[PPC64LE] More improvements to VSX swap optimization

This patch allows VSX swap optimization to succeed more frequently.
Specifically, it is concerned with common code sequences that occur
when copying a scalar floating-point value to a vector register.  This
patch currently handles cases where the floating-point value is
already in a register, but does not yet handle loads (such as via an
LXSDX scalar floating-point VSX load).  That will be dealt with later.

A typical case is when a scalar value comes in as a floating-point
parameter.  The value is copied into a virtual VSFRC register, and
then a sequence of SUBREG_TO_REG and/or COPY operations will convert
it to a full vector register of the class required by the context.  If
this vector register is then used as part of a lane-permuted
computation, the original scalar value will be in the wrong lane.  We
can fix this by adding a swap operation following any widening
SUBREG_TO_REG operation.  Additional COPY operations may be needed
around the swap operation in order to keep register assignment happy,
but these are pro forma operations that will be removed by coalescing.

If a scalar value is otherwise directly referenced in a computation
(such as by one of the many XS* vector-scalar operations), we
currently disable swap optimization.  These operations are
lane-sensitive by definition.  A MentionsPartialVR flag is added for
use in each swap table entry that mentions a scalar floating-point
register without having special handling defined.

A common idiom for PPC64LE is to convert a double-precision scalar to
a vector by performing a splat operation.  This ensures that the value
can be referenced as V[0], as it would be for big endian, whereas just
converting the scalar to a vector with a SUBREG_TO_REG operation
leaves this value only in V[1].  A doubleword splat operation is one
form of an XXPERMDI instruction, which takes one doubleword from a
first operand and another doubleword from a second operand, with a
two-bit selector operand indicating which doublewords are chosen.  In
the general case, an XXPERMDI can be permitted in a lane-swapped
region provided that it is properly transformed to select the
corresponding swapped values.  This transformation is to reverse the
order of the two input operands, and to reverse and complement the
bits of the selector operand (derivation left as an exercise to the
reader ;).

A new test case that exercises the scalar-to-vector and generalized
XXPERMDI transformations is added as CodeGen/PowerPC/swaps-le-5.ll.
The patch also requires a change to CodeGen/PowerPC/swaps-le-3.ll to
use CHECK-DAG instead of CHECK for two independent instructions that
now appear in reverse order.

There are two small unrelated changes that are added with this patch.
First, the XXSLDWI instruction was incorrectly omitted from the list
of lane-sensitive instructions; this is now fixed.  Second, I observed
that the same webs were being rejected over and over again for
different reasons.  Since it's sufficient to reject a web only once, I
added a check for this to speed up the compilation time slightly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242081 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCVSXSwapRemoval.cpp

@@ -80,6 +80,7 @@ struct PPCVSXSwapEntry {
   unsigned int IsSwap : 1;
   unsigned int MentionsPhysVR : 1;
   unsigned int IsSwappable : 1;
+  unsigned int MentionsPartialVR : 1;
   unsigned int SpecialHandling : 3;
   unsigned int WebRejected : 1;
   unsigned int WillRemove : 1;
@@ -91,7 +92,9 @@ enum SHValues {
   SH_INSERT,
   SH_NOSWAP_LD,
   SH_NOSWAP_ST,
-  SH_SPLAT
+  SH_SPLAT,
+  SH_XXPERMDI,
+  SH_COPYSCALAR
 };
 
 struct PPCVSXSwapRemoval : public MachineFunctionPass {
@@ -167,6 +170,21 @@ struct PPCVSXSwapRemoval : public MachineFunctionPass {
             isRegInClass(Reg, &PPC::VRRCRegClass));
   }
 
+  // Return true iff the given register is a partial vector register.
+  bool isScalarVecReg(unsigned Reg) {
+    return (isRegInClass(Reg, &PPC::VSFRCRegClass) ||
+            isRegInClass(Reg, &PPC::VSSRCRegClass));
+  }
+
+  // Return true iff the given register mentions all or part of a
+  // vector register.  Also sets Partial to true if the mention
+  // is for just the floating-point register overlap of the register.
+  bool isAnyVecReg(unsigned Reg, bool &Partial) {
+    if (isScalarVecReg(Reg))
+      Partial = true;
+    return isScalarVecReg(Reg) || isVecReg(Reg);
+  }
+
 public:
   // Main entry point for this pass.
   bool runOnMachineFunction(MachineFunction &MF) override {
@@ -223,12 +241,13 @@ bool PPCVSXSwapRemoval::gatherVectorInstructions() {
     for (MachineInstr &MI : MBB) {
 
       bool RelevantInstr = false;
+      bool Partial = false;
 
