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


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.

Attachment: patch.txt
Description: Text document


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