[Bug ada/96799] New: C and Ada frontends have a different interpretation of double constants

[Bug ada/96799] New: C and Ada frontends have a different interpretation of double constants

[demoonlit at panathenaia dot halfmoon.jp]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96799

            Bug ID: 96799
           Summary: C and Ada frontends have a different interpretation of
                    double constants
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
          Assignee: unassigned at gcc dot gnu.org
          Reporter: demoonlit at panathenaia dot halfmoon.jp
  Target Milestone: ---

Hello.
Each of the C and Ada frontends has a different interpretation of the some kind
of double constants.

Reproducing:
Write the same small double constants for each language.

double1.c
--------
const double d_min = 0x0.3ffffffffffffeP-1072;
--------

double2.ads
--------
package double2 is
   d_min : constant Long_Float := 16#0.3ffffffffffffe#e-268;
end double2;
--------

(Note: These constants are the hexadecimal of __DBL_DENORM_MIN__ and wanted to
be rounded to the minimal value of double. The precise value is 0x0.4P-1072,
but __DBL_DENORM_MIN__ is a written as decimal, so these constants are got from
it by the conversion. This value is unimportant because perhaps this bug can be
reproduced with other values.)

Compile them.

$ gcc -W -Wall -S double1.c
$ gcc -W -Wall -S double2.ads
double2.ads:2:35: warning: floating-point value underflows to 0.0

Ada frontend reports the underflow. C frontend does not.
And compare double1.s and double2.s.

double1.s
--------
        .text
        .globl _d_min
        .const
        .align 3
_d_min:
        .long   1                   # <- It has been rounded.
        .long   0
        .ident  "GCC: (GNU) 10.1.0"
        .subsections_via_symbols
--------

double2.s
--------
        .text
        .globl _double2_E
        .data
        .align 1
_double2_E:
        .space 2
        .globl _double2__d_min
        .const
        .align 3
_double2__d_min:
        .space 8                    # <- It has been truncated.
        .ident  "GCC: (GNU) 10.1.0"
        .subsections_via_symbols
--------

I first noticed it in the FreeBSD 32bit. FreeBSD 32bit has the lesser precision
of long double than other environments. However, then I tried it in FreeBSD
64bit, Linux, and MacOSX and it is reproducible too.

[Bug ada/96799] C and Ada frontends have a different interpretation of double constants

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96799

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |ebotcazou at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
C and Ada are different languages.