Hi All,

For IEEE 754 floating point formats we can replace a sequence of alternative
+/- with fneg of a wider type followed by an fadd.  This eliminated the need for
using a permutation.  This patch adds a math.pd rule to recognize and do this
rewriting.

For

void f (float *restrict a, float *restrict b, float *res, int n)
{
   for (int i = 0; i < (n & -4); i+=2)
    {
      res[i+0] = a[i+0] + b[i+0];
      res[i+1] = a[i+1] - b[i+1];
    }
}

we generate:

.L3:
        ldr     q1, [x1, x3]
        ldr     q0, [x0, x3]
        fneg    v1.2d, v1.2d
        fadd    v0.4s, v0.4s, v1.4s
        str     q0, [x2, x3]
        add     x3, x3, 16
        cmp     x3, x4
        bne     .L3

now instead of:

.L3:
        ldr     q1, [x0, x3]
        ldr     q2, [x1, x3]
        fadd    v0.4s, v1.4s, v2.4s
        fsub    v1.4s, v1.4s, v2.4s
        tbl     v0.16b, {v0.16b - v1.16b}, v3.16b
        str     q0, [x2, x3]
        add     x3, x3, 16
        cmp     x3, x4
        bne     .L3

Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.

Thanks to George Steed for the idea.

Ok for master?

Thanks,
Tamar

gcc/ChangeLog:

	* match.pd: Add fneg/fadd rule.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/simd/addsub_1.c: New test.
	* gcc.target/aarch64/sve/addsub_1.c: New test.

--- inline copy of patch -- 
diff --git a/gcc/match.pd b/gcc/match.pd
index 51b0a1b562409af535e53828a10c30b8a3e1ae2e..af1c98d4a2831f38258d6fc1bbe811c8ee6c7c6e 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -7612,6 +7612,49 @@ and,
   (simplify (reduc (op @0 VECTOR_CST@1))
     (op (reduc:type @0) (reduc:type @1))))
 
