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Re: Combiner fixes
On Tue, Aug 3, 2010 at 5:28 PM, Paolo Bonzini <bonzini@gnu.org> wrote:
> On 08/03/2010 05:16 PM, Jeff Law wrote:
>>
>> On 08/03/10 09:10, Bernd Schmidt wrote:
>>>
>>> On 08/03/2010 05:00 PM, Jeff Law wrote:
>>>>
>>>> On 08/03/10 08:46, Bernd Schmidt wrote:
>>>>>
>>>>> On 08/03/2010 09:24 AM, Paolo Bonzini wrote:
>>>>>>
>>>>>> Why not go one step further and try it on all operands:
>>>>>>
>>>>>> if (GET_CODE (op0) == NEG)
>>>>>> {
>>>>>> rtx temp = simplify_unary (NEG, mode, op1, mode);
>>>>>> if (temp)
>>>>>> return simplify_gen_binary (MULT, mode, XEXP (op0, 0), temp);
>>>>>> }
>>>>
>>>> OK.
>>>
>>> Actually, do I have to limit that to integer modes?
>>
>> Good point. Probably since you're effectively reassociating which can be
>> bad for FP.
>
> "-a * b" to "a * -b" is safe even for non-fast-math, try grepping -A.*B in
> fold-const.c. ?This of course assumes that simplify_unary_operation itself
> doesn't do any invalid transformation, but that's a different problem.
It's not safe on RTL, please do not add FP reassociation there.
(config/i386/i386.c:ix86_expand_{round,trunc,...} would start
to break).
Richard.