From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ulrich Drepper <drepper@ipd.info.uni-karlsruhe.de>
To: egcs@cygnus.com
Subject: next glibc showstopper
Date: Fri, 21 Nov 1997 21:50:00 -0000
Message-id: <x7yb2hpkyb.fsf@myware.rz.uni-karlsruhe.de>
X-SW-Source: 1997-11/msg00706.html

Hi,

Here's the next problem which prevent successfully compiling the glibc
with the current egcs on ix86:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
double r (double, double);

extern inline int
bar (double x)
{
  union { double d; int i[2]; } u = { d: x }; return u.i[1] < 0;
}

int
foo (double d1, double d2)
{
  double e = r (d1, d2);

  if (bar (d1) && bar (e))		<---- PROBLEM
    return 1;
  else
    return 5;

  return 0;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The fault part of the code is this:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  12:   e8 fc ff ff ff  call   13 <foo+0x13>
  17:   dd 45 08        fldl   0x8(%ebp)
  1a:   dd 5d f8        fstpl  0xfffffff8(%ebp)

  if (bar (d1) && bar (e))
  1d:   83 7d fc 00     cmpl   $0x0,0xfffffffc(%ebp)
  21:   7d 0d           jnl    30 <foo+0x30>
  23:   dd 5d f8        fstpl  0xfffffff8(%ebp)   \ here is something
  26:   7d 0a           jnl    32 <foo+0x32>      / missing
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After the call to `r' (address 12) the return value is in %st(0).  The
according to the inline function `d1' is tested.  It is copied on the
stack (why?) and the correct word is tested against $0.  This is ok.

But now the return value must be checked.  Here only the storing
happens but NO compare instruction sets the flags.  I.e., the flags as
determined for `d1' are used.


-- Uli
---------------.      drepper at gnu.org  ,-.   Rubensstrasse 5
Ulrich Drepper  \    ,-------------------'   \  76149 Karlsruhe/Germany
Cygnus Solutions `--' drepper at cygnus.com   `------------------------