+/* Simplify vector floating point operations of alternating sub/add pairs
+   into using an fneg of a wider element type followed by a normal add.
+   under IEEE 754 the fneg of the wider type will negate every even entry
+   and when doing an add we get a sub of the even and add of every odd
+   elements.  */
+(simplify
+ (vec_perm (plus:c @0 @1) (minus @0 @1) VECTOR_CST@2)
+ (if (!VECTOR_INTEGER_TYPE_P (type) && !BYTES_BIG_ENDIAN)
+  (with
+   {
+     /* Build a vector of integers from the tree mask.  */
+     vec_perm_builder builder;
+     if (!tree_to_vec_perm_builder (&builder, @2))
+       return NULL_TREE;
+
+     /* Create a vec_perm_indices for the integer vector.  */
+     poly_uint64 nelts = TYPE_VECTOR_SUBPARTS (type);
+     vec_perm_indices sel (builder, 2, nelts);
+   }
+   (if (sel.series_p (0, 2, 0, 2))
+    (with
+     {
+       machine_mode vec_mode = TYPE_MODE (type);
+       auto elem_mode = GET_MODE_INNER (vec_mode);
+       auto nunits = exact_div (GET_MODE_NUNITS (vec_mode), 2);
+       tree stype;
+       switch (elem_mode)
+	 {
+	 case E_HFmode:
+	   stype = float_type_node;
+	   break;
+	 case E_SFmode:
+	   stype = double_type_node;
+	   break;
+	 default:
+	   return NULL_TREE;
+	 }
+       tree ntype = build_vector_type (stype, nunits);
+       if (!ntype)
+	 return NULL_TREE;
+     }
+     (plus (view_convert:type (negate (view_convert:ntype @1))) @0))))))
+
 (simplify
  (vec_perm @0 @1 VECTOR_CST@2)
  (with
diff --git a/gcc/testsuite/gcc.target/aarch64/simd/addsub_1.c b/gcc/testsuite/gcc.target/aarch64/simd/addsub_1.c
new file mode 100644
index 0000000000000000000000000000000000000000..1fb91a34c421bbd2894faa0dbbf1b47ad43310c4
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/simd/addsub_1.c
@@ -0,0 +1,56 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target arm_v8_2a_fp16_neon_ok } */
+/* { dg-options "-Ofast" } */
+/* { dg-add-options arm_v8_2a_fp16_neon } */
+/* { dg-final { check-function-bodies "**" "" "" { target { le } } } } */
+
+#pragma GCC target "+nosve"
+
+/* 
+** f1:
+** ...
+**	fneg	v[0-9]+.2d, v[0-9]+.2d
+**	fadd	v[0-9]+.4s, v[0-9]+.4s, v[0-9]+.4s
+** ...
+*/
+void f1 (float *restrict a, float *restrict b, float *res, int n)
+{
+   for (int i = 0; i < (n & -4); i+=2)
+    {
+      res[i+0] = a[i+0] + b[i+0];
+      res[i+1] = a[i+1] - b[i+1];
+    }
+}
+
+/* 
+** d1:
+** ...
+** 	fneg	v[0-9]+.4s, v[0-9]+.4s
+** 	fadd	v[0-9]+.8h, v[0-9]+.8h, v[0-9]+.8h
+** ...
+*/
+void d1 (_Float16 *restrict a, _Float16 *restrict b, _Float16 *res, int n)
+{
+   for (int i = 0; i < (n & -8); i+=2)
+    {
+      res[i+0] = a[i+0] + b[i+0];
+      res[i+1] = a[i+1] - b[i+1];
+    }
+}
+
+/* 
+** e1:
+** ...
+** 	fadd	v[0-9]+.2d, v[0-9]+.2d, v[0-9]+.2d
+** 	fsub	v[0-9]+.2d, v[0-9]+.2d, v[0-9]+.2d
+** 	ins	v[0-9]+.d\[1\], v[0-9]+.d\[1\]
+** ...
+*/
+void e1 (double *restrict a, double *restrict b, double *res, int n)
+{
+   for (int i = 0; i < (n & -4); i+=2)
+    {
+      res[i+0] = a[i+0] + b[i+0];
+      res[i+1] = a[i+1] - b[i+1];
+    }
+}
diff --git a/gcc/testsuite/gcc.target/aarch64/sve/addsub_1.c b/gcc/testsuite/gcc.target/aarch64/sve/addsub_1.c
new file mode 100644
index 0000000000000000000000000000000000000000..ea7f9d9db2c8c9a3efe5c7951a314a29b7a7a922
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/sve/addsub_1.c
@@ -0,0 +1,52 @@
+/* { dg-do compile } */
+/* { dg-options "-Ofast" } */
+/* { dg-final { check-function-bodies "**" "" "" { target { le } } } } */
+
+/*
+** f1:
+** ...
+** 	fneg	z[0-9]+.d, p[0-9]+/m, z[0-9]+.d
+** 	fadd	z[0-9]+.s, z[0-9]+.s, z[0-9]+.s
+** ...
+*/
+void f1 (float *restrict a, float *restrict b, float *res, int n)
+{
+   for (int i = 0; i < (n & -4); i+=2)
+    {
+      res[i+0] = a[i+0] + b[i+0];
+      res[i+1] = a[i+1] - b[i+1];
+    }
+}
+
+/* 
+** d1:
+** ...
+** 	fneg	z[0-9]+.s, p[0-9]+/m, z[0-9]+.s
+** 	fadd	z[0-9]+.h, z[0-9]+.h, z[0-9]+.h
+** ...
+*/ 
+void d1 (_Float16 *restrict a, _Float16 *restrict b, _Float16 *res, int n)
+{
+   for (int i = 0; i < (n & -8); i+=2)
+    {
+      res[i+0] = a[i+0] + b[i+0];
+      res[i+1] = a[i+1] - b[i+1];
+    }
+}
+
+/*
+** e1:
+** ...
+** 	fsub	z[0-9]+.d, z[0-9]+.d, z[0-9]+.d
+** 	movprfx	z[0-9]+.d, p[0-9]+/m, z[0-9]+.d
+** 	fadd	z[0-9]+.d, p[0-9]+/m, z[0-9]+.d, z[0-9]+.d
+** ...
+*/
+void e1 (double *restrict a, double *restrict b, double *res, int n)
+{
+   for (int i = 0; i < (n & -4); i+=2)
+    {
+      res[i+0] = a[i+0] + b[i+0];
+      res[i+1] = a[i+1] - b[i+1];
+    }
+}




--