On Wed, Sep 2, 2026 at 7:59 AM <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> For unsigned X, ~X is MAX - X. MIN (~X, Y) limits the addend to the largest
> value that cannot overflow. X + MIN (~X, Y) is therefore saturating addition.
>
> Recognize this form. Require the MIN to have one use so that the replacement
> removes it.
>
> AArch64 -O2:
>
> before:
>
> mvn w2, w0
> cmp w2, w1
> csel w2, w2, w1, ls
> add w0, w2, w0
>
> after:
>
> adds w0, w0, w1
> csinv w0, w0, wzr, cc
>
> Four instructions become two.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
>
> Ok for trunk?
>
> gcc/ChangeLog:
>
> * match-sat-alu.pd (unsigned_integer_sat_add): Add the
> X + MIN (~X, Y) form.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.target/aarch64/sat_u_add_min_not-1.c: New test.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
> gcc/match-sat-alu.pd | 7 +++++++
> .../gcc.target/aarch64/sat_u_add_min_not-1.c | 18 ++++++++++++++++++
> 2 files changed, 25 insertions(+)
> create mode 100644 gcc/testsuite/gcc.target/aarch64/sat_u_add_min_not-1.c
>
> diff --git a/gcc/match-sat-alu.pd b/gcc/match-sat-alu.pd
> index 9564366d32f..b01989c3a1c 100644
> --- a/gcc/match-sat-alu.pd
> +++ b/gcc/match-sat-alu.pd
> @@ -68,6 +68,13 @@ along with GCC; see the file COPYING3. If not see
> wide_int sum = wi::add (cst_1, cst_2);
> }
> (if (wi::eq_p (max, sum))))))
> + (match (unsigned_integer_sat_add @0 @1)
> + /* SAT_U_ADD = X + MIN (~X, Y). ~X is MAX - X, so the MIN caps Y at the
> + largest addend that does not overflow. The MIN has to be single use:
> + while it stays live the saturating add is computed beside it instead of
> + replacing it, which costs an instruction. */
> + (plus:c (min:c@2 (bit_not @0) @1) @0)
> + (if (single_use (@2))))
Does using :cs on min work instead of the single_use? It might be
useful to add a testcase where the min is used twice.
Otherwise this is ok.
On a follow up maybe you can recognize `~X + MIN(X, Y)` as sat u add for `~X+Y`
> (match (unsigned_integer_sat_add @0 @1)
> /* SUM = ADD_OVERFLOW (X, Y)
> SAT_U_ADD = REALPART (SUM) | -IMAGPART (SUM) */
> diff --git a/gcc/testsuite/gcc.target/aarch64/sat_u_add_min_not-1.c
> b/gcc/testsuite/gcc.target/aarch64/sat_u_add_min_not-1.c
> new file mode 100644
> index 00000000000..58ed36897ab
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sat_u_add_min_not-1.c
> @@ -0,0 +1,18 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +typedef unsigned char u8;
> +typedef unsigned short u16;
> +typedef unsigned int u32;
> +typedef unsigned long long u64;
> +
> +#define DEF(T) \
> + T f_##T (T a, T b) { T t = ~a; return a + (b < t ? b : t); } \
> + T g_##T (T a, T b) { T t = ~a; return (t < b ? t : b) + a; }
> +
> +DEF (u8)
> +DEF (u16)
> +DEF (u32)
> +DEF (u64)
> +
> +/* { dg-final { scan-tree-dump-times "\\.SAT_ADD " 8 "optimized" } } */
> --
> 2.50.1 (Apple Git-155)
>
