[Bug testsuite/110419] [14 regression] new test case gfortran.dg/value_9.f90 in r14-2050-gd130ae8499e0c6 fails

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110419

David Edelsohn <dje at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |dje at gcc dot gnu.org
   Last reconfirmed|                            |2023-07-18
     Ever confirmed|0                           |1

--- Comment #16 from David Edelsohn <dje at gcc dot gnu.org> ---
As I wrote in issue 110360, the bug appears to be the memory layout and padding
assumed by GFortran that does not take into account endianness.

I have changed val() to print both c and x, and not halt.

  subroutine val (x, c)
    character(kind=1), intent(in) :: x  ! control: pass by reference
    character(kind=1), value      :: c
    print *, "by value(kind=1): ", x
    print *, "by value(kind=1): ", c
!    if (c /= x)   stop 1
    c = "*"
    if (c /= "*") stop 2
  end


The output is:

 by value(kind=1): B
 by value(kind=1): B
 by value(kind=1): A
 by value(kind=1): A
 by value(kind=1): A
 by value(kind=1):    <- c
 by value(kind=1): A
 by value(kind=1):    <- c
 by value(kind=1): A
 by value(kind=1):    <- c
 by value(kind=1): 1
 by value(kind=1):    <- c
 by value(kind=1): 1
 by value(kind=1):    <- c


The assembly language for the first few calls is

# call val  ("B","B")
        lwz 31,LC..5(2)  LOAD ADDRESS of x
        mr 3,31          COPY address to first parameter
        li 6,1
        li 5,1
        lbzu 4,148(3)    LOAD BYTE of c as second parameter
        slwi 4,4,24      SHIFT c 24 bits
        bl .val.4
# call val  ("A",char(65))
        mr 30,31         COPY ADDRESS of x
        li 6,1
        li 5,1
        lbzu 4,152(30)   LOAD BYTE of c as second parameter
        slwi 4,4,24      SHIFT c 24 bits
        mr 3,30          COPY address of first parameter
        bl .val.4
# call val  ("A",char(a))
        li 6,1
        li 5,1
        li 4,65  <- c NOT SHIFTED
        mr 3,30  <- x
        bl .val.4
# call val  ("A",mychar(65))
        li 6,1
        li 5,1
        li 4,65  <- c NOT SHIFTED
        mr 3,30  <- x
        bl .val.4
# call val  ("A",mychar(a))
        li 6,1
        li 5,1
        li 4,65  <- c NOT SHIFTED
        mr 3,30  <- x
        bl .val.4

GFortran is not taking account of endianness for the layout of values in memory
compared to constants loaded into registers.  This isn't an ABI issue of the
target, this is a memory layout and register layout issue of GFortran.

On a big endian system, a character / byte is loaded at the LSB, but GFortran
seems to be comparing it to a memory image with the character / byte stored at
the MSB, which would be correct for little endian.  In some cases, GFortran is
shifting the value and in other cases it is not.

GFortran does not seem to have a consistent view of the memory layout for
characters / bytes loaded into a larger object.