🚀 go-pugleaf

Martin D Kealey (martin@kcbbs.gen.nz) wrote:
[...]
: *Sigh*  This is not supposed to be a complete description, just an
: example.  So ok, change it to invalidate all registers which
: contain pointers to the same type as p, not just the single
: register which contains the value of p.  Also, *all* registers are
: invalidated over (non-inline) function calls, or any other time
: we're not 100% sure what they might contain.
: 
: I think now that accidental use of register aliasing is not
: possible -- one has explicitly to use p (or an alias for it) as a
: non-lvalue in the source code to cause this effect, and that means
: the program is non-conforming.

Agreed.  With a sufficiently conservative invalidation policy, the
implementation conforms.  So, would it continue to conform if a valid
program could read an invalid pointer (a pointer object at a time when
it happens to hold an invalid value)?  I think so.  The implementation
can freely assume that a pointer object holds a valid value from the
time it is assigned that value until it is invalidated, since, if the
assigned value was invalid, then the implementation is off the hook
via "undefined behavior."  It would make no difference whether that 
invalid value was generated by reading a pointer to a deleted object
or though assigning the presumably invalid value (int*)1.

: I know this is a perverse implementation that does some pretty
: fantastic peephole optimising without bothering to do any dataflow
: analysis, but that alone doesn't currently make it non-conforming.

Right!  Besides, it shows that one could possibly lose opportunities
for optimization if it were permitted to assign invalid values to
a pointer object.

Tom Payne (thp@cs.ucr.edu)
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