[AVX] Optimize x86 VSELECT instructions using SimplifyDemandedBits.

We know that the blend instructions only use the MSB, so if the mask is
sign-extended then we can convert it into a SHL instruction. This is a
common pattern because the type-legalizer sign-extends the i1 type which
is used by the LLVM-IR for the condition.

Added a new optimization in SimplifyDemandedBits for SIGN_EXTEND_INREG -> SHL.

llvm-svn: 148225

1 files changed, 37 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/blend-msb.ll b/llvm/test/CodeGen/X86/blend-msb.ll
new file mode 100644
index 00000000000..3a10c70ada8
--- /dev/null
+++ b/llvm/test/CodeGen/X86/blend-msb.ll
@@ -0,0 +1,37 @@
+; RUN: llc < %s -mtriple=x86_64-apple-darwin -mcpu=corei7 -promote-elements -mattr=+sse41 | FileCheck %s
+
+
+; In this test we check that sign-extend of the mask bit is performed by
+; shifting the needed bit to the MSB, and not using shl+sra.
+
+;CHECK: vsel_float
+;CHECK: pslld
+;CHECK-NEXT: blendvps
+;CHECK: ret
+define <4 x float> @vsel_float(<4 x float> %v1, <4 x float> %v2) {
+  %vsel = select <4 x i1> <i1 true, i1 false, i1 false, i1 false>, <4 x float> %v1, <4 x float> %v2
+  ret <4 x float> %vsel
+}
+
+;CHECK: vsel_4xi8
+;CHECK: pslld
+;CHECK-NEXT: blendvps
+;CHECK: ret
+define <4 x i8> @vsel_4xi8(<4 x i8> %v1, <4 x i8> %v2) {
+  %vsel = select <4 x i1> <i1 true, i1 false, i1 false, i1 false>, <4 x i8> %v1, <4 x i8> %v2
+  ret <4 x i8> %vsel
+}
+
+
+; We do not have native support for v8i16 blends and we have to use the
+; blendvb instruction or a sequence of NAND/OR/AND. Make sure that we do not r
+; reduce the mask in this case.
+;CHECK: vsel_8xi16
+;CHECK: psllw
+;CHECK: psraw
+;CHECK: pblendvb
+;CHECK: ret
+define <8 x i16> @vsel_8xi16(<8 x i16> %v1, <8 x i16> %v2) {
+  %vsel = select <8 x i1> <i1 true, i1 false, i1 false, i1 false, i1 true, i1 false, i1 false, i1 false>, <8 x i16> %v1, <8 x i16> %v2
+  ret <8 x i16> %vsel
+}