(j3.2006) Question about zero-sized array.
Daniel C Chen
cdchen
Thu Jun 22 10:22:42 EDT 2017
Thanks Walt and Tom!
I think my confusion is I still couldn't find a place in the standard that
specifies the behavior you guys described.
Daniel
XL Fortran Development, Fortran Standard Representative
IBM Toronto Software Lab
Phone: 905-413-3056
Tie: 969-3056
Email: cdchen at ca.ibm.com
http://www.ibm.com/software/awdtools/fortran/xlfortran
From: Walt Brainerd <walt.brainerd at gmail.com>
To: fortran standards email list for J3 <j3 at mailman.j3-fortran.org>
Date: 06/21/2017 07:24 PM
Subject: Re: (j3.2006) Question about zero-sized array.
Sent by: j3-bounces at mailman.j3-fortran.org
I think Tom correctly answered your Q3.
The result of the MATMUL is a rank-1 size 7 array of 0s,
since each element is the sum of an empty array.
You can generate all the code you want as long as you get the right answer,
but as Tom says, if somehow you figure out that the MATMUL result is all 0,
it certainly could be optimized away. So the whole assignment can be
optimized away (again, if all that can be determined by the compiler).
On Wed, Jun 21, 2017 at 1:18 PM, Clune, Thomas L. (GSFC-6101) <
thomas.l.clune at nasa.gov> wrote:
On Jun 21, 2017, at 2:51 PM, Daniel C Chen <cdchen at ca.ibm.com>
wrote:
Sure. I agree with Q1 and Q2.
As for Q3,
1. is the statement standard conforming?
y(:) = MATMUL( a(:,:), x(:) ) + 2*y(:)
Yes.
2. if it is standard conforming, would it be equivalent to
y(:) = 2*y(:)
Yes.
or it should be no code gen for the whole assignment?
The array y _does_ have elements. ? The expression changes their values.
The compiler would need to ?know? that all of the elements are initially
0 to skip the assignment, and this has nothing to do with zero-sized
things.
- Tom
Thanks,
Daniel
XL Fortran Development, Fortran Standard Representative
IBM Toronto Software Lab
Phone: 905-413-3056
Tie: 969-3056
Email: cdchen at ca.ibm.com
http://www.ibm.com/software/awdtools/fortran/xlfortran
<graycol.gif>Walt Brainerd ---06/21/2017 01:43:49 PM---Let me try
to help (maybe it won't) with a different point of view. I always
thought it was not the
From: Walt Brainerd <walt.brainerd at gmail.com>
To: fortran standards email list for J3 <j3 at mailman.j3-fortran.org>
Date: 06/21/2017 01:43 PM
Subject: Re: (j3.2006) Question about zero-sized array.
Sent by: j3-bounces at mailman.j3-fortran.org
Let me try to help (maybe it won't) with a different point of view.
I always thought it was not the best to say that x is always
defined,
but since the standard does . . .
For Q1, I would say the value of x is the empty array.
For Q2, I think you are asking the wrong question.
An *element* of the array can have the value 0.0 or 1.0, but
x has no elements, and the question is meaningless.
Also Q3.
Crystal clear?
On Wed, Jun 21, 2017 at 9:37 AM, Daniel C Chen <cdchen at ca.ibm.com>
wrote:
Thanks John and Malcolm for your response. I didn't make the
questions clear enough so I will try again.
As for Q1,
non-allocatable case:
real :: x(0) ! x is always defined by the standard. What is
its value at this point?
allocatable case:
real, allocatable :: x(:)
allocate(x0)) ! at this point, x is defined by allocation.
Again, what is the value after allocation?
As for Q2, I actualy meant to ask 'x' rather than 'a'.
x = 0.0 ! Does x have value 0.0?
x = 1.0 ! x Does x have value 1.0?
As for Q3, if x in Q2 is defined with the value 0.0 or 1.0
and used in the following statement respectively,
a(:,:) = 1.0
y(:) = MATMUL( a(:,:), x(:) ) + 2*y(:) ! Does the value of x
affect the result?
