[PowerPC] Try to simplify a Swap if it feeds a Splat

If we have the situation where a Swap feeds a Splat we can sometimes change the
index on the Splat and then remove the Swap instruction.

Differential Revision: https://reviews.llvm.org/D39009

llvm-svn: 316366

1 files changed, 47 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCMIPeephole.cpp b/llvm/lib/Target/PowerPC/PPCMIPeephole.cpp
index 80b7ac24345..beb40992900 100644
--- a/llvm/lib/Target/PowerPC/PPCMIPeephole.cpp
+++ b/llvm/lib/Target/PowerPC/PPCMIPeephole.cpp
@@ -375,6 +375,53 @@ bool PPCMIPeephole::simplifyCode(void) {
             MI.getOperand(2).setImm(NewElem);
           }
         }
+
+        // Splat is fed by a SWAP which is a permute of this form
+        //  XXPERMDI %VA, %VA, 2
+        // Since the splat instruction can use any of the vector elements to do
+        //  the splat we do not have to rearrange the elements in the vector
+        //  with a swap before we do the splat. We can simply do the splat from
+        //  a different index.
+        // If the swap has only one use (the splat) then we can completely
+        //  remove the swap too.
+        if (DefOpcode == PPC::XXPERMDI && MI.getOperand(1).isImm()) {
+          unsigned SwapRes = DefMI->getOperand(0).getReg();
+          unsigned SwapOp1 = DefMI->getOperand(1).getReg();
+          unsigned SwapOp2 = DefMI->getOperand(2).getReg();
+          unsigned SwapImm = DefMI->getOperand(3).getImm();
+          unsigned SplatImm = MI.getOperand(1).getImm();
+
+          // Break if this permute is not a swap.
+          if (SwapOp1 != SwapOp2 || SwapImm != 2)
+            break;
+
+          unsigned NewElem = 0;
+          // Compute the new index to use for the splat.
+          if (MI.getOpcode() == PPC::VSPLTB)
+            NewElem = (SplatImm + 8) & 0xF;
+          else if (MI.getOpcode() == PPC::VSPLTH)
+            NewElem = (SplatImm + 4) & 0x7;
+          else if (MI.getOpcode() == PPC::XXSPLTW)
+            NewElem = (SplatImm + 2) & 0x3;
+          else {
+            DEBUG(dbgs() << "Unknown splat opcode.");
+            DEBUG(MI.dump());
+            break;
+          }
+
+          if (MRI->hasOneNonDBGUse(SwapRes)) {
+            DEBUG(dbgs() << "Removing redundant swap: ");
+            DEBUG(DefMI->dump());
+            ToErase = DefMI;
+          }
+          Simplified = true;
+          DEBUG(dbgs() << "Changing splat immediate from " << SplatImm <<
+                " to " << NewElem << " in instruction: ");
+          DEBUG(MI.dump());
+          MI.getOperand(1).setImm(NewElem);
+          MI.getOperand(2).setReg(SwapOp1);
+        }
+
         break;
       }
       case PPC::XVCVDPSP: {