[patch AArch64] Do not perform a vector splat for vector initialisation if it is not useful

[James Greenhalgh <james dot greenhalgh at arm dot com>]

Hi,

In the testcase in this patch we create an SLP vector with only two
elements. Our current vector initialisation code will first duplicate
the first element to both lanes, then overwrite the top lane with a new
value.

This duplication can be clunky and wasteful.

Better would be to simply use the fact that we will always be overwriting
the remaining bits, and simply move the first element to the corrcet place
(implicitly zeroing all other bits).

This reduces the code generation for this case, and can allow more
efficient addressing modes, and other second order benefits for AArch64
code which has been vectorized to V2DI mode.

Note that the change is generic enough to catch the case for any vector
mode, but is expected to be most useful for 2x64-bit vectorization.

Unfortunately, on its own, this would cause failures in
gcc.target/aarch64/load_v2vec_lanes_1.c and
gcc.target/aarch64/store_v2vec_lanes.c , which expect to see many more
vec_merge and vec_duplicate for their simplifications to apply. To fix this,
add a special case to the AArch64 code if we are loading from two memory
addresses, and use the load_pair_lanes patterns directly.

We also need a new pattern in simplify-rtx.c:simplify_ternary_operation , to
catch:

  (vec_merge:OUTER
     (vec_duplicate:OUTER x:INNER)
     (subreg:OUTER y:INNER 0)
     (const_int N))

And simplify it to:

  (vec_concat:OUTER x:INNER y:INNER) or (vec_concat y x)

This is similar to the existing patterns which are tested in this function,
without requiring the second operand to also be a vec_duplicate.

Bootstrapped and tested on aarch64-none-linux-gnu and tested on
aarch64-none-elf.

Note that this requires https://gcc.gnu.org/ml/gcc-patches/2017-12/msg00614.html
if we don't want to ICE creating broken vector zero extends.

Are the non-AArch64 parts OK?

Thanks,
James

---
2017-12-11  James Greenhalgh  <james.greenhalgh@arm.com>

	* config/aarch64/aarch64.c (aarch64_expand_vector_init): Modify code
	generation for cases where splatting a value is not useful.
	* simplify-rtx.c (simplify_ternary_operation): Simplify vec_merge
	across a vec_duplicate and a paradoxical subreg forming a vector
	mode to a vec_concat.

2017-12-11  James Greenhalgh  <james.greenhalgh@arm.com>

	* gcc.target/aarch64/vect-slp-dup.c: New.

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 83d8607..8abb8e4 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -12105,9 +12105,51 @@ aarch64_expand_vector_init (rtx target, rtx vals)
 	    maxv = matches[i][1];
 	  }
 
-      /* Create a duplicate of the most common element.  */
-      rtx x = copy_to_mode_reg (inner_mode, XVECEXP (vals, 0, maxelement));
-      aarch64_emit_move (target, gen_vec_duplicate (mode, x));
+      /* Create a duplicate of the most common element, unless all elements
+	 are equally useless to us, in which case just immediately set the
+	 vector register using the first element.  */
+
+      if (maxv == 1)
+	{
+	  /* For vectors of two 64-bit elements, we can do even better.  */
+	  if (n_elts == 2
+	      && (inner_mode == E_DImode
+		  || inner_mode == E_DFmode))
+
+	    {
+	      rtx x0 = XVECEXP (vals, 0, 0);
+	      rtx x1 = XVECEXP (vals, 0, 1);
+	      /* Combine can pick up this case, but handling it directly
+		 here leaves clearer RTL.
+
+		 This is load_pair_lanes<mode>, and also gives us a clean-up
+		 for store_pair_lanes<mode>.  */
+	      if (memory_operand (x0, inner_mode)
+		  && memory_operand (x1, inner_mode)
+		  && !STRICT_ALIGNMENT
+		  && rtx_equal_p (XEXP (x1, 0),
+				  plus_constant (Pmode,
+						 XEXP (x0, 0),
+						 GET_MODE_SIZE (inner_mode))))
+		{
+		  rtx t;
+		  if (inner_mode == DFmode)
+		    t = gen_load_pair_lanesdf (target, x0, x1);
+		  else
+		    t = gen_load_pair_lanesdi (target, x0, x1);
+		  emit_insn (t);
+		  return;
+		}
+	    }
+	  rtx x = copy_to_mode_reg (inner_mode, XVECEXP (vals, 0, 0));
+	  aarch64_emit_move (target, lowpart_subreg (mode, x, inner_mode));
+	  maxelement = 0;
+	}
+      else
+	{
+	  rtx x = copy_to_mode_reg (inner_mode, XVECEXP (vals, 0, maxelement));
+	  aarch64_emit_move (target, gen_vec_duplicate (mode, x));
+	}
 
       /* Insert the rest.  */
       for (int i = 0; i < n_elts; i++)
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index 806c309..ed16f70 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -5785,6 +5785,36 @@ simplify_ternary_operation (enum rtx_code code, machine_mode mode,
 		return simplify_gen_binary (VEC_CONCAT, mode, newop0, newop1);
 	    }
 
+	  /* Replace:
+
+	      (vec_merge:outer (vec_duplicate:outer x:inner)
+			       (subreg:outer y:inner 0)
+			       (const_int N))
+
+	     with (vec_concat:outer x:inner y:inner) if N == 1,
+	     or (vec_concat:outer y:inner x:inner) if N == 2.
+
+	     Implicitly, this means we have a paradoxical subreg, but such
+	     a check is cheap, so make it anyway.
+
+	     Only applies for vectors of two elements.  */
+	  if (GET_CODE (op0) == VEC_DUPLICATE
+	      && GET_CODE (op1) == SUBREG
+	      && GET_MODE (op1) == GET_MODE (op0)
+	      && GET_MODE (SUBREG_REG (op1)) == GET_MODE (XEXP (op0, 0))
+	      && paradoxical_subreg_p (op1)
+	      && SUBREG_BYTE (op1) == 0
+	      && GET_MODE_NUNITS (GET_MODE (op0)) == 2
+	      && GET_MODE_NUNITS (GET_MODE (op1)) == 2
+	      && IN_RANGE (sel, 1, 2))
+	    {
+	      rtx newop0 = XEXP (op0, 0);
+	      rtx newop1 = SUBREG_REG (op1);
+	      if (sel == 2)
+		std::swap (newop0, newop1);
+	      return simplify_gen_binary (VEC_CONCAT, mode, newop0, newop1);
+	    }
+
 	  /* Replace (vec_merge (vec_duplicate x) (vec_duplicate y)
 				 (const_int n))
 	     with (vec_concat x y) or (vec_concat y x) depending on value
diff --git a/gcc/testsuite/gcc.target/aarch64/vect-slp-dup.c b/gcc/testsuite/gcc.target/aarch64/vect-slp-dup.c
new file mode 100644
index 0000000..0541e48
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/vect-slp-dup.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+
+/* { dg-options "-O3 -ftree-vectorize -fno-vect-cost-model" } */
+
+void bar (double);
+
+void
+foo (double *restrict in, double *restrict in2,
+     double *restrict out1, double *restrict out2)
+{
+  for (int i = 0; i < 1024; i++)
+    {
+      out1[i] = in[i] + 2.0 * in[i+128];
+      out1[i+1] = in[i+1] + 2.0 * in2[i];
+      bar (in[i]);
+    }
+}
+
+/* { dg-final { scan-assembler-not "dup\tv\[0-9\]+.2d, v\[0-9\]+" } } */
+