Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!wuarchive!udel!mmdf
From: SUGGS%freddy.hac.com@cunyvm.cuny.edu
Newsgroups: comp.sys.amiga
Subject: Lattice C 5.04 bug
Message-ID: <5453@nigel.udel.EDU>
Date: 6 Dec 89 14:54:52 GMT
Sender: mmdf@udel.EDU
Lines: 56

I've discovered the following bug in Lattice C 5.04:

When register variables are declared with an initial value, that
value is also written onto the stack.  That is, written over existing
values on the stack, not pushed on the stack.

Actually, the values are written to (A5), 4(A5) ..., but when a function
reserves variable space on the stack via   LINK A5,#nnnn   the values that
get overwritten are the saved value of A5 (for one register variable with
an initial value) and the return address of the subroutine call (with a
second register variable).

source:

test()
{
   register int i = 1234;
   register int j = 5678;
   int k; /* so the function does a LINK A5,#nnnn
             if this line isn't here, A5 could be pointing
             anywhere, so you don't always see what gets trashed */
}

generates the following assembly:
LINK     A5,#FFF4
CMPA.L   __base(A4),A7   \ stack
BCS.W    __xcovf         /      overflow checking
MOVEM.L  D6-D7,-(A7)
MOVE.L   #4D2,D7
LEA      0(A5),A0        \  this code shouldn't be here!
MOVE.L   D7,(A0)         /      (overwrites saved value of A5)
MOVE.L   #162E,D6
LEA      4(A5),A1        \  ditto for this!
MOVE.L   D6,(A1)         /      (overwrites subroutine call return address)
MOVEM.L  (A7)+,D6-D7
UNLK     A5
RTS

It seems that the compiler doesn't take into account the fact that the variable
is a register variable when handling the initial value of a declared variable.
I say this because the code is similar to what is done when the variable is
*not* a register variable (except the offset from A5 is negative to put
it into the space reserved by the LINK instruction.)

As I'm writing this, I just noticed that the LINK instruction also seems
to be reserving space for all three variables, it should only need to be
LINK A5,#-4 instead of -12.

Hope this helps someone avoid the frustration I've been through.
-Brian

P.S. Why do they do
         LEA      n(A5),A0
         MOVE.L   D7,(A0)
     instead of just
         MOVE.L   D7,n(A5)