[Patch ARM] Fix PR71778

[James Greenhalgh <james.greenhalgh@arm.com>]

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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.


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diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index e503891..b8d59c6 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -12124,6 +12124,11 @@ neon_make_constant (rtx vals)
       if (n_const == n_elts)
 	const_vec = gen_rtx_CONST_VECTOR (mode, XVEC (vals, 0));
     }
+  else if (vals == const0_rtx)
+    /* Something invalid, perhaps from expanding an intrinsic
+       which requires a constant argument, where a variable argument
+       was passed.  */
+     return const0_rtx;
   else
     gcc_unreachable ();
 
diff --git a/gcc/testsuite/gcc.target/arm/pr71778.c b/gcc/testsuite/gcc.target/arm/pr71778.c
new file mode 100644
index 0000000..d5b0d04
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr71778.c
@@ -0,0 +1,24 @@
+/* { dg-do compile }  */
+/* { dg-require-effective-target arm_neon_ok } */
+/* { dg-options "-O2" } */
+/* { dg-add-options arm_neon } */
+
+typedef __simd128_int32_t int32x4_t;
+
+__extension__ extern __inline int32x4_t
+__attribute__  ((__always_inline__, __gnu_inline__, __artificial__))
+vshrq_n_s32 (int32x4_t __a, const int __b)
+{
+  /* Errors for arm_neon.h intrinsics using constants end up on the line
+     in arm_neon.h rather than the source file line.  That means we
+     need to put the dg-error up here, rather than on line 22 where we'd
+     like it.  */
+  return (int32x4_t)__builtin_neon_vshrs_nv4si (__a, __b); /* { dg-error "argument 2 must be a constant immediate" } */
+}
+
+int32x4_t
+shift (int32x4_t a, int b)
+{
+  return vshrq_n_s32 (a, b);
+}
+