[llvm] [AMDGPU] fix up vop3p gisel errors (PR #136262)

[via llvm-commits]

================
@@ -4319,60 +4319,600 @@ AMDGPUInstructionSelector::selectVOP3NoMods(MachineOperand &Root) const {
   }};
 }
 
-std::pair<Register, unsigned>
-AMDGPUInstructionSelector::selectVOP3PModsImpl(
-  Register Src, const MachineRegisterInfo &MRI, bool IsDOT) const {
+enum class SrcStatus {
+  IS_SAME,
+  IS_UPPER_HALF,
+  IS_LOWER_HALF,
+  IS_UPPER_HALF_NEG,
+  // This means current op = [op_upper, op_lower] and src = -op_lower.
+  IS_LOWER_HALF_NEG,
+  IS_HI_NEG,
+  // This means current op = [op_upper, op_lower] and src = [op_upper,
+  // -op_lower].
+  IS_LO_NEG,
+  IS_BOTH_NEG,
+  INVALID,
+  NEG_START = IS_UPPER_HALF_NEG,
+  NEG_END = IS_BOTH_NEG,
+  HALF_START = IS_UPPER_HALF,
+  HALF_END = IS_LOWER_HALF_NEG
+};
+
+static bool isTruncHalf(const MachineInstr *MI,
+                        const MachineRegisterInfo &MRI) {
+  if (MI->getOpcode() != AMDGPU::G_TRUNC)
+    return false;
+
+  unsigned DstSize = MRI.getType(MI->getOperand(0).getReg()).getSizeInBits();
+  unsigned SrcSize = MRI.getType(MI->getOperand(1).getReg()).getSizeInBits();
+  return DstSize * 2 == SrcSize;
+}
+
+static bool isLshrHalf(const MachineInstr *MI, const MachineRegisterInfo &MRI) {
+  if (MI->getOpcode() != AMDGPU::G_LSHR)
+    return false;
+
+  Register ShiftSrc;
+  std::optional<ValueAndVReg> ShiftAmt;
+  if (mi_match(MI->getOperand(0).getReg(), MRI,
+               m_GLShr(m_Reg(ShiftSrc), m_GCst(ShiftAmt)))) {
+    unsigned SrcSize = MRI.getType(MI->getOperand(1).getReg()).getSizeInBits();
+    unsigned Shift = ShiftAmt->Value.getZExtValue();
+    return Shift * 2 == SrcSize;
+  }
+  return false;
+}
+
+static bool isShlHalf(const MachineInstr *MI, const MachineRegisterInfo &MRI) {
+  if (MI->getOpcode() != AMDGPU::G_SHL)
+    return false;
+
+  Register ShiftSrc;
+  std::optional<ValueAndVReg> ShiftAmt;
+  if (mi_match(MI->getOperand(0).getReg(), MRI,
+               m_GShl(m_Reg(ShiftSrc), m_GCst(ShiftAmt)))) {
+    unsigned SrcSize = MRI.getType(MI->getOperand(1).getReg()).getSizeInBits();
+    unsigned Shift = ShiftAmt->Value.getZExtValue();
+    return Shift * 2 == SrcSize;
+  }
+  return false;
+}
+
+static std::optional<std::pair<const MachineOperand *, SrcStatus>>
+retOpStat(const MachineOperand *Op, SrcStatus Stat,
----------------
Shoreshen wrote:

Hi, maybe only part of it. There are some places that using reference will cause error, such as:
```c++
static SmallVector<std::pair<const MachineOperand *, SrcStatus>>
getSrcStats(const MachineOperand *Op, const MachineRegisterInfo &MRI,
            searchOptions SearchOptions, int MaxDepth = 6) {
  ...
  while (Depth <= MaxDepth && Curr.has_value()) {
    ...
    Curr = calcNextStatus(Curr.value(), MRI);
  }
  ...
}
```

If I use reference in this place, the `=` will change the original operand. 

Currently I think it is simpler to use pointer in all place. 

https://github.com/llvm/llvm-project/pull/136262