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From: "kargl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/99561] gfortran reports an error for a truncation that is permitted by the standard
Date: Fri, 12 Mar 2021 18:51:53 +0000 [thread overview]
Message-ID: <bug-99561-4-PGTD0qUlKH@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-99561-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99561
--- Comment #2 from kargl at gcc dot gnu.org ---
(In reply to kargl from comment #1)
> (In reply to Michal Paszta from comment #0)
> > In this line of code:
> >
> > INTEGER(KIND=1) :: var8 = 257_2
> >
> > we try to cast an integer of kind 2 (16 bits) onto an integer of kind 1 (8
> > bits, value up to 256). This will result in a truncation of the value and is
> > allowed by the Fortran 2018 Standard, see Table 10.9, Fortran 2018 Standard.
> >
>
> The sentence preceding Table 10.9 and the table tell you
> what conversions are allowed and how the conversion is
> done via a built-in intrinsic subprogram.
>
> It does tell you anything about an out-of-range value.
> In fact, an INTEGER(KIND=1) entity has a range of
> [-128,127], so the value of 256 is still out-of-range.
>
> As you have found, gfortran offers a programmer a bullet
> to shoot their foot (i.e, the -fno-range-check option).
> On most (all?) targets supported by gfortran, you'll get
> two's complement wrap-around semantics. You do not get
> truncation, where I assume you mean that an out-of-range
> value is truncated to -128 or 127 as the situation would
> merit (e.g., var8 = 257_2 <-- huge(var8) = 127).
>
> As to the "no warning problem", you did not ask gfortran
> to generate warnings. You can use either the -Wall option
> or the -Wconversion option to get a warning when using
> the -fno-range-check option.
>
> %gfcx -o z -fno-range-check -Wall a.f90
> a.f90:2:33:
>
> 2 | integer(1), parameter :: var8 = 257_2
> | 1
> Warning: Conversion from 'INTEGER(2)' to 'INTEGER(1)' at (1) [-Wconversion]
I should probably not continue with this issue, but I found the
other text in the Standard. The intrinsic assignment of
var8 = 257_2
with regards to Table 10.9 is then
var8 = int(257_2, kind=1)
But, the Fortran standard contains (18-007r1, p.339)
A program shall not invoke an intrinsic procedure under circumstances
where a value to be assigned to a subroutine argument or returned as
a function result is not representable by objects of the specified
type and type parameters.
So, int(257_2, 1) is technically not permitted by the Fortran standard,
and gfortran dutifully issues an error if you write such a conversion.
next prev parent reply other threads:[~2021-03-12 18:51 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-12 13:33 [Bug fortran/99561] New: " michal.paszta at mobica dot com
2021-03-12 18:39 ` [Bug fortran/99561] " kargl at gcc dot gnu.org
2021-03-12 18:51 ` kargl at gcc dot gnu.org [this message]
2021-03-12 19:20 ` sgk at troutmask dot apl.washington.edu
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