On Thu, Jul 7, 2011 at 2:28 PM, Richard Guenther
<richard.guent...@gmail.com> wrote:
> On Thu, Jul 7, 2011 at 1:43 PM, Andrew Stubbs <andrew.stu...@gmail.com> wrote:
>> On 07/07/11 11:26, Andrew Stubbs wrote:
>>>
>>> On 07/07/11 10:58, Richard Guenther wrote:
>>>>
>>>> I think you should assume that series of widenings,
>>>> (int)(short)char_variable
>>>> are already combined. Thus I believe you only need to consider a single
>>>> conversion in valid_types_for_madd_p.
>>>
>>> Hmm, I'm not so sure. I'll look into it a bit further.
>>
>> OK, here's a test case that gives multiple conversions:
>>
>> long long
>> foo (long long a, signed char b, signed char c)
>> {
>> int bc = b * c;
>> return a + (short)bc;
>> }
>>
>> The dump right before the widen_mult pass gives:
>>
>> foo (long long int a, signed char b, signed char c)
>> {
>> int bc;
>> long long int D.2018;
>> short int D.2017;
>> long long int D.2016;
>> int D.2015;
>> int D.2014;
>>
>> <bb 2>:
>> D.2014_2 = (int) b_1(D);
>> D.2015_4 = (int) c_3(D);
>> bc_5 = D.2014_2 * D.2015_4;
>> D.2017_6 = (short int) bc_5;
>
> Ok, so you have a truncation that is a no-op value-wise. I would
> argue that this truncation should be removed independent on
> whether we have a widening multiply instruction or not.
>
> The technically most capable place to remove non-value-changing
> truncations (and combine them with a successive conversion)
> would be value-range propagation. Which already knows:
>
> Value ranges after VRP:
>
> b_1(D): VARYING
> D.2698_2: [-128, 127]
> c_3(D): VARYING
> D.2699_4: [-128, 127]
> bc_5: [-16256, 16384]
> D.2701_6: [-16256, 16384]
> D.2702_7: [-16256, 16384]
> a_8(D): VARYING
> D.2700_9: VARYING
>
> thus truncating bc_5 to short does not change the value.
>
> The simplification could be made when looking at the
> statement
>
>> D.2018_7 = (long long int) D.2017_6;
>
> in vrp_fold_stmt, based on the fact that this conversion
> converts from a value-preserving intermediate conversion.
> Thus the transform would replace the D.2017_6 operand
> with bc_5.
>
> So yes, the case appears - but it shouldn't ;)
>
> I'll cook up a quick patch for VRP.Like the attached. I'll finish and properly test it. Richard.
p
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