An aliasing problem

It's great that development on clisp has picked back up.  Thanks, Sam
and Bruno, for all your work.

Fedora recently rebuilt all packages with GCC 7.  The i386 and x86_64
builds succeeded, but the 32-bit ARM build failed, like so:

(PROGN (DEFMETHOD UPDATE-INSTANCE-FOR-REDEFINED-CLASS :BEFORE ((POS
X-Y-POSITION) ADDED DELETED PLIST &KEY) (LET ((X (GETF PLIST 'X)) (Y
(GETF PLIST 'Y))) (SETF (POSITION-RHO POS) (SQRT (+ (* X X) (* Y Y)))
(POSITION-THETA POS) (ATAN Y X)))) (DEFCLASS X-Y-POSITION
(ABSTRACT-POSITION) ((RHO :INITFORM 0 :ACCESSOR POSITION-RHO) (THETA
:INITFORM 0 :ACCESSOR POSITION-THETA))) (DEFMETHOD POSITION-X ((POS
X-Y-POSITION)) (WITH-SLOTS (RHO THETA) POS (* RHO (COS THETA))))
(DEFMETHOD (SETF POSITION-X) (NEW-X (POS X-Y-POSITION)) (WITH-SLOTS
(RHO THETA) POS (LET ((Y (POSITION-Y POS))) (SETQ RHO (SQRT (+ (*
NEW-X NEW-X) (* Y Y))) THETA (ATAN Y NEW-X)) NEW-X))) (DEFMETHOD
POSITION-Y ((POS X-Y-POSITION)) (WITH-SLOTS (RHO THETA) POS (* RHO
(SIN THETA)))) (DEFMETHOD (SETF POSITION-Y) (NEW-Y (POS X-Y-POSITION))
(WITH-SLOTS (RHO THETA) POS (LET ((X (POSITION-X POS))) (SETQ RHO
(SQRT (+ (* X X) (* NEW-Y NEW-Y))) THETA (ATAN NEW-Y X)) NEW-Y)))
(LIST (TYPE-OF I) (POSITION-X I) (POSITION-Y I) (POSITION-RHO I)
(POSITION-THETA I)))
WARNING: DEFCLASS: Class X-Y-POSITION (or one of its ancestors) is being
         redefined, instances are obsolete
WARNING: Removing method
         #1=#<STANDARD-WRITER-METHOD (#2=#<BUILT-IN-CLASS T>
#3=#<STANDARD-CLASS X-Y-POSITION :VERSION 1>)>
         from an already called generic function
         #<STANDARD-GENERIC-FUNCTION (SETF POSITION-Y)>
WARNING: Removing method
         #1=#<STANDARD-WRITER-METHOD (#2=#<BUILT-IN-CLASS T>
#3=#<STANDARD-CLASS X-Y-POSITION :VERSION 1>)>
         from an already called generic function
         #<STANDARD-GENERIC-FUNCTION (SETF POSITION-X)>
WARNING: DEFCLASS: Class X-Y-POSITION (or one of its ancestors) is being
         redefined, instances are obsolete
*** - Internal error: statement in file "../src/record.d", line 1543 has been
      reached!!
      Real time: 29.498787 sec.
Run time: 28.986 sec.
Space: 35364284 Bytes
GC: 70, GC time: 0.853 sec.
Please see <http://clisp.org/impnotes/faq.html#faq-bugs> for bug
      reporting instructions.
Bye.

After fruitlessly staring at the source code for awhile, trying to
divine how this could possibly happen, I decided to stare at the
preprocessed code instead.  I made it more readable by removing some
extra parentheses, reformatting, replacing some equivalent types, and
calculating some compile-time constants:

    obj = mv_space[0];
    ((Instance)((unsigned long)obj-1UL))->tfl |= 2048UL;
    {
      break_sems.einzeln[1] = 1;
      {
        Instance ptr = (Instance)((unsigned
long)STACK[-1-(long)(2+4+2*kept_slots)]-1UL);
        ptr->tfl |= 256UL;
        ptr->inst_class_version = obj;
        break_sems.einzeln[1] = 0;
      }
    }
    if (!(((((Record)((unsigned
long)STACK[-1-(long)(2+4+2*kept_slots)]-1UL))->tfl >> 8) & 0xFF) &
1UL))
      error_notreached("../src/record.d",1543);

It dawned on me that this might be an aliasing issue.  The pointer
derived from ((unsigned long)STACK[-1-(long)(2+4+2*kept_slots)]-1UL)
is visible in this scope twice, once cast to an Instance (pointer to
an anonymous struct) and once cast to a Record (record_ *).  I tried
building with -fno-strict-aliasing and sure enough, the build
succeeded.

What do you think about changing line 1543 to:

    ASSERT(record_flags(TheInstance(STACK_(2+4+2*kept_slots))) &
instflags_forwarded_B);

, or maybe making an Instance_flags(obj) macro that expands to
record_flags(TheInstance(obj))?  That makes the two pointers have the
same type, thereby solving the aliasing issue.  I have done a test
build with this change and it does indeed fix the problem.  Regards,
-- 
Jerry James
http://www.jamezone.org/

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