Steven,

this patch fixes:
- PR62004 (the if-conversion pass part, the tail-merge part is still todo), and
- PR62030.

In both cases, a valid program with a dead type-unsafe access is transformed by 
the if-conversion pass into an invalid program with a live type-unsafe access.

The transformation done by the if-conversion pass that suffers from this problem 
is if-merging, replacing the if-then-else with the if-block or the then then-block.

The patch fixes this problem by detecting when the if-block and the then-block 
are treated differently by alias analysis, and preventing the optimization in 
that case.

This part of the patch fixes PR62004.
...
@@ -2583,7 +2631,7 @@ noce_process_if_block (struct noce_if_info *if_info)

    /* Look and see if A and B are really the same.  Avoid creating silly
       cmove constructs that no one will fix up later.  */
-  if (rtx_equal_p (a, b))
+  if (rtx_interchangeable_p (a, b))
      {
        /* If we have an INSN_B, we don't have to create any new rtl.  Just
	 move the instruction that we already have.  If we don't have an
...

This part of the patch fixes PR62030:
...
@@ -2517,7 +2565,7 @@ noce_process_if_block (struct noce_if_info *if_info)
	  || BLOCK_FOR_INSN (insn_b) != BLOCK_FOR_INSN (if_info->cond_earliest)
	  || !NONJUMP_INSN_P (insn_b)
	  || (set_b = single_set (insn_b)) == NULL_RTX
-         || ! rtx_equal_p (x, SET_DEST (set_b))
+         || ! rtx_interchangeable_p (x, SET_DEST (set_b))
	  || ! noce_operand_ok (SET_SRC (set_b))
	  || reg_overlap_mentioned_p (x, SET_SRC (set_b))
	  || modified_between_p (SET_SRC (set_b), insn_b, jump)
...

I've added the other fixes after review of the if-conversion pass for the same 
problem, I hope this is complete (well, at least for the if-conversion pass. I 
wonder if cross-jumping suffers from the same problem).

The PR62030 test-case fails with trunk on MIPS. The PR62004 testcase fails with 
4.8 on x86_64. But I think the problem exists in trunk, 4.9 and 4.8, it's just 
hard to trigger.

Bootstrapped and reg-tested on x86_64, trunk. No issue found other than 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62060 , which looks like a 
test-case issue.

OK for trunk, 4.9, 4.8?

Thanks,
- Tom