[PATCH][simplify-rtx] PR 65235: Calculate element size correctly when simplifying (vec_select (vec_concat (const_int) (...)) [...])

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

Hi all,

This patch fixes PR rtl-optimization 65235.
As mentioned in bugzilla:
Combine tries to combine:
 (insn 72 71 73 2 (set (reg:V2DI 117 [ D.18177 ])
         (vec_concat:V2DI (reg:DI 176 [ D.18179 ])
           (reg:DI 114 [ D.18168 ])))
      (expr_list:REG_DEAD (reg:DI 176 [ D.18179 ])
      (expr_list:REG_DEAD (reg:DI 114 [ D.18168 ])

and

 (insn 104 102 105 2 (set (reg:DI 193 [ D.18168 ])
         (vec_select:DI (reg:V2DI 117 [ D.18177 ])
             (parallel [
                     (const_int 0 [0])
                 ])))
      (expr_list:REG_DEAD (reg:V2DI 117 [ D.18177 ])
         (nil)))

but ends up generating:
(set (reg:DI 193 [ D.18168 ])
    (reg:DI 114 [ D.18168 ]))


that is, it gets the wrong element of the vec_concat.

The problem is that in simplify-rtx it tries to get the size of the 
first element,
compare the requested offset against it and pick the second element if 
the offset is
greater than that size. If the first element is a const_int, the size is 
0, so it will
always pick the second part.

The patch fixes that by calculating the size of the first element by 
taking the size of
the outer mode and subtracting the size of the second element.

I've added an assert to make sure that the second element is not also a 
const_int, as a
vec_concat of const_ints doesn't make sense as far as I can see.

Bootstrapped and tested on aarch64-none-linux-gnu, 
arm-none-linux-gnueabihf, x86_64-linux-gnu.
This bug appears on trunk, 4.9 and 4.8, so it's not a regression on the 
release branches but it is a wrong-code bug.

Should this go in now? Or wait for stage 1?

Thanks,
Kyrill

2015-03-04  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR rtl-optimization 65235
    * simplify-rtx.c (simplify_binary_operation_1, VEC_SELECT case):
    When first element of vec_concat is const_int, calculate its size
    using second element.

2015-03-04  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR rtl-optimization 65235
    * gcc.target/aarch64/pr65235_1.c: New test.

commit 816f7592617681c48e5303aaa9e3b0ee8a858457
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date:   Thu Feb 26 16:40:52 2015 +0000

    [simplify-rtx] Calculate vector size correctly when simplifying (vec_select (vec_concat (const_int) (...)) [...])

diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index a003b41..3a6159f 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -3555,7 +3555,19 @@ simplify_binary_operation_1 (enum rtx_code code, machine_mode mode,
 	  while (GET_MODE (vec) != mode
 		 && GET_CODE (vec) == VEC_CONCAT)
 	    {
-	      HOST_WIDE_INT vec_size = GET_MODE_SIZE (GET_MODE (XEXP (vec, 0)));
+	      HOST_WIDE_INT vec_size;
+
+	      if (CONST_INT_P (XEXP (vec, 0)))
+	        {
+	          /* vec_concat of two const_ints doesn't make sense with
+	             respect to modes.  */
+	          gcc_assert (!CONST_INT_P (XEXP (vec, 1)));
+	          vec_size = GET_MODE_SIZE (GET_MODE (trueop0))
+	                     - GET_MODE_SIZE (GET_MODE (XEXP (vec, 1)));
+	        }
+	      else
+	        vec_size = GET_MODE_SIZE (GET_MODE (XEXP (vec, 0)));
+
 	      if (offset < vec_size)
 		vec = XEXP (vec, 0);
 	      else
diff --git a/gcc/testsuite/gcc.target/aarch64/pr65235_1.c b/gcc/testsuite/gcc.target/aarch64/pr65235_1.c
new file mode 100644
index 0000000..ca12cd5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr65235_1.c
@@ -0,0 +1,30 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+#include "arm_neon.h"
+
+int
+main (int argc, char** argv)
+{
+  int64x1_t val1;
+  int64x1_t val2;
+  int64x1_t val3;
+  uint64x1_t val13;
+  uint64x2_t val14;
+  uint64_t got;
+  uint64_t exp;
+  val1 = vcreate_s64(UINT64_C(0xffffffff80008000));
+  val2 = vcreate_s64(UINT64_C(0x0000f38d00000000));
+  val3 = vcreate_s64(UINT64_C(0xffff7fff0000809b));
+  /* Expect: "val13" = 8000000000001553.  */
+  val13 = vcreate_u64 (UINT64_C(0x8000000000001553));
+  /* Expect: "val14" = 0010 0000 0000 0002 0000 0000 0000 0000.  */
+  val14 = vcombine_u64(vcgt_s64(vqrshl_s64(val1, val2),
+				vshr_n_s64(val3, 18)),
+		       vshr_n_u64(val13, 11));
+  /* Should be 0000000000000000.  */
+  got = vgetq_lane_u64(val14, 0);
+  exp = 0;
+  if(exp != got)
+    __builtin_abort ();
+}