       for (const MachineOperand &MO : MI.operands()) {
         if (!MO.isReg())
           continue;
         unsigned Reg = MO.getReg();
-        if (isVecReg(Reg)) {
+        if (isAnyVecReg(Reg, Partial)) {
           RelevantInstr = true;
           break;
         }
@@ -250,8 +269,13 @@ bool PPCVSXSwapRemoval::gatherVectorInstructions() {
         // Unless noted otherwise, an instruction is considered
         // safe for the optimization.  There are a large number of
         // such true-SIMD instructions (all vector math, logical,
-        // select, compare, etc.).
-        SwapVector[VecIdx].IsSwappable = 1;
+        // select, compare, etc.).  However, if the instruction
+        // mentions a partial vector register and does not have
+        // special handling defined, it is not swappable.
+        if (Partial)
+          SwapVector[VecIdx].MentionsPartialVR = 1;
+        else
+          SwapVector[VecIdx].IsSwappable = 1;
         break;
       case PPC::XXPERMDI: {
         // This is a swap if it is of the form XXPERMDI t, s, s, 2.
@@ -269,25 +293,37 @@ bool PPCVSXSwapRemoval::gatherVectorInstructions() {
                                                VecIdx);
           if (trueReg1 == trueReg2)
             SwapVector[VecIdx].IsSwap = 1;
-        }
+          else {
+            // We can still handle these if the two registers are not
+            // identical, by adjusting the form of the XXPERMDI.
+            SwapVector[VecIdx].IsSwappable = 1;
+            SwapVector[VecIdx].SpecialHandling = SHValues::SH_XXPERMDI;
+          }
         // This is a doubleword splat if it is of the form
         // XXPERMDI t, s, s, 0 or XXPERMDI t, s, s, 3.  As above we
         // must look through chains of copy-likes to find the source
         // register.  We turn off the marking for mention of a physical
         // register, because splatting it is safe; the optimization
-        // will not swap the value in the physical register.
-        else if (immed == 0 || immed == 3) {
+        // will not swap the value in the physical register.  Whether
+        // or not the two input registers are identical, we can handle
+        // these by adjusting the form of the XXPERMDI.
+        } else if (immed == 0 || immed == 3) {
+
+          SwapVector[VecIdx].IsSwappable = 1;
+          SwapVector[VecIdx].SpecialHandling = SHValues::SH_XXPERMDI;
+
           unsigned trueReg1 = lookThruCopyLike(MI.getOperand(1).getReg(),
                                                VecIdx);
           unsigned trueReg2 = lookThruCopyLike(MI.getOperand(2).getReg(),
                                                VecIdx);
-          if (trueReg1 == trueReg2) {
-            SwapVector[VecIdx].IsSwappable = 1;
+          if (trueReg1 == trueReg2)
             SwapVector[VecIdx].MentionsPhysVR = 0;
-          }
+
+        } else {
+          // We can still handle these by adjusting the form of the XXPERMDI.
+          SwapVector[VecIdx].IsSwappable = 1;
+          SwapVector[VecIdx].SpecialHandling = SHValues::SH_XXPERMDI;
         }
-        // Any other form of XXPERMDI is lane-sensitive and unsafe
-        // for the optimization.
         break;
       }
       case PPC::LVX:
@@ -324,7 +360,32 @@ bool PPCVSXSwapRemoval::gatherVectorInstructions() {
         if (isVecReg(MI.getOperand(0).getReg()) &&
             isVecReg(MI.getOperand(1).getReg()))
           SwapVector[VecIdx].IsSwappable = 1;
+        // If we have a copy from one scalar floating-point register
+        // to another, we can accept this even if it is a physical
+        // register.  The only way this gets involved is if it feeds
+        // a SUBREG_TO_REG, which is handled by introducing a swap.
+        else if (isScalarVecReg(MI.getOperand(0).getReg()) &&
+                 isScalarVecReg(MI.getOperand(1).getReg()))
+          SwapVector[VecIdx].IsSwappable = 1;
+        break;
+      case PPC::SUBREG_TO_REG: {
+        // These are fine provided they are moving between full vector
+        // register classes.  If they are moving from a scalar
+        // floating-point class to a vector class, we can handle those
+        // as well, provided we introduce a swap.  It is generally the
+        // case that we will introduce fewer swaps than we remove, but
+        // (FIXME) a cost model could be used.  However, introduced
+        // swaps could potentially be CSEd, so this is not trivial.
+        if (isVecReg(MI.getOperand(0).getReg()) &&
+            isVecReg(MI.getOperand(2).getReg()))
+          SwapVector[VecIdx].IsSwappable = 1;
+        else if (isVecReg(MI.getOperand(0).getReg()) &&
+                 isScalarVecReg(MI.getOperand(2).getReg())) {
+          SwapVector[VecIdx].IsSwappable = 1;
+          SwapVector[VecIdx].SpecialHandling = SHValues::SH_COPYSCALAR;
+        }
         break;
+      }
       case PPC::VSPLTB:
       case PPC::VSPLTH:
       case PPC::VSPLTW:
@@ -425,6 +486,10 @@ bool PPCVSXSwapRemoval::gatherVectorInstructions() {
       case PPC::VUPKLSW:
       case PPC::XXMRGHW:
       case PPC::XXMRGLW:
+      // XXSLDWI could be replaced by a general permute with one of three
+      // permute control vectors (for shift values 1, 2, 3).  However,
+      // VPERM has a more restrictive register class.
+      case PPC::XXSLDWI:
       case PPC::XXSPLTW:
         break;
       }
@@ -501,18 +566,20 @@ void PPCVSXSwapRemoval::formWebs() {
     DEBUG(MI->dump());
 
