AArch64: Implement widen_[us]sum using 2-way [US]UDOT for SVE2p1 [PR122069]

SVE2p1 adds 2-way dotproduct which we can use when we have to do a single step
widening addition.  This is useful for instance when the value to be widened
does not come from a load.  For example for

int foo2_int(unsigned short *x, unsigned short * restrict y) {
  int sum = 0;
  for (int i = 0; i < 8000; i++)
    {
      x[i] = x[i] + y[i];
      sum += x[i];
    }
  return sum;
}

we used to generate

.L12:
        ld1h    z30.h, p7/z, [x0, x2, lsl 1]
        ld1h    z29.h, p7/z, [x1, x2, lsl 1]
        add     z30.h, z30.h, z29.h
        uaddwb  z31.s, z31.s, z30.h
        uaddwt  z31.s, z31.s, z30.h
        st1h    z30.h, p7, [x0, x2, lsl 1]
        mov     x3, x2
        inch    x2
        cmp     w2, w4
        bls     .L12
        inch    x3
        uaddv   d31, p7, z31.s

but with +sve2p1

.L12:
        ld1h    z31.h, p7/z, [x0, x2, lsl 1]
        ld1h    z29.h, p7/z, [x1, x2, lsl 1]
        add     z31.h, z31.h, z29.h
        udot    z30.s, z31.h, z28.h
        st1h    z31.h, p7, [x0, x2, lsl 1]
        mov     x3, x2
        inch    x2
        cmp     w2, w4
        bls     .L12
        inch    x3
        uaddv   d30, p7, z30.s

gcc/ChangeLog:

	PR middle-end/122069
	* config/aarch64/aarch64-sve2.md
	(widen_ssum<mode><Vnarrow>3): Update.
	(widen_usum<mode><Vnarrow>3): Update.

gcc/testsuite/ChangeLog:

	PR middle-end/122069
	* gcc.target/aarch64/sve2/pr122069_3.c: New test.
	* gcc.target/aarch64/sve2/pr122069_4.c: New test.

gcc/config/aarch64/aarch64-sve2.md

@@ -2390,7 +2390,19 @@
 	   (match_dup 1)]
 	  UNSPEC_SADDWT))]
   "TARGET_SVE2"
-)
+{
+  /* Use dot product to perform double widening sum reductions by
+     changing += a into += (a * 1).  i.e. we seed the multiplication with 1.  */
+  if (TARGET_SVE2p1_OR_SME2
+      && <VNARROW>mode == VNx8HImode
+      && <MODE>mode == VNx4SImode)
+    {
+      rtx ones = force_reg (VNx8HImode, CONST1_RTX (VNx8HImode));
+      emit_insn (gen_sdot_prodvnx4sivnx8hi (operands[0], operands[1],
+					    ones, operands[2]));
+      DONE;
+    }
+})
 
 ;; Define single step widening for widen_usum using UADDWB and UADDWT
 (define_expand "widen_usum<mode><Vnarrow>3"
@@ -2405,7 +2417,17 @@
 	   (match_dup 1)]
 	  UNSPEC_UADDWT))]
   "TARGET_SVE2"
-)
+{
+  if (TARGET_SVE2p1_OR_SME2
+      && <VNARROW>mode == VNx8HImode
+      && <MODE>mode == VNx4SImode)
+    {
+      rtx ones = force_reg (VNx8HImode, CONST1_RTX (VNx8HImode));
+      emit_insn (gen_udot_prodvnx4sivnx8hi (operands[0], operands[1],
+					    ones, operands[2]));
+      DONE;
+    }
+})
 
