On Thu, 2014-05-01 at 17:11 -0400, Larry Bradley wrote:
> Thanks. Somewhere I read that the bitfield stuff was not efficient.
>=20
> I tried it. The code using bitfields was the same size as using a mask.
> And the mask code is a bit (no pun intended) clearer!

I'm kind of surprised nobody compared the generated code.  The code
produced by XC16 is identical:

    if ( myBits & 0x0002 )
        myTestWord =3D 87;

    if (myBitfield.b1)
        myTestWord =3D 83;

53:                    if ( myBits & 0x0002 )
00029A  804000     MOV myBits, W0
00029C  600062     AND W0, #0x2, W0
00029E  500FE0     SUB W0, #0x0, [W15]
0002A0  320002     BRA Z, 0x2A6
54:                        myTestWord =3D 87;
0002A2  200570     MOV #0x57, W0
0002A4  884020     MOV W0, myTestWord
55:               =20
56:                    if (myBitfield.b1)
0002A6  BFC802     MOV.B myBitfield, WREG
0002A8  604062     AND.B W0, #0x2, W0
0002AA  504FE0     SUB.B W0, #0x0, [W15]
0002AC  320002     BRA Z, 0x2B2
57:                        myTestWord =3D 83;
0002AE  200530     MOV #0x53, W0
0002B0  884020     MOV W0, myTestWord

The real question is, does the optimizer have better luck with one over
the other.  Problem with that is that it is real hard to come up with
bogus code the optimizer won't simply optimize out of existence.

I would argue that the mask code is only clearer if
1) You are doing some bogus example like this or
2) You are an assembler programmer

With the bitfield you can give the bits names, so for example you could
say something like:

  if ( LCDstatus.LCDisBusy )

  rather than

  if ( LCDstatus & 0b0000000000100000 )

  Granted, you could say

  #define LCDBUSY 0x0020

  then

  if ( LCDstatus & LCDBUSY )

but I still don't find that as clear as the bitfield example, which is
probably why Microchip goes to the trouble of naming all the bits in the
status words.  You can even give the bit a macro name, making the code
even more clear.  Something Microchip does with many of the SFR bits.
So with a define I could simply say

  if ( BUSYBIT )

far more readable than

  if ( LCDstatus & 0b0000000000100000 )


--McD


--=20
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