     // It's sufficient to walk vector uses and join them to their unique
-    // definitions.  In addition, check *all* vector register operands
-    // for physical regs.
+    // definitions.  In addition, check full vector register operands
+    // for physical regs.  We exclude partial-vector register operands
+    // because we can handle them if copied to a full vector.
     for (const MachineOperand &MO : MI->operands()) {
       if (!MO.isReg())
         continue;
 
       unsigned Reg = MO.getReg();
-      if (!isVecReg(Reg))
+      if (!isVecReg(Reg) && !isScalarVecReg(Reg))
         continue;
 
       if (!TargetRegisterInfo::isVirtualRegister(Reg)) {
-        SwapVector[EntryIdx].MentionsPhysVR = 1;
+        if (!(MI->isCopy() && isScalarVecReg(Reg)))
+          SwapVector[EntryIdx].MentionsPhysVR = 1;
         continue;
       }
 
@@ -545,15 +612,21 @@ void PPCVSXSwapRemoval::recordUnoptimizableWebs() {
   for (unsigned EntryIdx = 0; EntryIdx < SwapVector.size(); ++EntryIdx) {
     int Repr = EC->getLeaderValue(SwapVector[EntryIdx].VSEId);
 
-    // Reject webs containing mentions of physical registers, or containing
-    // operations that we don't know how to handle in a lane-permuted region.
+    // If representative is already rejected, don't waste further time.
+    if (SwapVector[Repr].WebRejected)
+      continue;
+
+    // Reject webs containing mentions of physical or partial registers, or
+    // containing operations that we don't know how to handle in a lane-
+    // permuted region.
     if (SwapVector[EntryIdx].MentionsPhysVR ||
+        SwapVector[EntryIdx].MentionsPartialVR ||
         !(SwapVector[EntryIdx].IsSwappable || SwapVector[EntryIdx].IsSwap)) {
 
       SwapVector[Repr].WebRejected = 1;
 
       DEBUG(dbgs() <<
-            format("Web %d rejected for physreg, subreg, or not swap[pable]\n",
+            format("Web %d rejected for physreg, partial reg, or not swap[pable]\n",
                    Repr));
       DEBUG(dbgs() << "  in " << EntryIdx << ": ");
       DEBUG(SwapVector[EntryIdx].VSEMI->dump());
@@ -588,7 +661,7 @@ void PPCVSXSwapRemoval::recordUnoptimizableWebs() {
         }
       }
 
-    // Reject webs than contain swapping stores that are fed by something
+    // Reject webs that contain swapping stores that are fed by something
     // other than a swap instruction.
     } else if (SwapVector[EntryIdx].IsStore && SwapVector[EntryIdx].IsSwap) {
       MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
@@ -670,7 +743,8 @@ void PPCVSXSwapRemoval::markSwapsForRemoval() {
 // The identified swap entry requires special handling to allow its
 // containing computation to be optimized.  Perform that handling
 // here.
-// FIXME: This code is to be phased in with subsequent patches.
+// FIXME: Additional opportunities will be phased in with subsequent
+// patches.
 void PPCVSXSwapRemoval::handleSpecialSwappables(int EntryIdx) {
   switch (SwapVector[EntryIdx].SpecialHandling) {
 
@@ -704,6 +778,91 @@ void PPCVSXSwapRemoval::handleSpecialSwappables(int EntryIdx) {
     break;
   }
 
