IF conversion bug with CC0

[Dan <dgisselq at verizon dot net>]

Greetings!

GCC is usually so perfect, that I hate to write, but ... I think I'm
chasing down quite the bug in it and would appreciate some thought to
the following.

The code that causes the bug looks like:

if (ptr) {
  *ptr = 1;
}

This code evaluates, in the instruction set I am working with, into the
following RTL:

(set (cc0) (compare (reg 3) (reg 1)))
(set (pc) (if-then-else (eq (cc0) (const_int 0)) (label_ref 258) (pc)))
(set (mem (reg 3)) (reg 1))

(all modes are SI)

This would be all fine and dandy, up until the if_convert modifications.
if_convert() rightly decides that the store instruction is prime for a
conditional execution conversion.  Therefore, if_convert() transforms
this code into:

(cond_exec (ne (cc0) (const_int 0)) (set (mem (reg 3)) (reg 1)))

This, by itself, is a valid conversion.

The problem is that when cond_exec_process_if_block calls
merge_if_block(ce_info) to package up the changes and make them
permanent, merge_if_block then calls merge_block which deletes not only
the conditional jump statement (set (pc) ... etc.), but also the compare
that set the conditions prior to the jump statement (see
rtl_merge_blocks within cfgrtl.c).  Hence instead of the working RTL,

(set (cc0) (compare (reg 3) (reg 1)))
(cond_exec (ne (cc0) (const_int 0)) (set (mem (reg 3)) (reg 1)))

I'm left with the broken RTL:

(cond_exec (ne (cc0) (const_int 0)) (set (mem (reg 3)) (reg 1)))


Can someone tell me if I am missing something, or whether this really is
a bug in GCC?

Thanks!

Dan