Thanks,
Daniel
XL Fortran Development, Fortran Standard Representative
IBM Toronto Software Lab
Phone: 905-413-3056
Tie: 969-3056
Email: cdchen at ca.ibm.com
http://www.ibm.com/software/awdtools/fortran/xlfortran
<graycol.gif>"Malcolm Cohen" ---06/20/2017 07:52:35
PM---Daniel Chen asks: > The standard specifies, "Zero-sized
arrays and zero-length strings are
From: "Malcolm Cohen" <malcolm at nag-j.co.jp>
To: "'fortran standards email list for J3'" <
j3 at mailman.j3-fortran.org>
Date: 06/20/2017 07:52 PM
Subject: Re: (j3.2006) Question about zero-sized array.
Sent by: j3-bounces at mailman.j3-fortran.org
Daniel Chen asks:
> The standard specifies, "Zero-sized arrays and zero-length
strings are
> always defined".
> It also says "Allocation of a zero-sized array or
zero-length character
> variable causes the array or variable to become defined"
>
> Question1: Does it have initial value at all?
John Reid opines:
>Well, yes. It always has the same value, including
initially.
No.? Being allocatable or a pointer it is initially
undefined.? ?initially? means at the beginning of execution;
at that point the array does not have any shape.
Daniel continues::
> allocate(a(7,0),x(0),y(7))
> a(:,:) = 0.0 !Q2: Is this legal? What is the value of a?
John replies:
>Yes, you are broadcasting a scalar to every element of the
array. a has
>the value it always has when the size is zero.
?it always has? could be a little misleading here; being an
array, the shape is part of the value, so the value is the
7x0 empty array and not the 0x23 empty array.? I?m not sure
we have all this explicitly written down anywhere though,
except for derived types, where we say the array bounds of a
component are part of the value of the structure.? Outside of
a structure it is the shape that?s important rather than the
bounds.
Cheers,
--
..............Malcolm Cohen, NAG Oxford/Tokyo.
From: j3-bounces at mailman.j3-fortran.org [
mailto:j3-bounces at mailman.j3-fortran.org] On Behalf Of John
Reid
Sent: Wednesday, June 21, 2017 12:53 AM
To: fortran standards email list for J3 <
j3 at mailman.j3-fortran.org>
Subject: Re: (j3.2006) Question about zero-sized array.
Daniel C Chen wrote:
> Hello,
>
> The standard specifies, "Zero-sized arrays and zero-length
strings are
> always defined".
> It also says "Allocation of a zero-sized array or
zero-length character
> variable causes the array or variable to become defined"
>
> Question1: Does it have initial value at all?
Well, yes. It always has the same value, including initially.
> Question2: Can it appear as the LHS of an assignment?
Yes.
> Question3: Can it be used in an expression and have a
defined result?
Yes.
>
> Please consider the following example:
>
> program test
> real, dimension(:,:), allocatable:: a
> real, dimension(:), allocatable :: x,y
>
> allocate(a(7,0),x(0),y(7))
> a(:,:) = 0.0 !Q2: Is this legal? What is the value of a?
Yes, you are broadcasting a scalar to every element of the
array. a has
the value it always has when the size is zero.
> x(:) = 0.0
Yes, you are again broadcasting a scalar to every element of
the array.
> y(:) = 1.0
> y(:) = MATMUL( a(:,:), x(:) ) !Q3: Is this legal? what is
the value of y?
Yes. Every element of y has the value 0.0.
> end program test
>
> Non-allocatable version:
Nothing is reallocated in the allocatable version, so this is
the same.
Cheers,
John.
> program test2
> real :: a(7,0)
> real :: x(0),y(7)
>
> a = 0.0 !Q2: is this legal? What is the value of a?
> x = 0.0
> y = 1.0
> y = MATMUL( a, x ) !Q3: is this legal? what is the value of
y?
> end program test2
>
> Thanks,
>
> Daniel
>
> XL Fortran Development, Fortran Standard Representative
> IBM Toronto Software Lab
> Phone: 905-413-3056
> Tie: 969-3056
> Email: cdchen at ca.ibm.com
> http://www.ibm.com/software/awdtools/fortran/xlfortran
>
>
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