+  // For an XXPERMDI that isn't handled otherwise, we need to
+  // reverse the order of the operands.  If the selector operand
+  // has a value of 0 or 3, we need to change it to 3 or 0,
+  // respectively.  Otherwise we should leave it alone.  (This
+  // is equivalent to reversing the two bits of the selector
+  // operand and complementing the result.)
+  case SHValues::SH_XXPERMDI: {
+    MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
+
+    DEBUG(dbgs() << "Changing XXPERMDI: ");
+    DEBUG(MI->dump());
+
+    unsigned Selector = MI->getOperand(3).getImm();
+    if (Selector == 0 || Selector == 3)
+      Selector = 3 - Selector;
+    MI->getOperand(3).setImm(Selector);
+
+    unsigned Reg1 = MI->getOperand(1).getReg();
+    unsigned Reg2 = MI->getOperand(2).getReg();
+    MI->getOperand(1).setReg(Reg2);
+    MI->getOperand(2).setReg(Reg1);
+
+    DEBUG(dbgs() << "  Into: ");
+    DEBUG(MI->dump());
+    break;
+  }
+
+  // For a copy from a scalar floating-point register to a vector
+  // register, removing swaps will leave the copied value in the
+  // wrong lane.  Insert a swap following the copy to fix this.
+  case SHValues::SH_COPYSCALAR: {
+    MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
+
+    DEBUG(dbgs() << "Changing SUBREG_TO_REG: ");
+    DEBUG(MI->dump());
+
+    unsigned DstReg = MI->getOperand(0).getReg();
+    const TargetRegisterClass *DstRC = MRI->getRegClass(DstReg);
+    unsigned NewVReg = MRI->createVirtualRegister(DstRC);
+
+    MI->getOperand(0).setReg(NewVReg);
+    DEBUG(dbgs() << "  Into: ");
+    DEBUG(MI->dump());
+
+    MachineBasicBlock::iterator InsertPoint = MI->getNextNode();
+
+    // Note that an XXPERMDI requires a VSRC, so if the SUBREG_TO_REG
+    // is copying to a VRRC, we need to be careful to avoid a register
+    // assignment problem.  In this case we must copy from VRRC to VSRC
+    // prior to the swap, and from VSRC to VRRC following the swap.
+    // Coalescing will usually remove all this mess.
+
+    if (DstRC == &PPC::VRRCRegClass) {
+      unsigned VSRCTmp1 = MRI->createVirtualRegister(&PPC::VSRCRegClass);
+      unsigned VSRCTmp2 = MRI->createVirtualRegister(&PPC::VSRCRegClass);
+
+      BuildMI(*MI->getParent(), InsertPoint, MI->getDebugLoc(),
+              TII->get(PPC::COPY), VSRCTmp1)
+        .addReg(NewVReg);
+      DEBUG(MI->getNextNode()->dump());
+
+      BuildMI(*MI->getParent(), InsertPoint, MI->getDebugLoc(),
+              TII->get(PPC::XXPERMDI), VSRCTmp2)
+        .addReg(VSRCTmp1)
+        .addReg(VSRCTmp1)
+        .addImm(2);
+      DEBUG(MI->getNextNode()->getNextNode()->dump());
+
+      BuildMI(*MI->getParent(), InsertPoint, MI->getDebugLoc(),
+              TII->get(PPC::COPY), DstReg)
+        .addReg(VSRCTmp2);
+      DEBUG(MI->getNextNode()->getNextNode()->getNextNode()->dump());
+
+    } else {
+
+      BuildMI(*MI->getParent(), InsertPoint, MI->getDebugLoc(),
+              TII->get(PPC::XXPERMDI), DstReg)
+        .addReg(NewVReg)
+        .addReg(NewVReg)
+        .addImm(2);
+
+      DEBUG(MI->getNextNode()->dump());
+    }
+    break;
+  }
   }
 }
 
@@ -756,6 +915,8 @@ void PPCVSXSwapRemoval::dumpSwapVector() {
       DEBUG(dbgs() << "swap ");
     if (SwapVector[EntryIdx].MentionsPhysVR)
       DEBUG(dbgs() << "physreg ");
+    if (SwapVector[EntryIdx].MentionsPartialVR)
+      DEBUG(dbgs() << "partialreg ");
 
     if (SwapVector[EntryIdx].IsSwappable) {
       DEBUG(dbgs() << "swappable ");
@@ -780,6 +941,12 @@ void PPCVSXSwapRemoval::dumpSwapVector() {
       case SH_SPLAT:
         DEBUG(dbgs() << "special:splat ");
         break;
+      case SH_XXPERMDI:
+        DEBUG(dbgs() << "special:xxpermdi ");
+        break;
+      case SH_COPYSCALAR:
+        DEBUG(dbgs() << "special:copyscalar ");
+        break;
       }
     }
 

test/CodeGen/PowerPC/swaps-le-3.ll

@@ -17,8 +17,8 @@ entry:
 }
 
 ; CHECK-LABEL: @test
-; CHECK: xxspltd
-; CHECK: lxvd2x
+; CHECK-DAG: xxspltd
+; CHECK-DAG: lxvd2x
 ; CHECK: xvadddp
 ; CHECK: stxvd2x
 ; CHECK-NOT: xxswapd