Re: Combiner fixes

[Richard Guenther <richard.guenther@gmail.com>]

On Tue, Aug 3, 2010 at 6:35 PM, Paolo Bonzini <bonzini@gnu.org> wrote:
> On 08/03/2010 06:08 PM, Richard Guenther wrote:
>>
>> On Tue, Aug 3, 2010 at 6:06 PM, Richard Guenther
>> <richard.guenther@gmail.com>  wrote:
>>>
>>> On Tue, Aug 3, 2010 at 6:01 PM, Bernd Schmidt<bernds@codesourcery.com>
>>>  wrote:
>>>>
>>>> On 08/03/2010 05:44 PM, Richard Guenther wrote:
>>>>>
>>>>> 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).
>>>>
>>>> Not a problem (I guess a !FLOAT_MODE_P || flag_associative_math test is
>>>> needed), but I guess I don't understand how things would break - I see
>>>> no multiply operations in these i386 functions.  Can you elaborate?
>>>
>>> It was just a general comment to re-associations of FP on RTL,
>>> but more important is that we not start doing constant folding,
>>> re-associating should be fine as long as they are valid without
>>> any fancy math flags.
>>
>> Btw, doing more elaborate re-association on RTL would need
>> carrying PAREN_EXPR support down to RTL, which is an
>> explicit re-association barrier (dropped during expansion because
>> we do not re-associate FP on RTL - sofar).
>
> Looks like a bad idea...  If someone ever introduces some world-shaking
> transform on RTL that requires reassociation he/she'll have to live with
> requiring -ffast-math or whatnot.

No, the point is that even with -ffast-math reassociating over
PAREN_EXPR is invalid.

Richard.

> Paolo
>