[patch] Fix PR48183, NEON ICE in emit-rtl.c:immed_double_const() under -g

[Richard Sandiford]

Chung-Lin Tang <cltang@codesourcery.com> writes:
> PR48183 is a case where ARM NEON instrinsics, under -O -g, produce debug
> insns that tries to expand OImode (32-byte integer) zero constants, much
> too large to represent as two HOST_WIDE_INTs; as the internals manual
> indicates, such large constants are not supported in general, and ICEs
> on the GET_MODE_BITSIZE(mode) == 2*HOST_BITS_PER_WIDE_INT assertion.
>
> This patch allows the cases where the large integer constant is still
> representable using a single CONST_INT, such as zero(0). Bootstrapped
> and tested on i686 and x86_64, cross-tested on ARM, all without
> regressions. Okay for trunk?
>
> Thanks,
> Chung-Lin
>
> 2011-03-20  Chung-Lin Tang  <cltang@codesourcery.com>
>
> 	* emit-rtl.c (immed_double_const): Allow wider than
> 	2*HOST_BITS_PER_WIDE_INT mode constants when they are
> 	representable as a single const_int RTX.

I realise this might be seen as a good expedient fix, but it makes
me a bit uneasy.  Not a very constructive rationale, sorry.

For this particular case, the problem is that vst2q_s32 and the
like initialise a union directly:

  union { int32x4x2_t __i; __builtin_neon_oi __o; } __bu = { __b; };

and this gets translated into a zeroing of the whole union followed
by an assignment to __i:

  __bu = {};
  __bu.__i = __b;

We later optimise away the first assignment, but it still exists
in the debug info.

Another expedient fix might be to replace these initialisations with:

  union { int32x4x2_t __i; __builtin_neon_oi __o; } __bu;
  __bu.__i = __b;

so that we never get a zeroing statement.

I realise both "fixes" are papering over the real problem.  What we really
need is arbitrary-length constant integers, like we already have for vectors.
But that's going to be a much bigger patch.  It just seems to me that,
if we're going for a work-around, the arm_neon.h change is neutral,
while changing immed_double_const feels more risky.

Richard