[X86] Disable loop unrolling in loop vectorization pass when VF is 1.

The patch disabled unrolling in loop vectorization pass when VF==1 on x86 architecture,
by setting MaxInterleaveFactor to 1. Unrolling in loop vectorization pass may introduce
the cost of overflow check, memory boundary check and extra prologue/epilogue code when
regular unroller will unroll the loop another time. Disable it when VF==1 remove the
unnecessary cost on x86. The same can be done for other platforms after verifying
interleaving/memory bound checking to be not perf critical on those platforms.

Differential Revision: http://reviews.llvm.org/D9515

llvm-svn: 236613

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
index 5136619235b..17c86a7b9f0 100644
--- a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
+++ b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -66,7 +66,13 @@ unsigned X86TTIImpl::getRegisterBitWidth(bool Vector) {
 
 }
 
-unsigned X86TTIImpl::getMaxInterleaveFactor() {
+unsigned X86TTIImpl::getMaxInterleaveFactor(unsigned VF) {
+  // If the loop will not be vectorized, don't interleave the loop.
+  // Let regular unroll to unroll the loop, which saves the overflow
+  // check and memory check cost.
+  if (VF == 1)
+    return 1;
+
   if (ST->isAtom())
     return 1;