xref: /llvm-project-15.0.7/lldb/source/Plugins/Process/Utility/RegisterContext_x86.cpp (revision 8aea95f3)

//===-- RegisterContext_x86.cpp ---------------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "RegisterContext_x86.h"

using namespace lldb_private;

// Convert the 8-bit abridged FPU Tag Word (as found in FXSAVE) to the full
// 16-bit FPU Tag Word (as found in FSAVE, and used by gdb protocol).  This
// requires knowing the values of the ST(i) registers and the FPU Status Word.
uint16_t lldb_private::AbridgedToFullTagWord(uint8_t abridged_tw, uint16_t sw,
                                             llvm::ArrayRef<MMSReg> st_regs) {
  // Tag word is using internal FPU register numbering rather than ST(i).
  // Mapping to ST(i): i = FPU regno - TOP (Status Word, bits 11:13).
  // Here we start with FPU reg 7 and go down.
  int st = 7 - ((sw >> 11) & 7);
  uint16_t tw = 0;
  for (uint8_t mask = 0x80; mask != 0; mask >>= 1) {
    tw <<= 2;
    if (abridged_tw & mask) {
      // The register is non-empty, so we need to check the value of ST(i).
      uint16_t exp =
          st_regs[st].comp.sign_exp & 0x7fff; // Discard the sign bit.
      if (exp == 0) {
        if (st_regs[st].comp.mantissa == 0)
          tw |= 1; // Zero
        else
          tw |= 2; // Denormal
      } else if (exp == 0x7fff)
        tw |= 2; // Infinity or NaN
      // 0 if normal number
    } else
      tw |= 3; // Empty register

    // Rotate ST down.
    st = (st - 1) & 7;
  }

  return tw;
}

// Convert the 16-bit FPU Tag Word to the abridged 8-bit value, to be written
// into FXSAVE.
uint8_t lldb_private::FullToAbridgedTagWord(uint16_t tw) {
  uint8_t abridged_tw = 0;
  for (uint16_t mask = 0xc000; mask != 0; mask >>= 2) {
    abridged_tw <<= 1;
    // full TW uses 11 for empty registers, aTW uses 0
    if ((tw & mask) != mask)
      abridged_tw |= 1;
  }
  return abridged_tw;
}