[PATCH] Generate fused widening multiply-and-accumulate operations only when the widening multiply has single use

Yufeng Zhang Yufeng.Zhang@arm.com
Wed Oct 23 21:22:00 GMT 2013


Hi,

Thank you both for the reviewing.  I've updated the patch and also added 
a test (to the gcc.dg to avoid duplication).  I'll commit the patch shortly.

Thanks,
Yufeng

gcc/

         * tree-ssa-math-opts.c (convert_plusminus_to_widen): Call
         has_single_use () and not do the conversion if has_single_use ()
         returns false for the multiplication result.

gcc/testsuite/

         * gcc.dg/wmul-1.c: New test.


On 10/23/13 10:42, Richard Biener wrote:
> On Tue, Oct 22, 2013 at 12:01 AM, Yufeng Zhang<Yufeng.Zhang@arm.com>  wrote:
>> Hi,
>>
>> This patch changes the widening_mul pass to fuse the widening multiply with
>> accumulate only when the multiply has single use.  The widening_mul pass
>> currently does the conversion regardless of the number of the uses, which
>> can cause poor code-gen in cases like the following:
>>
>> typedef int ArrT [10][10];
>>
>> void
>> foo (ArrT Arr, int Idx)
>> {
>>    Arr[Idx][Idx] = 1;
>>    Arr[Idx + 10][Idx] = 2;
>> }
>>
>> On AArch64, after widening_mul, the IR is like
>>
>>    _2 = (long unsigned int) Idx_1(D);
>>    _3 = Idx_1(D) w* 40;<----
>>    _5 = Arr_4(D) + _3;
>>    *_5[Idx_1(D)] = 1;
>>    _8 = WIDEN_MULT_PLUS_EXPR<Idx_1(D), 40, 400>;<----
>>    _9 = Arr_4(D) + _8;
>>    *_9[Idx_1(D)] = 2;
>>
>> Where the arrows point, there are redundant widening multiplies.
>>
>> Bootstrap successfully on x86_64.
>>
>> The patch passes the regtest on aarch64, arm and x86_64.
>>
>> OK for the trunk?
>
>         if (!is_widening_mult_p (rhs1_stmt,&type1,&mult_rhs1,
> -&type2,&mult_rhs2))
> +&type2,&mult_rhs2)
> +  || !has_single_use (rhs1))
>
> please check has_single_use first, it's the cheaper check.
>
> Ok with that change (and possibly a testcase).
>
> Thanks,
> Richard.
>
>
>
>> Thanks,
>> Yufeng
>>
>> p.s. Note that x86_64 doesn't suffer from this issue as the corresponding
>> widening multiply accumulate op is not available on the target.
>>
>> gcc/
>>
>>          * tree-ssa-math-opts.c (convert_plusminus_to_widen): Call
>>          has_single_use () and not do the conversion if has_single_use ()
>>          returns false for the multiplication result.
>
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