On 18/11/15 09:11, Kyrill Tkachov wrote:
>
> On 17/11/15 23:11, Bernd Schmidt wrote:
>> On 11/17/2015 02:03 PM, Kyrill Tkachov wrote:
>>> +      || !reg_overlap_mentioned_p (tmp_reg, SET_SRC (PATTERN (cand->insn))))
>>>      return false;
>>
>>> Well, I think the statement we want to make is
>>> "return false from this function if the two expressions contain the same
>>> register number".
>>
>> I looked at it again and I think I'll need more time to really figure out what's going on in this pass.
>>
>> However, about the above... either I'm very confused, or the actual statement here is "return false _unless_ the two expressions contain the same register number". In the testcase, the regs in question are ax and bx, which is then 
>> rejected if the patch has been applied.
>
> I'm sorry, it is my mistake in explaining. I meant to say:
> "return false from this function unless the two expressions contain the same
>  register number"
>
>>
>> (gdb) p tmp_reg
>> (reg:SI 0 ax)
>> (gdb) p cand->insn
>> (insn 59 117 60 7 (set (reg:SI 4 si [orig:107 D.1813 ] [107])
>>         (sign_extend:SI (reg:HI 3 bx [orig:99 D.1813 ] [99])))
>>
>> And I think it would be better to strengthen that to "return false unless registers are identical". (From the above it's clear however that a plain rtx_equal_p wouldn't work since there's an extension in the operand).
>
> So the three relevant instructions are:
> I1: (set (reg:HI 0 ax)
>      (mem:HI (symbol_ref:DI ("f"))))
>
> I2: (set (reg:SI 3 bx)
>      (if_then_else:SI (eq (reg:CCZ 17 flags)
>                 (const_int 0))
>             (reg:SI 0 ax)
>             (reg:SI 3 bx)))
>
> I3: (set (reg:SI 4 si)
>      (sign_extend:SI (reg:HI 3 bx)))
>
> I1 is def_insn, I3 is cand->insn. tmp_reg is 'ax'. What we want to do is reject this transformation
> because the destination of def_insn (aka I1), that is 'ax', is not the operand of the extend operation
> in cand->insn (aka I3). As you said, rtx_equal won't work on just SET_SRC (PATTERN (cand->insn)) because
> it's an extend operation. So reg_overlap_mentioned should be appropriate.
> Hope this helps.
>
>>
>> Also, I had another look at the testcase. It uses __builtin_printf and dg-output, which is at least unusual. Try to use the normal "if (cond) abort ()".
>>
>
> I did not try to remove the __builtin_printf because the use of 'f' there is needed to trigger this.
> There are at least two other dups of this PR: 68328 and 68185 the testcases for which have an "if (cond) abort ();" form.
> I'll use them instead.
>

For completeness' sake here's the patch I'm proposing.
I've used the testcases from the other two PRs that exhibit the same problem and use
the if (cond) abort (); form.

Kyrill

2015-11-16  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR rtl-optimization/68194
     PR rtl-optimization/68328
     PR rtl-optimization/68185
     * ree.c (combine_reaching_defs): Reject copy_needed case if defining
     insn does not feed directly into candidate extension insn.

2015-11-16  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR rtl-optimization/68194
     * gcc.c-torture/execute/pr68185.c: Likewise.
     * gcc.c-torture/execute/pr68328.c: Likewise.

>
>>
>> Bernd
>>
>