[DAGCombine] Produce better code for constant splats

This solves PR22276.
Splats of constants would sometimes produce redundant shuffles, sometimes ridiculously so (see the PR for details). Fold these shuffles into BUILD_VECTORs early on instead.

Differential Revision: http://reviews.llvm.org/D7093

llvm-svn: 226811

1 files changed, 40 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/splat-const.ll b/llvm/test/CodeGen/X86/splat-const.ll
new file mode 100644
index 00000000000..19997b03ad5
--- /dev/null
+++ b/llvm/test/CodeGen/X86/splat-const.ll
@@ -0,0 +1,40 @@
+; RUN: llc < %s -mcpu=penryn | FileCheck %s --check-prefix=SSE
+; RUN: llc < %s -mcpu=sandybridge | FileCheck %s --check-prefix=AVX
+; RUN: llc < %s -mcpu=haswell | FileCheck %s --check-prefix=AVX2
+; This checks that lowering for creation of constant vectors is sane and
+; doesn't use redundant shuffles. (fixes PR22276)
+target triple = "x86_64-unknown-unknown"
+
+define <4 x i32> @zero_vector() {
+; SSE-LABEL: zero_vector:
+; SSE: xorps %xmm0, %xmm0
+; SSE-NEXT: retq
+; AVX-LABEL: zero_vector:
+; AVX: vxorps %xmm0, %xmm0, %xmm0
+; AVX-NEXT: retq
+; AVX2-LABEL: zero_vector:
+; AVX2: vxorps %xmm0, %xmm0, %xmm0
+; AVX2-NEXT: retq
+  %zero = insertelement <4 x i32> undef, i32 0, i32 0
+  %splat = shufflevector <4 x i32> %zero, <4 x i32> undef, <4 x i32> zeroinitializer
+  ret <4 x i32> %splat
+}
+
+; Note that for the "const_vector" versions, lowering that uses a shuffle
+; instead of a load would be legitimate, if it's a single broadcast shuffle.
+; (as opposed to the previous mess)
+; However, this is not the current preferred lowering.
+define <4 x i32> @const_vector() {
+; SSE-LABEL: const_vector:
+; SSE: movaps {{.*}}, %xmm0 # xmm0 = [42,42,42,42]
+; SSE-NEXT: retq
+; AVX-LABEL: const_vector:
+; AVX: vmovaps {{.*}}, %xmm0 # xmm0 = [42,42,42,42]
+; AVX-NEXT: retq
+; AVX2-LABEL: const_vector:
+; AVX2: vbroadcastss {{[^%].*}}, %xmm0
+; AVX2-NEXT: retq
+  %const = insertelement <4 x i32> undef, i32 42, i32 0
+  %splat = shufflevector <4 x i32> %const, <4 x i32> undef, <4 x i32> zeroinitializer
+  ret <4 x i32> %splat
+}