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.

llvm-svn: 171798

1 files changed, 50 insertions, 0 deletions

diff --git a/llvm/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll b/llvm/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll
new file mode 100644
index 00000000000..207598636c1
--- /dev/null
+++ b/llvm/test/Transforms/LoopVectorize/X86/unroll-small-loops.ll
@@ -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
+}