Bug 13719 - 0.2*5.0>1.0 even with -O0, missing fldl probably
Summary: 0.2*5.0>1.0 even with -O0, missing fldl probably
Alias: None
Product: gcc
Classification: Unclassified
Component: c++ (show other bugs)
Version: 3.3.2
: P2 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
Depends on:
Reported: 2004-01-17 14:24 UTC by Guilhem Bichot
Modified: 2005-07-23 22:49 UTC (History)
1 user (show)

See Also:
Known to work:
Known to fail:
Last reconfirmed:


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Description Guilhem Bichot 2004-01-17 14:24:39 UTC
I have read the "most reported non-bug" about floating-point precision, but I
think it's a real bug here; please take time to read.
I personally use 3.2.2 but colleague with 3.3.2 has the same bug too (with
same testcase and same compiler options). We use Linux (Mandrake & SuSE).

Here is test.cc:
#include <stdlib.h>

int main()
  return ((int)((atof("0.2")*atof("5.0"))>1.0));

Compile this with:
g++ -O0    -o test_bin test.cc
And in a shell execute:
./test_bin ; echo $?
I get 1, so comparison is done wrong.
Adding -ffloat-store does not remove the error.

Here is the problem in my humble opinion (using the output of g++ -S):
g++ -O0 -S test.cc:

        .file   "test.cc"
        .section        .rodata
        .string "0.2"
        .string "5.0"
        .align 2
.globl main
        .type   main,@function
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        andl    $-16, %esp
        movl    $0, %eax
        subl    %eax, %esp
        subl    $12, %esp
        pushl   $.LC0
        call    atof
        addl    $16, %esp
        fstpl   -8(%ebp)
        subl    $12, %esp
        pushl   $.LC1
        call    atof
        addl    $16, %esp
        fldl    -8(%ebp)
**multiplication of the 2 numbers
        fmulp   %st, %st(1)
**result of multiplication (1.000000000000000056)
**is left in FPU
**and directly used for comparison, hence the problem
        fxch    %st(1)
        fnstsw  %ax
        testb   $69, %ah
        sete    %al
        andl    $255, %eax
        .size   main,.Lfe1-main
        .ident  "GCC: (GNU) 3.2.2 (Mandrake Linux 9.1 3.2.2-3mdk)"

I guess there should be some rounding (using fldl) between multiplication and
comparison; it does not make sense to do the comparison with the extra precision
of the FPU. In the next example we see it better.

If I add a few printf(), the error appears only with -O1 (or more): here is
#include <stdlib.h>
#include <stdio.h>

  double e = atof("0.2")*atof("5.0");
  if (e>1.0)
    printf("0.2 * 5.0 > 1.0 (ERROR)\n");
    printf("0.2 * 5.0 <= 1.0 (OK)\n");

When built with g++ -O0 test2.cc it prints OK, with -01 it prints ERROR.
Here are snippets of the assembler code (the multiplication and comparison section):
with -O0:
        fmull   -16(%ebp)
        fstpl   -8(%ebp)
        fldl    -8(%ebp)
        fxch    %st(1)
(see fldl which does the rounding of the multiplication result from FPU
precision to double precision).
With -O1:
        fmulp   %st, %st(1)
        fxch    %st(1)
Again, the result of the multiplication (with precision of the FPU) is used
directly for comparison, without any rounding.

In the MySQL code which is were the problem originally arose (we had
SELECT ACOS(0.2*5.0) returning NULL), the problem appeared only with -O2 (or more).

I hope these testcases can be of use.
Guilhem Bichot (MySQL A.B.)
Comment 1 Guilhem Bichot 2004-01-17 14:25:51 UTC
Forgot to say: reproduced on processors AMD Athlon XP2000+ and Pentium III Xeon.
Comment 2 Andrew Pinski 2004-01-17 18:05:36 UTC
Not a bug, it is connect to the most reported non-bug but GCC is allowed to keep intermediate 
results in the excussive precision form which is happening here.  If you store the multiplication into 
a tempary variable it works as you are expecting, this is not a bug.
Comment 3 Guilhem Bichot 2004-01-17 19:54:23 UTC
Thanks for your answer. However, the 2nd example I gave in the bug report
#include <stdlib.h>
#include <stdio.h>

  double e = atof("0.2")*atof("5.0");
  if (e>1.0)
    printf("0.2 * 5.0 > 1.0 (ERROR)\n");
    printf("0.2 * 5.0 <= 1.0 (OK)\n");
does "store the multiplication into a temporary variable" (quoting your words)
('e'), but it certainly does not "work as I am expecting" when compiled with -O1.
Could you please explain to me?
Comment 4 Andrew Pinski 2004-01-17 20:02:48 UTC
Still need -ffloat-store for the second case, the first case it does not matter because of C rules and 
how -ffloat-store works.  Also you can use -mfpmath=sse to get the results you are expecting on