshold = 16;
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/// We don't unroll loops with a known constant trip count below this number.
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static const unsigned TinyTripCountUnrollThreshold = 128;
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/// We don't unroll loops that are larget than this threshold.
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static const unsigned MaxLoopSizeThreshold = 32;
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/// When performing a runtime memory check, do not check more than this
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/// When performing a runtime memory check, do not check more than this
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/// number of pointers. Notice that the check is quadratic!
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/// number of pointers. Notice that the check is quadratic!
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@ -2016,7 +2022,7 @@ bool LoopVectorizationLegality::canVectorize() {
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// Do not loop-vectorize loops with a tiny trip count.
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// Do not loop-vectorize loops with a tiny trip count.
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unsigned TC = SE->getSmallConstantTripCount(TheLoop, Latch);
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unsigned TC = SE->getSmallConstantTripCount(TheLoop, Latch);
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if (TC > 0u && TC < TinyTripCountThreshold) {
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if (TC > 0u && TC < TinyTripCountVectorThreshold) {
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DEBUG(dbgs() << "LV: Found a loop with a very small trip count. " <<
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DEBUG(dbgs() << "LV: Found a loop with a very small trip count. " <<
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"This loop is not worth vectorizing.\n");
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"This loop is not worth vectorizing.\n");
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return false;
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return false;
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@ -2678,6 +2684,12 @@ LoopVectorizationCostModel::selectUnrollFactor(bool OptForSize,
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if (OptForSize)
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if (OptForSize)
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return 1;
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return 1;
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// Do not unroll loops with a relatively small trip count.
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unsigned TC = SE->getSmallConstantTripCount(TheLoop,
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TheLoop->getLoopLatch());
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if (TC > 1 && TC < TinyTripCountUnrollThreshold)
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return 1;
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unsigned TargetVectorRegisters = TTI.getNumberOfRegisters(true);
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unsigned TargetVectorRegisters = TTI.getNumberOfRegisters(true);
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DEBUG(dbgs() << "LV: The target has " << TargetVectorRegisters <<
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DEBUG(dbgs() << "LV: The target has " << TargetVectorRegisters <<
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" vector registers\n");
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" vector registers\n");
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@ -2698,7 +2710,7 @@ LoopVectorizationCostModel::selectUnrollFactor(bool OptForSize,
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// We don't want to unroll the loops to the point where they do not fit into
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// We don't want to unroll the loops to the point where they do not fit into
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// the decoded cache. Assume that we only allow 32 IR instructions.
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// the decoded cache. Assume that we only allow 32 IR instructions.
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UF = std::min(UF, (32 / R.NumInstructions));
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UF = std::min(UF, (MaxLoopSizeThreshold / R.NumInstructions));
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// Clamp the unroll factor ranges to reasonable factors.
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// Clamp the unroll factor ranges to reasonable factors.
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if (UF > MaxUnrollSize)
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if (UF > MaxUnrollSize)
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50
test/Transforms/LoopVectorize/X86/unroll-small-loops.ll
Normal file
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test/Transforms/LoopVectorize/X86/unroll-small-loops.ll
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@ -0,0 +1,50 @@
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; RUN: opt < %s -loop-vectorize -mtriple=x86_64-apple-macosx10.8.0 -mcpu=corei7-avx2 -force-vector-width=4 -force-vector-unroll=0 -dce -S | FileCheck %s
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target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
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target triple = "x86_64-apple-macosx10.8.0"
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;CHECK: @foo
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;CHECK: load <4 x i32>
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;CHECK-NOT: load <4 x i32>
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;CHECK: store <4 x i32>
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;CHECK-NOT: store <4 x i32>
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;CHECK: ret
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define i32 @foo(i32* nocapture %A) nounwind uwtable ssp {
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br label %1
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; <label>:1 ; preds = %1, %0
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%indvars.iv = phi i64 [ 0, %0 ], [ %indvars.iv.next, %1 ]
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%2 = getelementptr inbounds i32* %A, i64 %indvars.iv
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%3 = load i32* %2, align 4
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%4 = add nsw i32 %3, 6
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store i32 %4, i32* %2, align 4
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%indvars.iv.next = add i64 %indvars.iv, 1
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%lftr.wideiv = trunc i64 %indvars.iv.next to i32
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%exitcond = icmp eq i32 %lftr.wideiv, 100
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br i1 %exitcond, label %5, label %1
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; <label>:5 ; preds = %1
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ret i32 undef
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}
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;CHECK: @bar
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;CHECK: store <4 x i32>
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;CHECK: store <4 x i32>
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;CHECK: ret
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define i32 @bar(i32* nocapture %A, i32 %n) nounwind uwtable ssp {
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%1 = icmp sgt i32 %n, 0
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br i1 %1, label %.lr.ph, label %._crit_edge
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.lr.ph: ; preds = %0, %.lr.ph
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%indvars.iv = phi i64 [ %indvars.iv.next, %.lr.ph ], [ 0, %0 ]
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%2 = getelementptr inbounds i32* %A, i64 %indvars.iv
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%3 = load i32* %2, align 4
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%4 = add nsw i32 %3, 6
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store i32 %4, i32* %2, align 4
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%indvars.iv.next = add i64 %indvars.iv, 1
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%lftr.wideiv = trunc i64 %indvars.iv.next to i32
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%exitcond = icmp eq i32 %lftr.wideiv, %n
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br i1 %exitcond, label %._crit_edge, label %.lr.ph
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._crit_edge: ; preds = %.lr.ph, %0
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ret i32 undef
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}
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