[llvm] r243519 - [PPC] Fix PR24216: Don't generate splat for misaligned shuffle mask

[Bill Schmidt]

Author: wschmidt
Date: Wed Jul 29 09:31:57 2015
New Revision: 243519

URL: http://llvm.org/viewvc/llvm-project?rev=243519&view=rev
Log:
[PPC] Fix PR24216: Don't generate splat for misaligned shuffle mask

Given certain shuffle-vector masks, LLVM emits splat instructions
which splat the wrong bytes from the source register.  The issue is
that the function PPC::isSplatShuffleMask() in PPCISelLowering.cpp
does not ensure that the splat pattern found is requesting bytes that
are aligned on an EltSize boundary.  This patch detects this situation
as not a valid splat mask, resulting in a permute being generated
instead of a splat.

Patch and test case by Tyler Kenney, cleaned up a bit by me.

This is a simple bug fix that would be good to incorporate into 3.7.

Added:
    llvm/trunk/test/CodeGen/PowerPC/pr24216.ll
Modified:
    llvm/trunk/lib/Target/PowerPC/PPCISelLowering.cpp

Modified: llvm/trunk/lib/Target/PowerPC/PPCISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCISelLowering.cpp?rev=243519&r1=243518&r2=243519&view=diff
==============================================================================
--- llvm/trunk/lib/Target/PowerPC/PPCISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/PowerPC/PPCISelLowering.cpp Wed Jul 29 09:31:57 2015
@@ -1430,6 +1430,11 @@ bool PPC::isSplatShuffleMask(ShuffleVect
   assert(N->getValueType(0) == MVT::v16i8 &&
          (EltSize == 1 || EltSize == 2 || EltSize == 4));
 
+  // The consecutive indices need to specify an element, not part of two
+  // different elements.  So abandon ship early if this isn't the case.
+  if (N->getMaskElt(0) % EltSize != 0)
+    return false;
+
   // This is a splat operation if each element of the permute is the same, and
   // if the value doesn't reference the second vector.
   unsigned ElementBase = N->getMaskElt(0);

Added: llvm/trunk/test/CodeGen/PowerPC/pr24216.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/pr24216.ll?rev=243519&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/PowerPC/pr24216.ll (added)
+++ llvm/trunk/test/CodeGen/PowerPC/pr24216.ll Wed Jul 29 09:31:57 2015
@@ -0,0 +1,14 @@
+; RUN: llc -mcpu=pwr8 -mtriple=powerpc64le-unknown-linux-gnu < %s | FileCheck %s
+
+; Test case adapted from PR24216.
+
+define void @foo(<16 x i8>* nocapture readonly %in, <16 x i8>* nocapture %out) {
+entry:
+  %0 = load <16 x i8>, <16 x i8>* %in, align 16
+  %1 = shufflevector <16 x i8> %0, <16 x i8> undef, <16 x i32> <i32 2, i32 3, i32 4, i32 5, i32 2, i32 3, i32 4, i32 5, i32 2, i32 3, i32 4, i32 5, i32 2, i32 3, i32 4, i32 5>
+  store <16 x i8> %1, <16 x i8>* %out, align 16
+  ret void
+}
+
+; CHECK: vperm
+; CHECK-NOT: vspltw