[PATCH 1/2] match.pd: recognise A > B ? A - B : B - A as abs (A - B) [PR50856]

The absolute-difference idiom is already folded to abs (A - B) when the
comparison is written against the subtraction itself, e.g.
(A - B) > 0 ? (A - B) : (B - A).  The very common alternative spelling that
compares the operands directly, A > B ? A - B : B - A, was not recognised, so
phiopt kept two independent subtractions which the backend if-converted into a
compare plus two predicated subtracts (three instructions on Arm) instead of
the two-instruction "subs; rsb<cc>" abs sequence.

For a signed, non-wrapping integer type A > B is equivalent to A - B > 0, so the
idiom (and its >=, < and <= variants) folds to [-]abs (A - B).  Add the two
simplifications next to the existing (A - B) cmp 0 family, guarded on
!TYPE_OVERFLOW_WRAPS so that -fwrapv does not trigger the fold.

As a two-patch series with the following widened-operand extension,
bootstrapped on x86_64-pc-linux-gnu with the stage2/stage3 comparison
successful, and regtested there with gcc.dg/dg.exp and
gcc.dg/tree-ssa/tree-ssa.exp: no unexpected results.

Assisted-by: Claude Opus 5 (Anthropic)

	PR tree-optimization/50856

gcc/ChangeLog:

	* match.pd: Fold A > B ? A - B : B - A (and the >=, < and <=
	spellings) into [-]abs (A - B) for signed non-wrapping integer
	types.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/pr50856.c: New test.
	* gcc.dg/tree-ssa/absdiff-1.c: New test.

Signed-off-by: Dominic P <[email protected]>
---
 gcc/match.pd                              | 26 +++++++++++++++++
 gcc/testsuite/gcc.dg/tree-ssa/absdiff-1.c | 12 ++++++++
 gcc/testsuite/gcc.target/arm/pr50856.c    | 35 +++++++++++++++++++++++
 3 files changed, 73 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/absdiff-1.c
 create mode 100644 gcc/testsuite/gcc.target/arm/pr50856.c

diff --git a/gcc/match.pd b/gcc/match.pd
index 18d295ad4cb..04ddb82c76e 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -7601,6 +7601,32 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
       (convert (negate (absu:utype @0))))
       (negate (abs @0)))))
  )
+ /* A > B ? A - B : B - A   and   A >= B ? A - B : B - A
+    same as abs (A - B) for signed non-wrapping types, where the
+    no-overflow assumption gives A > B <=> A - B > 0.  (PR50856)  */
+ (for cmp (gt ge)
+  (simplify
+   (cnd (cmp @1 @2) (minus@0 @1 @2) (minus @2 @1))
+   (if (!HONOR_SIGNED_ZEROS (type)
+	&& !TYPE_UNSIGNED (type)
+	&& ANY_INTEGRAL_TYPE_P (type)
+	&& !TYPE_OVERFLOW_WRAPS (type)
+	&& (!VECTOR_TYPE_P (type)
+	    || target_supports_op_p (type, ABS_EXPR, optab_vector)))
+    (abs @0))))
+ /* A < B ? A - B : B - A   and   A <= B ? A - B : B - A
+    same as -abs (A - B) for signed non-wrapping types.  (PR50856)  */
+ (for cmp (lt le)
+  (simplify
+   (cnd (cmp @1 @2) (minus@0 @1 @2) (minus @2 @1))
+   (if (!HONOR_SIGNED_ZEROS (type)
+	&& !TYPE_UNSIGNED (type)
+	&& ANY_INTEGRAL_TYPE_P (type)
+	&& !TYPE_OVERFLOW_WRAPS (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)))))
 )
 
 /* X >=/> 0 ? Y + X : Y - X
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/absdiff-1.c b/gcc/testsuite/gcc.dg/tree-ssa/absdiff-1.c
new file mode 100644
index 00000000000..4d59b93429b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/absdiff-1.c
@@ -0,0 +1,12 @@
+/* A > B ? A - B : B - A folds to abs (A - B) for signed non-wrapping
+   operands, in all four comparison directions.  PR50856.  */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+int ad_gt (int a, int b) { return a > b ? a - b : b - a; }
+int ad_ge (int a, int b) { return a >= b ? a - b : b - a; }
+long ad_lt (long a, long b) { return a < b ? a - b : b - a; }
+int ad_le (int a, int b) { return a <= b ? a - b : b - a; }
+
+/* { dg-final { scan-tree-dump-times "ABS_EXPR" 4 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "= -" 2 "optimized" } } */
diff --git a/gcc/testsuite/gcc.target/arm/pr50856.c b/gcc/testsuite/gcc.target/arm/pr50856.c
new file mode 100644
index 00000000000..adae32b09ea
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr50856.c
@@ -0,0 +1,35 @@
+/* PR tree-optimization/50856: the signed absolute-difference idiom
+   a > b ? a - b : b - a  (and the >=, and the mirror < / <= giving
+   -abs) should be recognised as ABS_EXPR (a - b) so the ARM backend
+   emits the 2-instruction "subs; rsb<cc>" sequence instead of a
+   compare followed by two predicated subtracts.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O2 -marm" } */
+/* { dg-require-effective-target arm32 } */
+/* { dg-final { scan-tree-dump-times "ABS_EXPR" 3 "optimized" } } */
+/* { dg-additional-options "-fdump-tree-optimized" } */
+
+int
+absdiff_gt (int a, int b)
+{
+  return a > b ? a - b : b - a;
+}
+
+int
+absdiff_ge (int a, int b)
+{
+  return a >= b ? a - b : b - a;
+}
+
+long
+labsdiff_gt (long a, long b)
+{
+  return a > b ? a - b : b - a;
+}
+
+/* The abs sequence is "subs rN, ..; rsb<cc> rN, rN, #0"; make sure we
+   no longer emit the compare + two predicated subtracts form.  */
+/* { dg-final { scan-assembler-not "subgt" } } */
+/* { dg-final { scan-assembler-not "suble" } } */
+/* { dg-final { scan-assembler "rsb" } } */
-- 
2.55.0