[Bug ada/41416] New: Conversion Float to fixed-point behaves differently for static expressions

[Bug ada/41416] New: Conversion Float to fixed-point behaves differently for static expressions

[dirk dot herrmann-privat at gmx dot de]

Hi,

the following complete sample code shows different conversion behaviours
between fixed-point types and Float to fixed-point depending on whether the
expressions are static or not.  In the static case, GNAT uses a conversion
strategy comparable to truncation, otherwise rounding is used.

I assume that this difference in behaviour is unintended.

----------- start -----------
with Ada.Text_IO;
use Ada.Text_IO;

procedure Conversion is

   type FpA is delta 0.5 range -10.0 .. +10.0;
   for FpA'Small use 0.5;
   type FpB is delta 0.4 range -10.0 .. +10.0;
   for FpB'Small use 0.4;

   function MakeB(F: in Float) return FpB is
   begin
      return FpB(F);
   end MakeB;

   function MakeBFromA(F: in FpA) return FpB is
   begin
      return FpB(F);
   end MakeBFromA;

   function MakeBViaA(F: in Float) return FpB is
   begin
      return FpB(FpA(F));
   end MakeBViaA;

   function Get_Minus_1_5 return Float is
   begin
      return -1.5;
   end Get_Minus_1_5;

begin

   Put_Line(FpA'Image(FpA(-1.5)));
   Put_Line(
      FpB'Image(FpB(-1.5)) & " "                   --> -1.2  (truncation)
      & FpB'Image(FpB(Float(-1.5))) & " "          --> -1.2  (truncation)
      & FpB'Image(FpB(Get_Minus_1_5)) & " "        --> -1.6  (rounding)
      & FpB'Image(MakeB(-1.5)));                   --> -1.6  (rounding)
   Put_Line(
      FpB'Image(FpB(FpA(-1.5))) & " "              --> -1.2  (truncation)
      & FpB'Image(FpB(FpA(Float(-1.5)))) & " "     --> -1.2  (truncation)
      & FpB'Image(FpB(FpA'Succ(FpA(-2.0)))) & " "  --> -1.2  (truncation)
      & FpB'Image(FpB(FpA(Get_Minus_1_5))) & " "   --> -1.6  (rounding)
      & FpB'Image(MakeBFromA(FpA(-1.5))) & " "     --> -1.6  (rounding)
      & FpB'Image(MakeBViaA(-1.5)));               --> -1.6  (rounding)

end Conversion;
------------ end ------------

The command line I am using for building:
> gnatmake -f -gnatVa -gnata -gnatwadhl.o -O3 -save-temps conversion.adb

The output from GNAT:
----------- start -----------
conversion.adb:35:17: warning: static fixed-point value is not a multiple of
Small
conversion.adb:36:19: warning: static fixed-point value is not a multiple of
Small
conversion.adb:40:17: warning: static fixed-point value is not a multiple of
Small
conversion.adb:41:19: warning: static fixed-point value is not a multiple of
Small
conversion.adb:42:19: warning: static fixed-point value is not a multiple of
Small
gnatbind -x conversion.ali
gnatlink conversion.ali
------------ end ------------

The version of GNAT I am using:
> gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.3.4-2'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--enable-multiarch --enable-linker-build-id --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--enable-nls --with-gxx-include-dir=/usr/include/c++/4.3 --program-suffix=-4.3
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--enable-targets=all --with-tune=generic --enable-checking=release
--build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.3.4 (Debian 4.3.4-2)

Friendly regards,
Dirk Herrmann


-- 
           Summary: Conversion Float to fixed-point behaves differently for
                    static expressions
           Product: gcc
           Version: 4.3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dirk dot herrmann-privat at gmx dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41416

[Bug ada/41416] Conversion Float to fixed-point behaves differently for static expressions

[sam at gcc dot gnu dot org]

------- Comment #1 from sam at gcc dot gnu dot org  2009-10-03 11:43 -------
It would help if you provided the RM reference that you think is violated here.


-- 

sam at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sam at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41416

[Bug ada/41416] Conversion Float to fixed-point behaves differently for static expressions

[dirk dot herrmann-privat at gmx dot de]

------- Comment #2 from dirk dot herrmann-privat at gmx dot de  2009-10-05 16:48 -------
Hi,

thanks for considering my report.

The relevant sections seem to be (these were pointed out to me on comp.lang.ada
by Adam Beneschan):

4.9(38) : "For a real static expression that is not part of a larger static
expression ... the implementation shall round or truncate the value (according
to the Machine_Rounds attribute of the expected type) to the nearest machine
number of the expected type...".

4.9(38.1/2): "The rounding of static floating point expressions is
implementation-defined. It is recommended that the rounding be the same as the
default rounding on the target system."

A.5.4(3): S'Machine_Rounds: Yields the value True if rounding is performed on
inexact results of every predefined operation that yields a result of the type
T; yields the value False otherwise. The value of this attribute is of the
predefined type Boolean.

Here's the interpretation (for which I also used some input from comp.lang.ada
by Adam Beneschan):

4.9(38) demands rounding or truncation for real static expressions, depending
on the value of Machine_Rounds.  A.5.4(3) is not limited to static expressions,
but relates "Machine_Rounds = True" to rounding.  The behaviour in case of
"Machine_Rounds = False" is left open by A.5.4(3), though.  The implementation
advice in 4.9(38.1/2) recommends that static rounding should be the same as the
default rounding.

There is no explicit statement with respect to how non-static expressions
should be handled in case of "Machine_Rounds = False".  But, 4.9(38.1/2) can be
interpreted such that it is generally desired that static and non-static
behaviour should match.  Therefore: 4.9(38) demands truncation for static
expressions in case of "Machine_Rounds = False", and it seems plausible to me
that this should then also apply for non-static expressions.

Thanks again,
Dirk


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41416