|> Hi folks
|> I noticed recently that Digital fortran compiler has introduced
|>incompatibility to the previous DEC fortran compiler and and also
|>other vender's platforms.
|> Following is a sample test program to show this incompatibility.
|>c sample program: test3.for
|> integer*4 i4a,i4b,i4c
|> i4a= '120000'O
|> i4b= '120000'O / 2
|> i4c= '120000'O / int(2)
|> write(*,*) i4a,i4b,i4c
|>(A) Result using the following environment
|> o DEC Fortran V6.2-108 (VAX) on Open VMS VAX V6.1
|> o DEC Fortran V6.3-711 (AXP) on Open VMS AXP V6.2
|> o Digital Fortran 77 V7.0-6 on Open VMS AXP V6.2
|> o DEC Fortran V3.8-711 on OSF/1 3.2A
|> o HP FORTRAN 77 Ver: B.10.20
|> o IBM AIX XL Fortran Compiler Version 03.02.0002.0000
|> o IBM AIX XL Fortran Compiler Version 03.02.0004.0000
|> o Silicon Graphics Fortran 77 version 5.2
|> o Absoft FORTRAN 77 on Linux
|> o AND, MANY OTHER PLATFORMS
|> 40960 20480 20480
|> I think this result is normal.
|>(B) result using the following environment (most recent Digital Fortran)
|> o DEC Fortran V6.3-155 (VAX) on Open VMS V6.2 VAX,
|> o Digital Fortran V6.4-165 (VAX) on Open VMS V6.2 VAX,
|> o AND, ALL FUTURE RELEASE OF DIGITAL FORTRAN (as DEC told us...)
|> 40960 -12288 20480
|> Local DEC support center told us that DEC has changed the
|>specification of the Fortran Compiler with the release of DEC
|>Fortran VAX V6.3 (as can be found "sys$help:FORT063.RELEASE_NOTES :
|>126.96.36.199 Sign Extension of Untyped Bit Constants" ) and hereafter all
|>the DEC fortman compiler (including AXP and VAX, VMS and UNIX) is
|>going to be resulted as (B).
|> I cannot believe why this silly change was introduced. This
|>change made a lot of confusion for the compatibility with the
|>previous DEC/VAX fortran. Also this change introduced serious
|>incompatibility with the other vender's fortran.
We considered the older behavior to be incorrect, but we admit that it relies
on a lot of not-well-specified behavior.
What is the type of a bit constant? They're not in Fortran 77 at all,
and are allowed in Fortran 90 only in initialization (DATA) expressions,
so none of that is relevant.
Here's what the Digital Fortran 77 Language Reference Manual has to say about
Bit constants have no data type until they are used. When used,
they assume a data type based on their use.
When the bit constant is used with a binary operator, including
the assignment operator, the datatype of the constant is the
data type of the other operand.
The case in question concerns the expression:
'120000'O / 2
What is its type? Well, what is the type of the constant 2? As has been
implemented in Digital Fortran back to VAX-11 FORTRAN-IV-PLUS V1.0 in 1978,
The value of an integer constant is normally INTEGER*2, INTEGER*4
or INTEGER*8 (Alpha only). If a value is within the INTEGER*2
range (-32768:32767 or 0:65535), it is treated as an INTEGER*2 value...
Therefore, the bit constant assumes the type INTEGER*2. Since in this
representation, the sign bit is on, the constant represents a negative
value, and the division has a negative result (Fortran doesn't have
unsigned arithmetic.) That negative result is then sign-extended to
INTEGER*4 for the assignment.
Amusingly, if the expression had been written instead as:
'120000'O / '2'O
this would be an INTEGER*4 expression because of the other rule that says if
there is no other typed operand, INTEGER*4 is assumed.
Now I'll be the first to admit that this "typing" of integer constants is
not obvious, and it often gets us into trouble, but that's the documented
behavior and we wanted to make sure that we were consistent. Other
compilers might not have this notion of INTEGER*2 typing for small integer
constants and would not run into this problem. But I maintain that the
current VAX compiler behavior is reasonable and consistent with the
documentation and behavior of the compiler in other areas for the past 19 years.
Our Alpha Fortran 77 compiler does not currently do this - it treats the
bit constant as a zero-extended INTEGER*4 (or perhaps INTEGER*8) value.
To add to the confusion, our Fortran 90 compiler now honors the Fortran 90
integer constant typing rules which say that the constant 2 is "default
integer", meaning INTEGER*4 (by default), so there you get the result you
seem to expect. It's not clear yet whether or not we'll change the
Alpha Fortran 77 compiler to match the VAX behavior - this is something
we're still discussing. I'd certainly be interested in observations from
others on the issue, keeping in mind our documented treatment of small
Fortran Development http://www.digital.com/info/slionel.html
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