Re: [PATCH] match.pd: combine a pair of vector reductions

On 8/4/2026 4:01 AM, [email protected] wrote:
> From: Kyrylo Tkachov <[email protected]>
>
> Two reductions feeding the matching scalar operation are one reduction of
> the elementwise operation:
>
>    REDUC (a) op REDUC (b)  ->  REDUC (a op b)
>
> The two sides accumulate the same multiset of lanes, just grouped
> differently, so for the associative and commutative reductions the result
> is identical.  This trades a lane-crossing reduction, which is the
> expensive part, for an elementwise operation.
>
> MIN, MAX, AND, IOR and XOR need no flag.  The sum needs one only for
> floating point, where the regrouping is a reassociation; the integer sum
> is done in the unsigned type, since the reduction wraps while the signed
> vector type has undefined overflow.
>
>    #include <arm_neon.h>
>    int f (int32x4_t a, int32x4_t b) { return vaddvq_s32 (a) + vaddvq_s32 (b); }
>
> aarch64 -O3 before:
>
> 	addv	s1, v1.4s
> 	addv	s0, v0.4s
> 	fmov	w1, s1
> 	fmov	w0, s0
> 	add	w0, w0, w1
>
> after:
>
> 	add	v0.4s, v0.4s, v1.4s
> 	addv	s0, v0.4s
> 	fmov	w0, s0
>
> A pair of maxima goes from 8 instructions to 3.  On a target without a
> horizontal reduce instruction the reduction is open coded as a chain of
> shuffles, so the saving is larger there, never smaller.
>
> For integers, preserve trapping and sanitized scalar overflow.
> For floating point, require reassociation, insignificant zero signs, and non-trapping exceptions.
>
> Restrict the reassociative alternative to floating-point types.
> Floating-point options must not license fixed-point saturation changes.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for trunk?
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
> 	* match.pd (REDUC (a) op REDUC (b)): New simplifications combining
> 	two reductions into one.
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.dg/tree-ssa/vec-reduc-pair-1.c: New test.
> 	* gcc.dg/tree-ssa/vec-reduc-pair-2.c: New test.
> 	* gcc.dg/tree-ssa/vec-reduc-pair-3.c: New test.
> 	* gcc.dg/tree-ssa/vec-reduc-pair-4.c: New test.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
Nice.  Given the often multi-cycle nature of reductions, particularly on 
vectors with many elements, this seems particularly helpful.

OK
jeff