[llvm-commits] [llvm] r171798 - in /llvm/trunk: lib/Transforms/Vectorize/LoopVectorize.cpp test/Transforms/LoopVectorize/X86/unroll-small-loops.ll

[Nadav Rotem]

Author: nadav
Date: Mon Jan  7 15:54:51 2013
New Revision: 171798

URL: http://llvm.org/viewvc/llvm-project?rev=171798&view=rev
Log:
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.


Added:
    llvm/trunk/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll
Modified:
    llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp

Modified: llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp?rev=171798&r1=171797&r2=171798&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp (original)
+++ llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp Mon Jan  7 15:54:51 2013
@@ -101,7 +101,13 @@
                    cl::desc("Enable if-conversion during vectorization."));
 
 /// We don't vectorize loops with a known constant trip count below this number.
-static const unsigned TinyTripCountThreshold = 16;
+static const unsigned TinyTripCountVectorThreshold = 16;
+
+/// We don't unroll loops with a known constant trip count below this number.
+static const unsigned TinyTripCountUnrollThreshold = 128;
+
+/// We don't unroll loops that are larget than this threshold.
+static const unsigned MaxLoopSizeThreshold = 32;
 
 /// When performing a runtime memory check, do not check more than this
 /// number of pointers. Notice that the check is quadratic!
@@ -2016,7 +2022,7 @@
 
   // Do not loop-vectorize loops with a tiny trip count.
   unsigned TC = SE->getSmallConstantTripCount(TheLoop, Latch);
-  if (TC > 0u && TC < TinyTripCountThreshold) {
+  if (TC > 0u && TC < TinyTripCountVectorThreshold) {
     DEBUG(dbgs() << "LV: Found a loop with a very small trip count. " <<
           "This loop is not worth vectorizing.\n");
     return false;
@@ -2678,6 +2684,12 @@
   if (OptForSize)
     return 1;
 
+  // Do not unroll loops with a relatively small trip count.
+  unsigned TC = SE->getSmallConstantTripCount(TheLoop,
+                                              TheLoop->getLoopLatch());
+  if (TC > 1 && TC < TinyTripCountUnrollThreshold)
+    return 1;
+
   unsigned TargetVectorRegisters = TTI.getNumberOfRegisters(true);
   DEBUG(dbgs() << "LV: The target has " << TargetVectorRegisters <<
         " vector registers\n");
@@ -2698,7 +2710,7 @@
 
   // We don't want to unroll the loops to the point where they do not fit into
   // the decoded cache. Assume that we only allow 32 IR instructions.
-  UF = std::min(UF, (32 / R.NumInstructions));
+  UF = std::min(UF, (MaxLoopSizeThreshold / R.NumInstructions));
 
   // Clamp the unroll factor ranges to reasonable factors.
   if (UF > MaxUnrollSize)

Added: llvm/trunk/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll?rev=171798&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll (added)
+++ llvm/trunk/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll Mon Jan  7 15:54:51 2013
@@ -0,0 +1,50 @@
+; 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
+
+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"
+target triple = "x86_64-apple-macosx10.8.0"
+;CHECK: @foo
+;CHECK: load <4 x i32>
+;CHECK-NOT: load <4 x i32>
+;CHECK: store <4 x i32>
+;CHECK-NOT: store <4 x i32>
+;CHECK: ret
+define i32 @foo(i32* nocapture %A) nounwind uwtable ssp {
+  br label %1
+
+; <label>:1                                       ; preds = %1, %0
+  %indvars.iv = phi i64 [ 0, %0 ], [ %indvars.iv.next, %1 ]
+  %2 = getelementptr inbounds i32* %A, i64 %indvars.iv
+  %3 = load i32* %2, align 4
+  %4 = add nsw i32 %3, 6
+  store i32 %4, i32* %2, align 4
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, 100
+  br i1 %exitcond, label %5, label %1
+
+; <label>:5                                       ; preds = %1
+  ret i32 undef
+}
+
+;CHECK: @bar
+;CHECK: store <4 x i32>
+;CHECK: store <4 x i32>
+;CHECK: ret
+define i32 @bar(i32* nocapture %A, i32 %n) nounwind uwtable ssp {
+  %1 = icmp sgt i32 %n, 0
+  br i1 %1, label %.lr.ph, label %._crit_edge
+
+.lr.ph:                                           ; preds = %0, %.lr.ph
+  %indvars.iv = phi i64 [ %indvars.iv.next, %.lr.ph ], [ 0, %0 ]
+  %2 = getelementptr inbounds i32* %A, i64 %indvars.iv
+  %3 = load i32* %2, align 4
+  %4 = add nsw i32 %3, 6
+  store i32 %4, i32* %2, align 4
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %._crit_edge, label %.lr.ph
+
+._crit_edge:                                      ; preds = %.lr.ph, %0
+  ret i32 undef
+}