On Tue, Oct 13, 2026 at 10:32 PM Alexey Lapshin
<[email protected]> wrote:
>
> PR tree-optimization/101039 folded A CMP 0 ? A : -A after that form
> was expanded into an if, by matching it in match.pd (r12-2041). The
> true arm there is A and the false arm is -A.
>
> The swapped form still does not fold for floating point. Integers
> reach the existing pattern by inverting the comparison. A floating
> comparison that may trap on NaNs cannot be inverted: the inverse of
> < is UNGE, and UNGE does not raise invalid.
>
> Fold A </<= 0 ? -A : A to ABS (A) and A >/>= 0 ? -A : A to -ABS (A)
> when signed zeros need not be preserved. NaNs stay honored. A NaN
> selects either arm, and both arms are NaNs. Vectors are folded only
> when the target can do ABS (and NEGATE for the -ABS form).
>
> gcc:
> * match.pd (A </<= 0 ? -A : A, A >/>= 0 ? -A : A): New patterns.
> * testsuite/gcc.dg/tree-ssa/abs-5.c: New test.
> * testsuite/gcc.dg/tree-ssa/phi-opt-53.c: New test.
> ---
> gcc/match.pd | 20 +++++++++++++
> gcc/testsuite/gcc.dg/tree-ssa/abs-5.c | 24 +++++++++++++++
> gcc/testsuite/gcc.dg/tree-ssa/phi-opt-53.c | 34 ++++++++++++++++++++++
> 3 files changed, 78 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/abs-5.c
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/phi-opt-53.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 6984af44582..7bf4d9e618c 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -7858,6 +7858,26 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (convert (negate (absu:utype (convert:stype @2))))))
> (convert (negate (abs @2))))))
> )
> + /* A </<= 0 ? -A : A same as abs (A)
> + A >/>= 0 ? -A : A same as -abs (A)
> + For integers these are reached by inverting the comparison of the
> + forms above, but a floating-point comparison that may trap on NaNs
> + cannot be inverted. */
> + (for cmp (lt le)
> + (simplify
> + (cnd (cmp @0 real_zerop) (negate @0) @0)
> + (if (!HONOR_SIGNED_ZEROS (type)
> + && (!VECTOR_TYPE_P (type)
> + || target_supports_op_p (type, ABS_EXPR, optab_vector)))
> + (abs @0))))
so ABS does not trap. So this is still invalid to do for
-ftrapping-math. Because you are removing a possible trapping
comparison (lt/le) and replacing it with something which does not
trap.
> + (for cmp (gt ge)
> + (simplify
> + (cnd (cmp @0 real_zerop) (negate @0) @0)
> + (if (!HONOR_SIGNED_ZEROS (type)
> + && (!VECTOR_TYPE_P (type)
> + || (target_supports_op_p (type, ABS_EXPR, optab_vector)
> + && target_supports_op_p (type, NEGATE_EXPR, optab_vector))))
> + (negate (abs @0)))))
Likewise.
>
> /* (A - B) == 0 ? (A - B) : (B - A) same as (B - A) */
> (for cmp (eq uneq)
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/abs-5.c
> b/gcc/testsuite/gcc.dg/tree-ssa/abs-5.c
> new file mode 100644
> index 00000000000..54b2dc40def
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/abs-5.c
> @@ -0,0 +1,24 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fno-signed-zeros -fdump-tree-optimized" } */
> +
> +/* Fold, when signed zeros need not be preserved:
> + A </<= 0 ? -A : A -> ABS (A)
> + A >/>= 0 ? -A : A -> -ABS (A)
> +
> + Matched directly. With NaNs honored, < cannot be inverted: its
> + inverse is UNGE, and UNGE does not raise invalid. Both arms are
> + NaN when A is NaN. */
> +
> +float abs_lt (float a) { return a < 0 ? -a : a; }
> +float abs_le (float a) { return a <= 0 ? -a : a; }
> +float abs_if (float a) { if (a < 0) a = -a; return a; }
> +double abs_lt_d (double a) { return a < 0 ? -a : a; }
> +
> +float nabs_gt (float a) { return a > 0 ? -a : a; }
> +float nabs_ge (float a) { return a >= 0 ? -a : a; }
> +
> +/* { dg-final { scan-tree-dump-times "= ABS_EXPR" 6 "optimized" } } */
> +/* { dg-final { scan-tree-dump-not " < " "optimized" } } */
> +/* { dg-final { scan-tree-dump-not " <= " "optimized" } } */
> +/* { dg-final { scan-tree-dump-not " > " "optimized" } } */
> +/* { dg-final { scan-tree-dump-not " >= " "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/phi-opt-53.c
> b/gcc/testsuite/gcc.dg/tree-ssa/phi-opt-53.c
> new file mode 100644
> index 00000000000..7483c574c6b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/phi-opt-53.c
> @@ -0,0 +1,34 @@
> +/* { dg-options "-O2 -fno-signed-zeros -fdump-tree-phiopt" } */
> +
> +/* Swapped arms of phi-opt-24.c. A < 0 ? -A : A is abs (A).
> + A floating compare that may trap on NaNs is not inverted into the
> + A >= 0 ? A : -A form, so these folds need their own patterns. */
> +
> +float f0(float A)
> +{
> +// A < 0 ? -A : A same as abs (A)
> + if (A < 0) return -A;
> + return A;
> +}
> +float f1(float A)
> +{
> +// A <= 0 ? -A : A same as abs (A)
> + if (A <= 0) return -A;
> + return A;
> +}
> +float f2(float A)
> +{
> +// A > 0 ? -A : A same as -abs (A)
> + if (A > 0) return -A;
> + return A;
> +}
> +float f3(float A)
> +{
> +// A >= 0 ? -A : A same as -abs (A)
> + if (A >= 0) return -A;
> + return A;
> +}
> +
> +/* f2 and f3 are -abs, not allowed in early phi-opt. */
> +/* { dg-final { scan-tree-dump-times "if" 2 "phiopt1" } } */
> +/* { dg-final { scan-tree-dump-not "if" "phiopt2" } } */
> --
> 2.55.0
>
