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* [Bug fortran/99561] New: gfortran reports an error for a truncation that is permitted by the standard
@ 2021-03-12 13:33 michal.paszta at mobica dot com
2021-03-12 18:39 ` [Bug fortran/99561] " kargl at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: michal.paszta at mobica dot com @ 2021-03-12 13:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99561
Bug ID: 99561
Summary: gfortran reports an error for a truncation that is
permitted by the standard
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: michal.paszta at mobica dot com
Target Milestone: ---
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.
$ ~/gcc/trunk/bin/gfortran issue.f90
issue.f90:17:27:
17 | INTEGER(KIND=1) :: var8 = 257_2
| 1
Error: Arithmetic overflow converting INTEGER(2) to INTEGER(1) at (1). This
check can be disabled with the option ‘-fno-range-check’
We believe the operation should be allowed. It could warn the programmer about
the truncation, but should not prevent a successful compilation.
The obvious workaround is to follow the suggestion for the error message and
add '-fno-range-check' - in that case I got no warning or error.
The report originates from a flang github issue:
https://github.com/flang-compiler/flang/issues/992 and was pointed out by Bryan
Chan.
I tried gfortran 7.5.0 and 10.2.0 and both acted the same.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug fortran/99561] gfortran reports an error for a truncation that is permitted by the standard
2021-03-12 13:33 [Bug fortran/99561] New: gfortran reports an error for a truncation that is permitted by the standard michal.paszta at mobica dot com
@ 2021-03-12 18:39 ` kargl at gcc dot gnu.org
2021-03-12 18:51 ` kargl at gcc dot gnu.org
2021-03-12 19:20 ` sgk at troutmask dot apl.washington.edu
2 siblings, 0 replies; 4+ messages in thread
From: kargl at gcc dot gnu.org @ 2021-03-12 18:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99561
kargl at gcc dot gnu.org changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
CC| |kargl at gcc dot gnu.org
--- Comment #1 from kargl at gcc dot gnu.org ---
(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]
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug fortran/99561] gfortran reports an error for a truncation that is permitted by the standard
2021-03-12 13:33 [Bug fortran/99561] New: gfortran reports an error for a truncation that is permitted by the standard 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
2021-03-12 19:20 ` sgk at troutmask dot apl.washington.edu
2 siblings, 0 replies; 4+ messages in thread
From: kargl at gcc dot gnu.org @ 2021-03-12 18:51 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug fortran/99561] gfortran reports an error for a truncation that is permitted by the standard
2021-03-12 13:33 [Bug fortran/99561] New: gfortran reports an error for a truncation that is permitted by the standard 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
@ 2021-03-12 19:20 ` sgk at troutmask dot apl.washington.edu
2 siblings, 0 replies; 4+ messages in thread
From: sgk at troutmask dot apl.washington.edu @ 2021-03-12 19:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99561
--- Comment #3 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Fri, Mar 12, 2021 at 06:39:49PM +0000, kargl at gcc dot gnu.org wrote:
> --- Comment #1 from kargl at gcc dot gnu.org ---
> (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.
It does NOT tell
> In fact, an INTEGER(KIND=1) entity has a range of
> [-128,127], so the value of 256 is still out-of-range.
Sigh, "not" was a rather important omission. :(
^ permalink raw reply [flat|nested] 4+ messages in thread
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2021-03-12 13:33 [Bug fortran/99561] New: gfortran reports an error for a truncation that is permitted by the standard 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
2021-03-12 19:20 ` sgk at troutmask dot apl.washington.edu
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