> On 3 Sep 2026, at 08:00, Andrea Pinski <[email protected]> wrote:
>
> 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))`
The :s does not enforce single_use in a named predicate matcher.
I haven’t looked into whether that’s by design or an oversight though.
Is that something that’s implementable?
>
>> (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))
> {
> ...
Thanks, I’ll do that in the v2.
Kyrill
>
>> {
>> match_arith_overflow (&gsi, stmt, code, m_cfg_changed_p);
>> if (gsi_stmt (gsi) == stmt)
>> --
>> 2.50.1 (Apple Git-155)
