[llvm] [AMDGPU] Fold multiple aligned v_mov_b32 to v_mov_b64 on gfx942 (PR #138843)

[Matt Arsenault via llvm-commits]

================
@@ -2127,6 +2128,99 @@ bool SIFoldOperandsImpl::tryFoldOMod(MachineInstr &MI) {
   return true;
 }
 
+// gfx942+ can use V_MOV_B64 for materializing constant immediates.
+// For example:
+// %0:vgpr_32 = V_MOV_B32 0, implicit $exec
+// %1:vreg_64_align2 = REG_SEQUENCE %0, %subreg.sub0, %0, %subreg.sub1
+//  ->
+// %1:vreg_64_align2 = V_MOV_B64_PSEUDO 0, implicit $exec
+bool SIFoldOperandsImpl::tryFoldImmRegSequence(MachineInstr &MI) {
+  assert(MI.isRegSequence());
+  auto Reg = MI.getOperand(0).getReg();
+  const TargetRegisterClass *DefRC = MRI->getRegClass(Reg);
+
+  if (!ST->hasMovB64() || !TRI->isVGPR(*MRI, Reg) ||
+      !MRI->hasOneNonDBGUse(Reg) || !TRI->isProperlyAlignedRC(*DefRC))
+    return false;
+
+  SmallVector<std::pair<MachineOperand *, unsigned>, 32> Defs;
+  if (!getRegSeqInit(Defs, Reg, MCOI::OPERAND_REGISTER))
+    return false;
+
+  // Only attempting to fold immediate materializations.
+  if (!Defs.empty() &&
+      !std::all_of(Defs.begin(), Defs.end(),
+                   [](const std::pair<MachineOperand *, unsigned> &Op) {
+                     return Op.first->isImm();
+                   }))
+    return false;
+
+  SmallVector<uint64_t, 8> ImmVals;
+  uint64_t ImmVal = 0;
+  uint64_t ImmSize = 0;
+  for (unsigned i = 0; i < Defs.size(); ++i) {
+    auto &[Op, SubIdx] = Defs[i];
+    unsigned SubRegSize = TRI->getSubRegIdxSize(SubIdx);
+    unsigned Shift = (TRI->getChannelFromSubReg(SubIdx) % 2) * SubRegSize;
+    ImmSize += SubRegSize;
+    ImmVal |= Op->getImm() << Shift;
+
+    if (ImmSize > 64 || SubRegSize == 64)
+      return false;
+
+    if (ImmSize == 64) {
+      // Only 32 bit literals can be encoded.
+      if (!isUInt<32>(ImmVal))
+        return false;
+      ImmVals.push_back(ImmVal);
+      ImmVal = 0;
+      ImmSize = 0;
+    }
+  }
+
+  assert(ImmVals.size() > 0 &&
+         "REG_SEQUENCE should have at least 1 operand pair");
+
+  // Can only combine REG_SEQUENCE into one 64b immediate materialization mov.
+  if (DefRC == TRI->getVGPR64Class()) {
+    BuildMI(*MI.getParent(), MI, MI.getDebugLoc(),
+            TII->get(AMDGPU::V_MOV_B64_PSEUDO), Reg)
+        .addImm(ImmVals[0]);
+    MI.eraseFromParent();
+    return true;
+  }
+
+  if (ImmVals.size() == 1)
+    return false;
+
+  // Can't bail from here on out: modifying the MI.
+
+  // Remove source operands.
+  for (unsigned i = MI.getNumOperands() - 1; i > 0; --i)
+    MI.removeOperand(i);
+
+  for (unsigned i = 0; i < ImmVals.size(); ++i) {
+    const TargetRegisterClass *RC = TRI->getVGPR64Class();
+    auto MovReg = MRI->createVirtualRegister(RC);
----------------
arsenm wrote:

```suggestion
    Register MovReg = MRI->createVirtualRegister(RC);
```

No auto 

https://github.com/llvm/llvm-project/pull/138843