c/2582: implicit cast assignment from int to float problem with x86

Joe Murray jmurray@dsrnet.com
Wed Apr 18 15:35:00 GMT 2001


Note: Don't compile with any optimization because it precomputes all of the
values and masks the problem. i.e. don't use any flavor of -O

Joe.

-----Original Message-----
From: jmurray@dsrnet.com [ mailto:jmurray@dsrnet.com ]
Sent: Wednesday, April 18, 2001 5:30 PM
To: nobody@gcc.gnu.org
Cc: gcc-prs@gcc.gnu.org; gcc-bugs@gcc.gnu.org
Subject: c/2582: implicit cast assignment from int to float problem with
x86



>Number:         2582
>Category:       c
>Synopsis:       implicit cast assignment from int to float problem with x86
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Wed Apr 18 14:36:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Joseph Murray
>Release:        gcc version egcs-2.91.66 19990314/Linux (egcs-1.1.2
release)
>Organization:
>Environment:
Red Hat Linux 6.2 w/ the Linux 2.2.18 Kernel
However, this is present on all versions of Linux for x86 that we have
found. We have tested on single processor, dual processor, quad processor,
and octal processor with the 2.2.xx kernels and the 2.4.xx kernels and
several versions of gcc and g++.

uname -a results:
Linux compaq1a 2.2.18 #1 SMP Fri Apr 6 14:55:49 EDT 2001 i686 unknown.
>Description:
We had an extensive discussion with a number of different groups on this
problem but never found a good solution. We have tested this on Win98 w/ a
Borland Compiler, on several versions of Linux with several versions of GCC,
and on several RISC based workstations with GCC and other compilers.

This routine works as anticipated on all RISC machines regardless of the
compiler but fails on all x86 based machines (that we have) with GCC. If we
declare the variables volatile it works. My problem is that I have 600000
lines of code and I can't declare variables volatile everywhere or find all
of the potential locations of this problem.

A routine that demonstrates the problem is below. I can provide several
other examples. It was suggested previously that the floating point unit was
being put into truncate mode from its default round-to mode. I don't know
how to overcome this without major ramifications on the code baseline.

A summary of the routine is:

1. Assign some values to some integer variables.
2. Assign a floating point value from the division of two cast integers.
3. Assign a floating point value from the multiplication of two floats.
4. Assign an integer from a cast of a floating point product.
5. Assign an integer from a cast of a floating point number.

Result:

This is equiv to num = .35 * 20

3 results in: 7.0000000000 (actually probably 6.999999?rounded to
7.00000000)
4 results in: 6 <------- PROBLEM WITH INPLACE ASSIGNMENT
5 results in: 7 (correct answer)

Example routine:
-------------------------------------------------------
#include <stdio.h>

int main(void) {

  /* Assign some values to ints */
  int iaa = 28;
  int ibb = 80;
  int icc = 20;

  int imm, immO;
  float fll;
  float ftmp;

  fll = (float)iaa / (float)ibb;

  imm = ftmp = fll * (float)icc;

  printf("Direct assign float to float = %.10f\n", ftmp);
  printf(" Inplace assign float to int = %d\n", imm);

  immO = ftmp;
  printf("  Direct assign float to int = %d\n", immO);

  return(0);
}
-------------------------------------------------------

We can not find all of the instances of this problem and often it is
undetectable in many of the normalization processes of our systems. However,
the results are wrong and lead to the wrong solution.
>How-To-Repeat:
#include <stdio.h>

int main(void) {

  /* Assign some values to ints */
  int iaa = 28;
  int ibb = 80;
  int icc = 20;

  int imm, immO;
  float fll;
  float ftmp;

  fll = (float)iaa / (float)ibb;

  imm = ftmp = fll * (float)icc;

  printf("Direct assign float to float = %.10f\n", ftmp);
  printf(" Inplace assign float to int = %d\n", imm);

  immO = ftmp;
  printf("  Direct assign float to int = %d\n", immO);

  return(0);
}
>Fix:
A fix is to declare the variables volatile which ultimately changes the
output assembly. Alternately, you can program to force all intermediate
variables to be set to an appropriate type before truncation. This is
difficult for existing baselines and inplace checks.
>Release-Note:
>Audit-Trail:
>Unformatted:



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