 ;; -------------------------------------------------------------------------
 ;; ---- [INT] Long binary arithmetic

gcc/testsuite/gcc.target/aarch64/sve2/pr122069_3.c

@@ -0,0 +1,39 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -march=armv8-a+sve2p1 -mautovec-preference=sve-only --param vect-epilogues-nomask=0 -fno-schedule-insns -fno-reorder-blocks -fno-schedule-insns2 -fdump-tree-vect-details" }*/
+/* { dg-final { check-function-bodies "**" "" } } */
+
+inline char char_abs(char i) {
+  return (i < 0 ? -i : i);
+}
+
+/*
+** foo_int:
+** 	...
+** 	sub	z[0-9]+.b, z[0-9]+.b, z[0-9]+.b
+** 	udot	z[0-9]+.s, z[0-9]+.b, z[0-9]+.b
+** 	...
+*/
+int foo_int(unsigned char *x, unsigned char * restrict y) {
+  int sum = 0;
+  for (int i = 0; i < 8000; i++)
+     sum += char_abs(x[i] - y[i]);
+  return sum;
+}
+
+/* 
+** foo2_int:
+** 	...
+** 	udot	z[0-9]+.s, z[0-9]+.h, z[0-9]+.h
+** 	...
+*/
+int foo2_int(unsigned short *x, unsigned short * restrict y) {
+  int sum = 0;
+  for (int i = 0; i < 8000; i++)
+    {
+      x[i] = x[i] + y[i];
+      sum += x[i];
+    }
+  return sum;
+}
+
+/* { dg-final { scan-tree-dump-times "LOOP VECTORIZED" 2 "vect" } } */

gcc/testsuite/gcc.target/aarch64/sve2/pr122069_4.c

@@ -0,0 +1,81 @@
+/* { dg-do run } */
+/* { dg-require-effective-target aarch64_sve2p1_hw }  */
+/* { dg-options "-O3 -march=armv8-a+sve2p1 -mautovec-preference=sve-only -fdump-tree-vect-details" }*/
+
+inline char char_abs(char i) {
+  return (i < 0 ? -i : i);
+}
+
+__attribute__((noipa))
+int foo_int(unsigned char *x, unsigned char * restrict y) {
+  int sum = 0;
+  for (int i = 0; i < 100; i++)
+     sum += char_abs(x[i] - y[i]);
+  return sum;
+}
+
+__attribute__((noipa))
+int foo2_int(unsigned short *x, unsigned short * restrict y,
+	     unsigned short * restrict z) {
+  int sum = 0;
+  for (int i = 0; i < 100; i++)
+    {
+      z[i] = x[i] + y[i];
+      sum += z[i];
+    }
+  return sum;
+}
+
+__attribute__((noipa))
+int foo_int2(unsigned char *x, unsigned char * restrict y) {
+  int sum = 0;
+#pragma GCC novector
+  for (int i = 0; i < 100; i++)
+     sum += char_abs(x[i] - y[i]);
+  return sum;
+}
+
+__attribute__((noipa))
+int foo2_int2(unsigned short *x, unsigned short * restrict y,
+	      unsigned short * restrict z) {
+  int sum = 0;
+#pragma GCC novector
+  for (int i = 0; i < 100; i++)
+    {
+      z[i] = x[i] + y[i];
+      sum += z[i];
+    }
+  return sum;
+}
+
+int main ()
+{
+  unsigned short a[100];
+  unsigned short b[100];
+  unsigned short r1[100];
+  unsigned short r2[100];
+  unsigned char c[100];
+  unsigned char d[100];
+#pragma GCC novector
+  for (int i = 0; i < 100; i++)
+    {
+      a[i] = c[i] = i;
+      b[i] = d[i] = 100 - i;
+    }
+
+  if (foo_int (c, d) != foo_int2 (c, d))
+    __builtin_abort();
+
+
+  if (foo2_int (a, b, r1) != foo2_int2 (a, b, r2))
+    __builtin_abort();
+
+#pragma GCC novector
+  for (int i = 0; i < 100; i++)
+    if (r1[i] != r2[i])
+      __builtin_abort ();
+
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump-times "LOOP VECTORIZED" 2 "vect" } } */