[PowerPC] Simplify a Swap if it feeds a Splat

[stefanp]

Summary

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.

[echristo]

First pass on review.

lib/Target/PowerPC/PPCMIPeephole.cpp

379

A bit more elaborate description here please.

389–391

Style wise we try to avoid single statements in braces.

test/CodeGen/PowerPC/ppc64-peephole-swap.ll

2

Please add a description of what the testcase is supposed to be testing for each case.

[stefanp]

Fixed style and comment issues based on previous review.

[echristo]

lib/Target/PowerPC/PPCMIPeephole.cpp

393

Early break rather than more indenting please?

403

This seems problematic if a new splat opcode is ever added. Should we just break and possibly DEBUG?

[stefanp]

Sorry for the late reply Eric. I was on vacation Thursday and Friday last week.

I've made the changes you mentioned.

[echristo]

LGTM. Thanks!

-eric

lib/Target/PowerPC/PPCMIPeephole.cpp

for (MachineInstr &MI : MBB) {
             Simplified = true;
             DEBUG(dbgs() << "Changing splat immediate from " << SplatImm <<
                   " to " << NewElem << " in instruction: ");
             DEBUG(MI.dump());
             MI.getOperand(1).setReg(ShiftOp1);
             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();
+          if (SwapOp1 == SwapOp2 && SwapImm == 2) {
+            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
+              llvm_unreachable("Unknown splat opcode.");
+
+            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: {
         // If this is a DP->SP conversion fed by an FRSP, the FRSP is redundant.
         unsigned TrueReg = lookThruCopyLike(MI.getOperand(1).getReg());
         if (!TargetRegisterInfo::isVirtualRegister(TrueReg))
           break;
         MachineInstr *DefMI = MRI->getVRegDef(TrueReg);

