浮点指令异常 - FLDZ故障？ [英] floating point instruction anomaly -- FLDZ malfunctioning?

问题描述

我试图调试我早些时候在这里发布的问题：
字段）。由于文档说：

如果将成为新ST（0）的ST（7）数据寄存器不为空，则检测到堆栈故障和无效操作异常，设置这两个标志在状态字。状态字中的TOP寄存器指针将仍然被递减，并且ST（0）中的新值将是INDEFINITE NAN。

顺便说一下，你的根本问题是因为这只是浮点的性质。这不是精确的。请参见，例如，此gcc错误报告 - 和这一个。

I am trying to debug the problem I posted earlier here: C++ and pin tool -- very weird DOUBLE variable issue with IF statement. I tracked down the moment when the weird behavior occurred using gdb. What I found is shown in the figure below that shows the gdb screenshot displaying the disassembled code and floating pointer register values. (larger image here) Left-hand side image shows the screenshot before the highlighted FLDZ instruction is executed and the right-hand side image is after the instructions is executed. I looked up the x86 ISA and FLDZ is for loading +0.0 into ST(0). However, what I get is -nan instead of +0.0. Does anybody know why this happens? The system I am using is Intel xeon 5645 running 64-bit CentOS, but the target program I am trying to debug is 32-bit application. Also, as I mentioned in the earlier post, I tried two versions of gcc, 4.2.4 and 4.1.2 and observed the same problem. Thanks.

--added-- By the way, below is the source code.

void Router::Evaluate( )
{
  if (_id == 0) aaa++;

  if ( _partial_internal_cycles != 0 )
  {
    aaa += 12345;
    cout << "this is not a zero : " << endl;
    on = true;
  }

  _partial_internal_cycles += (double) 1.0;

  if ( _partial_internal_cycles >= (double)1.0 ) {
    _InternalStep( );
    _partial_internal_cycles -= (double)1.0;
  }

  if (GetSimTime() > 8646000 && _id == 0) cout << "aaa = " << aaa << endl;
  if ( on)
  {
    cout << "break. id = " << _id << endl;
    assert(false);
  }

}

解决方案

An exception was generated (notice the I bit is set in the stat field). As the documentation says:

If the ST(7) data register which would become the new ST(0) is not empty, both a Stack Fault and an Invalid operation exceptions are detected, setting both flags in the Status Word. The TOP register pointer in the Status Word would still be decremented and the new value in ST(0) would be the INDEFINITE NAN.

By the way, your underlying issue is because this is just the nature of floating point. It's not exact. See, for example, this gcc bug report -- and this one.

这篇关于浮点指令异常 - FLDZ故障？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！