[Bug c/51656] New: C front end and strtold handle hexadecimal floating differently

["exponent-bias at yandex dot ru" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51656

             Bug #: 51656
           Summary: C front end and strtold handle hexadecimal floating
                    differently
    Classification: Unclassified
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: exponent-bias@yandex.ru


Consider this program:

----
#include <stdlib.h>
#include <stdio.h>

long double f;

int main(void)
{
    f = 0x9.1a1c9420419a1a08p0L;
    printf("%.15La\n", f);

    f = strtold("0x9.1a1c9420419a1a08p0", NULL);
    printf("%.15La\n", f);

    return EXIT_SUCCESS;
}
----

That program prints:
0x9.1a1c9420419a1a0p+0
0x9.1a1c9420419a1a1p+0

Note the difference in the last mantissas' digits. I believe that difference
can be considered a bug by at least two reasons: 1) the "strtold()" function
seems to be rounding hexadecimals by rules that don't match neither of the
standard rounding mode (including the to-nearest mode for which it should
return 0x9.1a1c9420419a1a0p+0) and 2) that difference breaks user's
expectations in that these two ways of acquiring floating-point values should
behave identically under the same rounding mode.

Command-line options used:
gcc --float-store -g -O0 -o test test.c

Compiler version:
gcc-4.4.real (Ubuntu 4.4.3-4ubuntu5) 4.4.3

Host and target platforms:
32-bit x86 system