On Wed, Sep 2, 2026 at 7:54 AM <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> X - MIN (X, Y) is unsigned saturating subtraction, but subtraction
> statements were not checked for this form.
>
> Recognize the form and check MINUS_EXPR statements for saturation. Require
> the MIN to have one use so that the replacement removes it. If a replacement
> changes the current statement, do not try to widen that statement.
>
> AArch64 -O2:
>
> before:
>
> cmp w1, w0
> csel w1, w1, w0, ls
> sub w0, w0, w1
>
> after:
>
> subs w0, w0, w1
> csel w0, w0, wzr, cs
>
> The vector form becomes one UQSUB instead of a comparison, a select, and a
> subtraction.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
>
> Ok for trunk?
>
> gcc/ChangeLog:
>
> * match-sat-alu.pd (unsigned_integer_sat_sub): Add the
> X - MIN (X, Y) form.
> * tree-ssa-math-opts.cc
> (math_opts_dom_walker::before_dom_children): Call
> match_unsigned_saturation_sub for MINUS_EXPR as well, and
> only widen while the statement is unchanged.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.target/aarch64/sat_u_sub_minmax-1.c: New test.
> * gcc.target/aarch64/sat_u_sub_minmax-2.c: New test.
> * gcc.target/aarch64/sat_u_sub_minmax-3.c: New test.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
> gcc/match-sat-alu.pd | 6 ++++
> .../gcc.target/aarch64/sat_u_sub_minmax-1.c | 18 +++++++++++
> .../gcc.target/aarch64/sat_u_sub_minmax-2.c | 32 +++++++++++++++++++
> .../gcc.target/aarch64/sat_u_sub_minmax-3.c | 27 ++++++++++++++++
> gcc/tree-ssa-math-opts.cc | 5 +--
> 5 files changed, 86 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-1.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-2.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-3.c
>
> diff --git a/gcc/match-sat-alu.pd b/gcc/match-sat-alu.pd
> index 7156529f589..1b35b35cff8 100644
> --- a/gcc/match-sat-alu.pd
> +++ b/gcc/match-sat-alu.pd
> @@ -131,6 +131,12 @@ along with GCC; see the file COPYING3. If not see
> /* SAT_U_SUB = (X - Y) * (X >= Y) */
> (mult:c (minus @0 @1) (convert (ge @0 @1)))
> (if (types_match (type, @0, @1))))
> + (match (unsigned_integer_sat_sub @0 @1)
> + /* SAT_U_SUB = X - MIN (X, Y). The MIN has to be single use: while it
> + stays live the saturating subtract is computed beside it instead of
> + replacing it, which costs an instruction. */
> + (minus @0 (min:c@2 @0 @1))
> + (if (single_use (@2))))
:s on the min.
So `(minus @0 (min:cs @0 @1))`
> (match (unsigned_integer_sat_sub @0 @1)
> /* DIFF = SUB_OVERFLOW (X, Y)
> SAT_U_SUB = REALPART (DIFF) | (IMAGPART (DIFF) + (-1)) */
> diff --git a/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-1.c
> b/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-1.c
> new file mode 100644
> index 00000000000..aa1d7b6f031
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-1.c
> @@ -0,0 +1,18 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +#define DEF_MIN(T) \
> + T min_##T (T a, T b) { return a - (a < b ? a : b); } \
> + T nim_##T (T a, T b) { return a - (b < a ? b : a); }
> +
> +typedef unsigned char u8;
> +typedef unsigned short u16;
> +typedef unsigned int u32;
> +typedef unsigned long long u64;
> +
> +DEF_MIN (u8)
> +DEF_MIN (u16)
> +DEF_MIN (u32)
> +DEF_MIN (u64)
> +
> +/* { dg-final { scan-tree-dump-times "\\.SAT_SUB " 8 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-2.c
> b/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-2.c
> new file mode 100644
> index 00000000000..dcf8ab46309
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-2.c
> @@ -0,0 +1,32 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -fdump-tree-optimized" } */
> +
> +typedef unsigned char u8;
> +typedef unsigned int u32;
> +
> +/* The recognition must reach the vectoriser, so that the loop becomes a
> + single uqsub rather than a compare, a select and a subtraction. */
> +
> +void
> +min_loop (u8 *__restrict d, u8 *__restrict a, u8 *__restrict b, int n)
> +{
> + for (int i = 0; i < n; i++)
> + {
> + u8 x = a[i], y = b[i];
> + d[i] = x - (x < y ? x : y);
> + }
> +}
> +
> +void
> +min_loop32 (u32 *__restrict d, u32 *__restrict a, u32 *__restrict b, int n)
> +{
> + for (int i = 0; i < n; i++)
> + {
> + u32 x = a[i], y = b[i];
> + d[i] = x - (y < x ? y : x);
> + }
> +}
> +
> +/* { dg-final { scan-tree-dump "\\.SAT_SUB " "optimized" } } */
> +/* { dg-final { scan-assembler "uqsub\tv\[0-9\]+\\.16b" } } */
> +/* { dg-final { scan-assembler "uqsub\tv\[0-9\]+\\.4s" } } */
> diff --git a/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-3.c
> b/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-3.c
> new file mode 100644
> index 00000000000..029c136b7d0
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sat_u_sub_minmax-3.c
> @@ -0,0 +1,27 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +typedef unsigned int u32;
> +
> +/* While the MIN stays live the saturating subtract would be computed beside
> + it rather than instead of it, so the rules do not fire. */
> +
> +u32
> +min_live (u32 a, u32 b, u32 *o)
> +{
> + u32 m = a < b ? a : b;
> + *o = m;
> + return a - m;
> +}
> +
> +u32
> +add_min_live (u32 a, u32 b, u32 *o)
> +{
> + u32 t = ~a;
> + u32 m = b < t ? b : t;
> + *o = m;
> + return a + m;
> +}
> +
> +/* { dg-final { scan-tree-dump-not "\\.SAT_SUB " "optimized" } } */
> +/* { dg-final { scan-tree-dump-not "\\.SAT_ADD " "optimized" } } */
> diff --git a/gcc/tree-ssa-math-opts.cc b/gcc/tree-ssa-math-opts.cc
> index b371b5b7cff..ed3abb7d5c4 100644
> --- a/gcc/tree-ssa-math-opts.cc
> +++ b/gcc/tree-ssa-math-opts.cc
> @@ -7332,10 +7332,11 @@ math_opts_dom_walker::after_dom_children (basic_block
> bb)
>
> case PLUS_EXPR:
> match_saturation_add_with_assign (&gsi, as_a<gassign *> (stmt));
> - match_unsigned_saturation_sub (&gsi, as_a<gassign *> (stmt));
> /* fall-through */
> case MINUS_EXPR:
> - if (!convert_plusminus_to_widen (&gsi, stmt, code))
> + match_unsigned_saturation_sub (&gsi, as_a<gassign *> (stmt));
> + if (gsi_stmt (gsi) == stmt
> + && !convert_plusminus_to_widen (&gsi, stmt, code))
Instead of `gsi_stmt (gsi) == stmt` instead return true from
match_unsigned_saturation_sub if something was done.
And do:
if (!match_unsigned_saturation_sub (&gsi, as_a<gassign *> (stmt))
&& !convert_plusminus_to_widen (&gsi, stmt, code))
{
...
> {
> match_arith_overflow (&gsi, stmt, code, m_cfg_changed_p);
> if (gsi_stmt (gsi) == stmt)
> --
> 2.50.1 (Apple Git-155)
>
