[X86] When commuting SSE immediate blend, make sure that the new blend mask is a valid imm8.

Example:
define <4 x i32> @test(<4 x i32> %a, <4 x i32> %b) {
  %shuffle = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 4, i32 5, i32 6, i32 3>
  ret <4 x i32> %shuffle
}

Before llc (-mattr=+sse4.1), produced the following assembly instruction:
  pblendw $4294967103, %xmm1, %xmm0

After
  pblendw $63, %xmm1, %xmm0


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221455 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 15 insertions, 1 deletions

diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index dd463f15b32..a49dcc7d160 100644
--- a/lib/Target/X86/X86InstrInfo.cpp
+++ b/lib/Target/X86/X86InstrInfo.cpp
@@ -2449,7 +2449,8 @@ X86InstrInfo::commuteInstruction(MachineInstr *MI, bool NewMI) const {
     case X86::VPBLENDDYrri:  Mask = 0xFF; break;
     case X86::VPBLENDWYrri:  Mask = 0xFF; break;
     }
-    unsigned Imm = MI->getOperand(3).getImm();
+    // Only the least significant bits of Imm are used.
+    unsigned Imm = MI->getOperand(3).getImm() & Mask;
     if (NewMI) {
       MachineFunction &MF = *MI->getParent()->getParent();
       MI = MF.CloneMachineInstr(MI);
diff --git a/test/CodeGen/X86/commuted-blend-mask.ll b/test/CodeGen/X86/commuted-blend-mask.ll
new file mode 100644
index 00000000000..e6322cbb7a1
--- /dev/null
+++ b/test/CodeGen/X86/commuted-blend-mask.ll
@@ -0,0 +1,13 @@
+; RUN: llc -mtriple=x86_64-unknown-unknown -mattr=+sse4.1 < %s | FileCheck %s
+
+; When commuting the operands of a SSE blend, make sure that the resulting blend
+; mask can be encoded as a imm8.
+; Before, when commuting the operands to the shuffle in function @test, the backend
+; produced the following assembly:
+;   pblendw $4294967103, %xmm1, %xmm0
+
+define <4 x i32> @test(<4 x i32> %a, <4 x i32> %b) {
+  ;CHECK: pblendw $63, %xmm1, %xmm0
+  %shuffle = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 4, i32 5, i32 6, i32 3>
+  ret <4 x i32> %shuffle
+}