test/CodeGen/PowerPC/ppc64-peephole-swap.ll

+; RUN: llc < %s -mtriple=powerpc64le-unknown-linux-gnu -mcpu=pwr9 -verify-machineinstrs | FileCheck %s
+; RUN: llc < %s -mtriple=powerpc64le-unknown-linux-gnu -mcpu=pwr8 -verify-machineinstrs | FileCheck %s -check-prefix=CHECK-PWR8
+
+; The strightforward expansion of this code will result in a swap followed by a
+;  splat. However, the swap is not needed since in this case the splat is the
+;  only use.
+; We want to check that we are not using the swap and that we have indexed the
+;  splat to the correct location.
+; 8 Bit Signed Version of the test.
+; Function Attrs: norecurse nounwind readnone
+define <16 x i8> @splat_8_plus(<16 x i8> %v, i8 signext %c) local_unnamed_addr {
+entry:
+  %splat.splatinsert.i = insertelement <16 x i8> undef, i8 %c, i32 0
+  %splat.splat.i = shufflevector <16 x i8> %splat.splatinsert.i, <16 x i8> undef, <16 x i32> zeroinitializer
+  %add = add <16 x i8> %splat.splat.i, %v
+  ret <16 x i8> %add
+; CHECK-LABEL: splat_8_plus
+; CHECK-NOT: xxswapd
+; CHECK: vspltb {{[0-9]+}}, {{[0-9]+}}, 7
+; CHECK: blr
+; CHECK-PWR8-LABEL: splat_8_plus
+; CHECK-PWR8-NOT: xxswapd
+; CHECK-PWR8: vspltb {{[0-9]+}}, {{[0-9]+}}, 7
+; CHECK-PWR8: blr
+}
+
+; 8 Bit Unsigned Version of the test.
+; Function Attrs: norecurse nounwind readnone
+define <16 x i8> @splat_u8_plus(<16 x i8> %v, i8 zeroext %c) local_unnamed_addr {
+entry:
+  %splat.splatinsert.i = insertelement <16 x i8> undef, i8 %c, i32 0
+  %splat.splat.i = shufflevector <16 x i8> %splat.splatinsert.i, <16 x i8> undef, <16 x i32> zeroinitializer
+  %add = add <16 x i8> %splat.splat.i, %v
+  ret <16 x i8> %add
+; CHECK-LABEL: splat_u8_plus
+; CHECK-NOT: xxswapd
+; CHECK: vspltb {{[0-9]+}}, {{[0-9]+}}, 7
+; CHECK: blr
+; CHECK-PWR8-LABEL: splat_u8_plus
+; CHECK-PWR8-NOT: xxswapd
+; CHECK-PWR8: vspltb {{[0-9]+}}, {{[0-9]+}}, 7
+; CHECK-PWR8: blr
+}
+
+; 16 Bit Signed Version of the test.
+; Function Attrs: norecurse nounwind readnone
+define <8 x i16> @splat_16_plus(<8 x i16> %v, i16 signext %c) local_unnamed_addr {
+entry:
+  %0 = shl i16 %c, 8
+  %conv.i = ashr exact i16 %0, 8
+  %splat.splatinsert.i = insertelement <8 x i16> undef, i16 %conv.i, i32 0
+  %splat.splat.i = shufflevector <8 x i16> %splat.splatinsert.i, <8 x i16> undef, <8 x i32> zeroinitializer
+  %add = add <8 x i16> %splat.splat.i, %v
+  ret <8 x i16> %add
+; CHECK-LABEL: splat_16_plus
+; CHECK-NOT: xxswapd
+; CHECK: vsplth {{[0-9]+}}, {{[0-9]+}}, 3
+; CHECK: blr
+; CHECK-PWR8-LABEL: splat_16_plus
+; CHECK-PWR8-NOT: xxswapd
+; CHECK-PWR8: vsplth {{[0-9]+}}, {{[0-9]+}}, 3
+; CHECK-PWR8: blr
+}
+
+; 16 Bit Unsigned Version of the test.
+; Function Attrs: norecurse nounwind readnone
+define <8 x i16> @splat_u16_plus(<8 x i16> %v, i16 zeroext %c) local_unnamed_addr {
+entry:
+  %0 = shl i16 %c, 8
+  %conv.i = ashr exact i16 %0, 8
+  %splat.splatinsert.i = insertelement <8 x i16> undef, i16 %conv.i, i32 0
+  %splat.splat.i = shufflevector <8 x i16> %splat.splatinsert.i, <8 x i16> undef, <8 x i32> zeroinitializer
+  %add = add <8 x i16> %splat.splat.i, %v
+  ret <8 x i16> %add
+; CHECK-LABEL: splat_u16_plus
+; CHECK-NOT: xxswapd
+; CHECK: vsplth {{[0-9]+}}, {{[0-9]+}}, 3
+; CHECK: blr
+; CHECK-PWR8-LABEL: splat_u16_plus
+; CHECK-PWR8-NOT: xxswapd
+; CHECK-PWR8: vsplth {{[0-9]+}}, {{[0-9]+}}, 3
+; CHECK-PWR8: blr
+}
+
+; 32 Bit Signed Version of the test.
+; The 32 bit examples work differently than the 8 and 16 bit versions of the
+;  test. On Power 9 we have the mtvsrws instruction that does both the move to
+;  register and the splat so it does not really test the newly implemented code.
+; On Power 9 for the 32 bit case we don't need the new simplification. It is
+;  just here for completeness.
+; Function Attrs: norecurse nounwind readnone
+define <4 x i32> @splat_32_plus(<4 x i32> %v, i32 signext %c) local_unnamed_addr {
+entry:
+  %sext = shl i32 %c, 24
+  %conv.i = ashr exact i32 %sext, 24
+  %splat.splatinsert.i = insertelement <4 x i32> undef, i32 %conv.i, i32 0
+  %splat.splat.i = shufflevector <4 x i32> %splat.splatinsert.i, <4 x i32> undef, <4 x i32> zeroinitializer
+  %add = add <4 x i32> %splat.splat.i, %v
+  ret <4 x i32> %add
+; CHECK-LABEL: splat_32_plus
+; CHECK-NOT: xxswapd
+; CHECK: mtvsrws {{[0-9]+}}, {{[0-9]+}}
+; CHECK: blr
+; CHECK-PWR8-LABEL: splat_32_plus
+; CHECK-PWR8-NOT: xxswapd
+; CHECK-PWR8: xxspltw {{[0-9]+}}, {{[0-9]+}}, 1
+; CHECK-PWR8: blr
+}
+
+; 32 Bit Unsigned Version of the test.
+; The 32 bit examples work differently than the 8 and 16 bit versions of the
+;  test. On Power 9 we have the mtvsrws instruction that does both the move to
+;  register and the splat so it does not really test the newly implemented code.
+; On Power 9 for the 32 bit case we don't need the new simplification. It is
+;  just here for completeness.
+; Function Attrs: norecurse nounwind readnone
+define <4 x i32> @splat_u32_plus(<4 x i32> %v, i32 zeroext %c) local_unnamed_addr {
+entry:
+  %sext = shl i32 %c, 24
+  %conv.i = ashr exact i32 %sext, 24
+  %splat.splatinsert.i = insertelement <4 x i32> undef, i32 %conv.i, i32 0
+  %splat.splat.i = shufflevector <4 x i32> %splat.splatinsert.i, <4 x i32> undef, <4 x i32> zeroinitializer
+  %add = add <4 x i32> %splat.splat.i, %v
+  ret <4 x i32> %add
+; CHECK-LABEL: splat_u32_plus
+; CHECK-NOT: xxswapd
+; CHECK: mtvsrws {{[0-9]+}}, {{[0-9]+}}
+; CHECK: blr
+; CHECK-PWR8-LABEL: splat_u32_plus
+; CHECK-PWR8-NOT: xxswapd
+; CHECK-PWR8: xxspltw {{[0-9]+}}, {{[0-9]+}}, 1
+; CHECK-PWR8: blr
+}
+