Re: [PATCH][v2] match: fold the sum of a min/max pair

On 8/5/2026 5:53 AM, [email protected] wrote:
> From: Kyrylo Tkachov <[email protected]>
>
> The minimum and the maximum of two values add up to the sum of those
> values, so subtracting one of them from the sum yields the other one.
> That identity holds in modular arithmetic.  It also holds for floating
> point when reassociation is enabled and signed zeros and traps are not
> honoured.
>
>    int f (int a, int b) { int mn = a < b ? a : b; return (a + b) - mn; }
>
> aarch64 -O2:
>
>    before                          after
>      cmp   w1, w0                    cmp   w1, w0
>      add   w2, w1, w0                csel  w0, w1, w0, ge
>      csel  w0, w1, w0, le
>      sub   w0, w2, w0
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for trunk?
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
> 	* match.pd ((x + y) - minmax (x, y)): New simplification.
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.dg/tree-ssa/minmax-sum-1.c: New test.
> 	* gcc.dg/tree-ssa/minmax-sum-fp-1.c: Likewise.
> 	* gcc.dg/tree-ssa/minmax-sum-fp-2.c: Likewise.
> 	* g++.target/aarch64/minmax-sum-1.C: Likewise.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
>   gcc/match.pd                                   | 15 +++++++++++++++
>   .../g++.target/aarch64/minmax-sum-1.C          | 17 +++++++++++++++++
>   gcc/testsuite/gcc.dg/tree-ssa/minmax-sum-1.c   | 18 ++++++++++++++++++
>   .../gcc.dg/tree-ssa/minmax-sum-fp-1.c          | 15 +++++++++++++++
>   .../gcc.dg/tree-ssa/minmax-sum-fp-2.c          | 10 ++++++++++
>   5 files changed, 75 insertions(+)
>   create mode 100644 gcc/testsuite/g++.target/aarch64/minmax-sum-1.C
>   create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/minmax-sum-1.c
>   create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/minmax-sum-fp-1.c
>   create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/minmax-sum-fp-2.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 728fa8a8d63..35cc995ebf9 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -2051,6 +2051,21 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>          && !TYPE_SATURATING (type))
>      (res @0 @1))))
>   
> +/* (x + y) - min (x, y) -> max (x, y)
> +   (x + y) - max (x, y) -> min (x, y)
> +   The sum of the minimum and the maximum is the sum of the operands.  */
> +(for minmax (min max)
> +     maxmin (max min)
> + (simplify
> +  (minus (plus @0 @1) (minmax @0 @1))
> +  (if ((ANY_INTEGRAL_TYPE_P (type)
> +	&& !TYPE_OVERFLOW_SANITIZED (type) && !TYPE_OVERFLOW_TRAPS (type))
> +	|| (FLOAT_TYPE_P (type)
> +	    && flag_associative_math
> +	    && !HONOR_SIGNED_ZEROS (type)
> +	    && !flag_trapping_math))
> +   (maxmin @0 @1))))
> +
LOL.  I was going to suggest a :c on the plus and minmax expressions, 
but it looks like that was something Andrea explicitly indicated you 
didn't need :-)



OK for the trunk.

jeffh