[flang] Incorrect COMMON block storage sequence

[DanielCChen]

Consider the following code:

   INTEGER i,j,k

   INTEGER i7(10)
   INTEGER i8(10)
   INTEGER i9(20)

   COMMON /BLOCK1/ i7,i8
   EQUIVALENCE (i7,i9)


   DO i = 1,20
      i9(i) = i
   END DO

   FORALL ( i = 1:10 )
      i8(i) = i9(9+i)
   END FORALL
   PRINT *,i8
   PRINT *,i9

END

Expected output:

 10 10 10 10 10 10 10 10 10 10
 1 2 3 4 5 6 7 8 9 10 10 10 10 10 10 10 10 10 10 10

Flang outputs:

 10 11 12 13 14 15 16 17 18 19
 1 2 3 4 5 6 7 8 9 10 10 11 12 13 14 15 16 17 18 19

The standard requires the storage sequence of a COMMON block to be the order of its object list.

The order of the storage sequences is the same as the order of the appearance of the common block object lists in the scoping unit.

Therefore, the assignment to i8 in FORALL should affect both i8 and i9.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` INTEGER i,j,k

INTEGER i7(10)
INTEGER i8(10)
INTEGER i9(20)

COMMON /BLOCK1/ i7,i8
EQUIVALENCE (i7,i9)

DO i = 1,20
i9(i) = i
END DO

FORALL ( i = 1:10 )
i8(i) = i9(9+i)
END FORALL
PRINT *,i8
PRINT *,i9

END

Expected output:

10 10 10 10 10 10 10 10 10 10
1 2 3 4 5 6 7 8 9 10 10 10 10 10 10 10 10 10 10 10

Flang outputs:

10 11 12 13 14 15 16 17 18 19
1 2 3 4 5 6 7 8 9 10 10 11 12 13 14 15 16 17 18 19

The standard requires the storage sequence of a COMMON block to be the order of its object list. 

The order of the storage sequences is the same as the order of the appearance of the common block object lists in the scoping unit.

Therefore, the assignment to `i8` in `FORALL` should affect both `i8` and `i9`.

</details>

[jeanPerier]

@DanielCChen, I think you can fill a bug against xlf and gfortran FORALL implementation here instead. I believe flang output (which matches the one of ifx and nag) is correct.

i8 and i9 correctly overlaps (you can check that with LOC), but FORALL requires (Fortran 2023 section 10.2.4.2.4):

Execution of a forall-assignment-stmt that is an assignment-stmt causes the evaluation of expr and all expressions within variable
for all active combinations of index-name values. These evaluations may be done in any order. After all these evaluations have been
performed, each expr value is assigned to the corresponding variable. The assignments may occur in any order.

So the FORALL in your code is equivalent to the following code that produces flang output when compiled with gfortran and XLF:

   INTEGER temp(10)
   DO i = 1,10
      temp(i) = i9(9+i)
   END DO

   DO i = 1,10
     i8(i) = temp(i)
   END DO

I am under the impression FORALL is often mistaken as a fancy loop/one liner loop while it has a drastic execution order requirement for the compiler that actually makes it not so easy to optimize (maybe that is why it was deprecated, I do not know the story here).

[eugeneepshteyn]

Just want to add that ifx produces the same output as flang and it seems that it splits forall into the loops similar to those that Jean presented in the previous comment.

[klausler]

agree, flang-new's output is correct here.

[DanielCChen]

Thanks for the explanation! I indeed missed the ordering requirement of FORALL.