FIX: ISHFT and ISHL Always Assume 4-Byte Integers (111234)

SYMPTOMS

The ISHL and ISHFT intrinsic functions may return incorrect results when shifting INTEGER*1 or INTEGER*2 variables. The incorrect results occur only when right-shifting negative values.

CAUSE

The compiler converts the value to be shifted to a 4-byte value. For 1- and 2-byte negative integers, the higher order bits are filled with 1s, which can be incorrectly shifted into the low order bytes.

RESOLUTION

Convert the bit pattern of the negative INTEGER*1 or INTEGER*2 value to an unsigned INTEGER*4 value. This is done by adding 256 to a negative INTEGER*1 value or by adding 65536 to a negative INTEGER*2 value. Then shift the resulting INTEGER*4 value.

STATUS

Microsoft has confirmed this to be a problem in FORTRAN PowerStation 32 for Windows NT version 1.0 and MS-DOS version 1.0 and 1.0a. This problem was fixed in FORTRAN PowerStation 32, version 4.0.

MORE INFORMATION

The following sample illustrates the problem:

Sample Code #1

c Compile options needed: none
c
      integer*2 i2
      integer*1 i1
      i1 = -123
      i2 = i1
      write(*,'('' Before shift '',Z2,2x,z4)') i1, i2
      write(*,'('' After shift  '',Z2,2x,z4)') ishl(i1,-2), ishl(i2,-2)
      end

Output

Before shift 85 FF85
After shift E1 FFE1

The following code illustrates the article's resolution:

Sample Code #2

c Compile options needed: none
c
      integer*4 j,k
      integer*2 i2
      integer*1 i1
      i1 = -123
      i2 = i1
      j = 256 + i1
      k = 65536 + i2
      write(*,'('' Before shift '',Z2,2x,z4)') i1, i2
      write(*,'('' After shift  '',Z2,2x,z4)') ishl(j,-2), ishl(k,-2)
      end

Output

Before shift 85 FF85
After shift 21 3FE1