fyl2x modifying bottom of stack

Question

Hello, I don't know if this is the right forum to post this in, but I'll try here anyway.  I have some inline assembly code I'm using to calculate the y^x but after several iterations it's returning a QNaN.  I narrowed down the cause to a stack fault because the fpu stack is incorrectly modified by the fyl2x instruction.  I'm just wondering if there is something wierd I'm missing here. I'm using VS2008.

Here is a sample from my code:

#include <stdio.h>

int main()
{
	__asm
	{
		fld1 // Nothing on fpu stack
		fld1 // st(0) = 1
		fyl2x // st(0) = 1, st(1) = 1
		wait // st(0) = 0, st(7) = 1 ??????
	}
	return 0;
}

Answer 1

In our experiments this fragment shows a correct result:

 

      double x = 8.0;

      double y = 10.0;

      double result;

      __asm

      {

            fld y

            fld x

            fyl2x // y * log2(x)

            fwait

            fstp result // result = 10 * log2(8) = 30

      }

  

It uses two elements of the stack; therefore two bottom values are lost.

 

Answer 2

Thank you for your help. I realize now that I had a misconception about the nature of the floating point stack.  I thought that fyl2x was the source of the fpu stack corruption in my original source, but I am now trying to pin the true source down.

My Original Code:

#include <stdio.h>

double Pow(double x, double y)
{
	bool neg = false;
	if(x < 0)
	{
		if(y != double((int)y))
			throw "For negative bases y must be an integer";
		if(int(y) % 2 != 0)
			neg = true;
	}
	short controlword;
	__asm
	{
		fld y
		fld x
		fabs
		fyl2x
		lea eax, controlword
		fstcw   word ptr [eax]
		or      byte ptr [eax+1], 0Ch
		fldcw   word ptr [eax]
		fld     st(0)
		fld     st(0)
		frndint
		fsubp st(1), st(0)
		f2xm1
		fld1
		fadd
		fscale
		fstp x
		lea eax, controlword
		fldcw word ptr [eax]
	}
	return (neg)? -x : x;
}


int main()
{
	for(int i = 0;;i++)
	{
		printf("%f", Pow(5, 2));
		getchar();
	}
	return 0;
}

This prints out 25 the first 5 iterations but then starts to print -1.#IND00.

Answer 3

Never mind I found the problem. I was leaving a value on the stack at the end. I just had to add fstp st(1) after everything else. Thanks for the help.