[PATCH][ARM] PR target/71270 fix neon_valid_immediate for big-endian

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

Hi all,

In this wrong-code issue the RTL tries to load a const_vector:
(const_vector:V8QI [
        (const_int 1 [0x1])
        (const_int 0 [0])
        (const_int 1 [0x1])
        (const_int 0 [0])
        (const_int 1 [0x1])
        (const_int 0 [0])
        (const_int 1 [0x1])
        (const_int 0 [0])
    ])

into a NEON register. The logic for that is in neon_valid_immediate which does a number of tricks
to decide what value and of what size to do a VMOV on to load the correct vector immediate into the register.
It goes wrong on big-endian. On both big and little-endian this outputs:
vmov.i16    d18, #0x1

This is wrong on big-endian. The vector layout has to be such as if loaded from memory.
I've tried various approaches of fixing neon_valid_immediate to generate the correct immediate but have been unsuccessful,
resulting in regressions in various parts of the testsuite or making a big mess of the function.

Given that armeb is not a target of major concern I believe the safest route at this stage is to only allow vector constants
that will obviously work on big-endian, that is the ones that are just a single element duplicated in all lanes.

This patch fixes the execution failures:
FAIL: gfortran.dg/intrinsic_pack_1.f90
FAIL: gfortran.dg/c_f_pointer_logical.f03
FAIL: gcc.dg/torture/pr60183.c
FAIL: gcc.dg/vect/pr51581-4.c

on armeb-none-eabi.

Ok for trunk?

Thanks,
Kyrill

2017-01-06  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR target/71270
    * config/arm/arm.c (neon_valid_immediate): Reject vector constants
    in big-endian mode when they are not a single duplicated value.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 45e0a3cd11fa650f456f7382ffbbcd1c932b95eb..2beee8ebe94eeadd6fb1d5b119e3b474e1ab902a 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -11542,6 +11542,12 @@ neon_valid_immediate (rtx op, machine_mode mode, int inverse,
 	return 18;
     }
 
+  /* The tricks done in the code below apply for little-endian vector layout.
+     For big-endian vectors only allow vectors of the form { a, a, a..., a }.
+     FIXME: Implement logic for big-endian vectors.  */
+  if (BYTES_BIG_ENDIAN && vector && !const_vec_duplicate_p (op))
+    return -1;
+
   /* Splat vector constant out into a byte vector.  */
   for (i = 0; i < n_elts; i++)
     {