[llvm] [X86][CodeGen] Support long instruction fixup for APX NDD instructions (PR #83578)

[Phoebe Wang via llvm-commits]

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@@ -613,6 +613,89 @@ bool X86ExpandPseudo::expandMI(MachineBasicBlock &MBB,
   case X86::CALL64m_RVMARKER:
     expandCALL_RVMARKER(MBB, MBBI);
     return true;
+  case X86::ADD32mi_ND:
+  case X86::ADD64mi32_ND:
+  case X86::SUB32mi_ND:
+  case X86::SUB64mi32_ND:
+  case X86::AND32mi_ND:
+  case X86::AND64mi32_ND:
+  case X86::OR32mi_ND:
+  case X86::OR64mi32_ND:
+  case X86::XOR32mi_ND:
+  case X86::XOR64mi32_ND:
+  case X86::ADC32mi_ND:
+  case X86::ADC64mi32_ND:
+  case X86::SBB32mi_ND:
+  case X86::SBB64mi32_ND: {
+    // It's possible for an EVEX-encoded legacy instruction to reach the 15-byte
+    // instruction length limit: 4 bytes of EVEX prefix + 1 byte of opcode + 1
+    // byte of ModRM + 1 byte of SIB + 4 bytes of displacement + 4 bytes of
+    // immediate = 15 bytes in total, e.g.
+    //
+    //  subq    $184, %fs:257(%rbx, %rcx), %rax
+    //
+    // In such a case, no additional (ADSIZE or segment override) prefix can be
+    // used. To resolve the issue, we split the “long” instruction into 2
+    // instructions:
+    //
+    //  movq %fs:257(%rbx, %rcx)，%rax
+    //  subq $184, %rax
+    const MachineOperand &ImmOp =
+        MI.getOperand(MI.getNumExplicitOperands() - 1);
+    // If the immediate is a expr, conservatively estimate 4 bytes.
+    if (ImmOp.isImm() && isInt<8>(ImmOp.getImm()))
+      return false;
+    int MemOpNo = X86::getFirstAddrOperandIdx(MI);
+    const MachineOperand &DispOp = MI.getOperand(MemOpNo + X86::AddrDisp);
+    // If the displacement is a expr, conservatively estimate 4 bytes.
+    if (DispOp.isImm() && isInt<8>(DispOp.getImm()))
+      return false;
+    // There can only be one of three: SIB, segment override register, ADSIZE
+    Register Segment = MI.getOperand(MemOpNo + X86::AddrSegmentReg).getReg();
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phoebewang wrote:

unsigned Count = !!MI.getOperand(MemOpNo + X86::AddrSegmentReg).getReg();

https://github.com/llvm/llvm-project/pull/83578