[llvm] AMDGPU: Generalize instruction shrinking code (PR #93810)

[Jay Foad via llvm-commits]

================
@@ -4486,46 +4486,47 @@ static void copyFlagsToImplicitVCC(MachineInstr &MI,
 MachineInstr *SIInstrInfo::buildShrunkInst(MachineInstr &MI,
                                            unsigned Op32) const {
   MachineBasicBlock *MBB = MI.getParent();
+
+  const MCInstrDesc &Op32Desc = get(Op32);
   MachineInstrBuilder Inst32 =
-    BuildMI(*MBB, MI, MI.getDebugLoc(), get(Op32))
+    BuildMI(*MBB, MI, MI.getDebugLoc(), Op32Desc)
     .setMIFlags(MI.getFlags());
 
   // Add the dst operand if the 32-bit encoding also has an explicit $vdst.
   // For VOPC instructions, this is replaced by an implicit def of vcc.
-  if (AMDGPU::hasNamedOperand(Op32, AMDGPU::OpName::vdst)) {
-    // dst
-    Inst32.add(MI.getOperand(0));
-  } else if (AMDGPU::hasNamedOperand(Op32, AMDGPU::OpName::sdst)) {
-    // VOPCX instructions won't be writing to an explicit dst, so this should
-    // not fail for these instructions.
-    assert(((MI.getOperand(0).getReg() == AMDGPU::VCC) ||
-            (MI.getOperand(0).getReg() == AMDGPU::VCC_LO)) &&
-           "Unexpected case");
-  }
-
-  Inst32.add(*getNamedOperand(MI, AMDGPU::OpName::src0));
 
-  const MachineOperand *Src1 = getNamedOperand(MI, AMDGPU::OpName::src1);
-  if (Src1)
-    Inst32.add(*Src1);
+  // We assume the defs of the shrunk opcode are in the same order, and the
+  // shrunk opcode loses the last def (SGPR def, in the VOP3->VOPC case).
+  for (int I = 0, E = Op32Desc.getNumDefs(); I != E; ++I)
+    Inst32.add(MI.getOperand(I));
 
   const MachineOperand *Src2 = getNamedOperand(MI, AMDGPU::OpName::src2);
 
-  if (Src2) {
-    int Op32Src2Idx = AMDGPU::getNamedOperandIdx(Op32, AMDGPU::OpName::src2);
-    if (Op32Src2Idx != -1) {
-      Inst32.add(*Src2);
-    } else {
-      // In the case of V_CNDMASK_B32_e32, the explicit operand src2 is
-      // replaced with an implicit read of vcc or vcc_lo. The implicit read
-      // of vcc was already added during the initial BuildMI, but we
-      // 1) may need to change vcc to vcc_lo to preserve the original register
-      // 2) have to preserve the original flags.
-      fixImplicitOperands(*Inst32);
-      copyFlagsToImplicitVCC(*Inst32, *Src2);
+  int Idx = MI.getNumExplicitDefs();
+  for (const MachineOperand &Use : MI.explicit_uses()) {
+    int OpTy = MI.getDesc().operands()[Idx++].OperandType;
+    if (OpTy == AMDGPU::OPERAND_INPUT_MODS || OpTy == MCOI::OPERAND_IMMEDIATE)
----------------
jayfoad wrote:

What cases does `OPERAND_IMMEDIATE` cover? Does this mean we don't copy any immediate operands, and if so why not?

https://github.com/llvm/llvm-project/pull/93810