[Bug c/17778] New: regression in evaluating long double hexadecimal constants

[das at FreeBSD dot ORG]

On FreeBSD/i386, FPU precision is set to 53 bits to avoid problems with extra
precision and double rounding for doubles.  gcc understands this and evaluates
FP constants in 53-bit precision, even for long doubles.  (See
ieee_extended_intel_96_round_53_format in real.c.)  However, gcc occasionally
evaluates hexadecimal (and possibly decimal) floating-point constants
incorrectly starting with version 3.4:

        das@VARK:~> cat bar.c
        #include <stdio.h>
                                                                                
        int main(int argc, char *argv[]) {
            volatile long double d = 0x3.243f6a8885a31p0L;
                                                                                
            printf("%La\n", d);
            d = 2.0 * d / 2.0;
            printf("%La\n", d);
            return (0);
        }
        das@VARK:~> gcc33 bar.c
        das@VARK:~> ./a.out
        0xc.90fdaa22168cp-2
        0xc.90fdaa22168cp-2
        das@VARK:~> gcc bar.c
        das@VARK:~> ./a.out
        0xc.90fdaa22168c4p-2
        0xc.90fdaa22168cp-2
        das@VARK:~> gcc --version
        gcc (GCC) 3.4.2 [FreeBSD] 20040728
        das@VARK:~> gcc33 --version
        gcc33 (GCC) 3.3.5 20040630 (prerelease) [FreeBSD]

In the tests I've tried, the symptoms were always setting the 57th mantissa bit,
even when the number would not have that bit set when rounded to 57 bits.  Given
the position of the incorrect bit, I suspect that the bug is due to either a
mistake in the way rounding is handled, or, more likely, an endianness mistake
in the handling of the sign bit that doesn't show up in the 96-bit format since
the 97th bit is ignored.

-- 
           Summary: regression in evaluating long double hexadecimal
                    constants
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: das at FreeBSD dot ORG
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i386-unknown-freebsd6.0
  GCC host triplet: i386-unknown-freebsd6.0
GCC target triplet: i386-unknown-freebsd6.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17778