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

[Chris Jackson via llvm-commits]

================
@@ -2331,6 +2332,113 @@ 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);
+  const MCInstrDesc &MovDesc = TII->get(AMDGPU::V_MOV_B64_PSEUDO);
+  const TargetRegisterClass *RC =
+      TII->getRegClass(MovDesc, 0, TRI, *MI.getMF());
+
+  if (!ST->hasMovB64() || !TRI->isVGPR(*MRI, Reg) ||
+      !MRI->hasOneNonDBGUse(Reg) ||
+      (!TRI->getCompatibleSubRegClass(DefRC, RC, AMDGPU::sub0_sub1) &&
+       DefRC != RC))
+    return false;
+
+  SmallVector<std::pair<MachineOperand *, unsigned>, 32> Defs;
+  if (!getRegSeqInit(Defs, Reg))
+    return false;
+
+  // Only attempting to fold immediate materializations.
+  if (!Defs.empty() &&
+      llvm::any_of(Defs, [](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;
+  uint64_t RemainderSize = TRI->getRegSizeInBits(*DefRC);
+  SmallVector<std::pair<MachineOperand *, unsigned>, 4> Remainders;
+  for (auto &[Op, SubIdx] : Defs) {
+    unsigned SubRegSize = TRI->getSubRegIdxSize(SubIdx);
+    unsigned Shift = (TRI->getChannelFromSubReg(SubIdx) % 2) * SubRegSize;
+    ImmSize += SubRegSize;
+    ImmVal |= Op->getImm() << Shift;
+
+    if (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;
+      RemainderSize -= 64;
+    } else if ((RemainderSize / 64) == 0 && (RemainderSize % 64)) {
----------------
chrisjbris wrote:

```suggestion
    } else if ((RemainderSize / 64 == 0) && (RemainderSize % 64 != 0)) {
```

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