Hi,

PR71778 is an ICE when you pass a non-constant argument to an intrinsic
which requires a constant.

This ICE was introduced after we rewrote some of the builtin handling for
Neon intrinsics, the issue is that after throwing an error in
arm_expand_builtin_args, we return const0_rtx to indicate the expand
has failed

      if (!(*insn_data[icode].operand[opno].predicate)
	  (op[argc], mode[argc]))
	{
	  error ("%Kargument %d must be a constant immediate",
		 exp, argc + 1);
	  return const0_rtx;
	}

At this point we're safely in to invalid code, but the mid-end continues
trying to resolve the assignment, with const0_rtx on the right-hand
side. That gets in to trouble in movv2di, which sees the constant and
tries to expand through neon_make_constant, which doesn't expect to see
a const0_rtx in the assignment (it wants a vector), and we hit a
gcc_unreachable () and take the ICE.

There are a few moving parts in the back end, so it isn't clear to me that
the fix I've come up with is 100% in the right place. AArch64 doesn't
bother with a similar construct, expanding straight to a mov with whatever
you've given it, so I don't see a right place over there.

The change is defensible, but I don't really know the ARM back end.

Bootstrapped on arm-none-linux-gnueabihf.

OK?

Thanks,
James

---
gcc/

2017-06-12  James Greenhalgh  <james.greenhalgh@arm.com>

	PR target/71778
	* config/arm/arm.c (neon_make_constant): Return const0_rtx for
	const0_rtx input.

gcc/testsuite/

2017-06-12  James Greenhalgh  <james.greenhalgh@arm.com>

	PR target/71778
	* gcc.target/arm/pr71778.c: New.