Fix ICE with thunks taking scalars passed by reference

[Jakub Jelinek]

On Wed, Apr 01, 2015 at 08:32:04AM +0200, Jan Hubicka wrote:
> this patch solves ICE in the attached testcase on mingw32.  The problem is that
> on Windows API long double is passed & returned by reference and while expanidng
> the tunk tail call, we get lost because we turn the parameter into SSA name and
> later need its address to pass it further.
> 
> The patch extends hack https://gcc.gnu.org/ml/gcc-patches/2014-11/msg00423.html
> to handle not only non-registers but also registers.
> 
> Bootstrapped/regtested ppc64-linux. OK?

I think it might be better to handle thunks the same way as other tail
calls, but
--- gcc/cgraphunit.c	2015-04-03 15:32:31.000000000 +0200
+++ gcc/cgraphunit.c	2015-04-07 13:02:16.537723725 +0200
@@ -1767,7 +1767,17 @@ cgraph_node::expand_thunk (bool output_a
 
 	  /* Build return value.  */
 	  if (!DECL_BY_REFERENCE (resdecl))
-	    ret = gimple_build_return (restmp);
+	    {
+	      if (is_gimple_reg_type (restype)
+		  && aggregate_value_p (resdecl, TREE_TYPE (thunk_fndecl)))
+		{
+		  gimple stmt = gimple_build_assign (resdecl, restmp);
+		  gsi_insert_after (&bsi, stmt, GSI_NEW_STMT);
+		  ret = gimple_build_return (resdecl);
+		}
+	      else
+		ret = gimple_build_return (restmp);
+	    }
 	  else
 	    ret = gimple_build_return (resdecl);
(which generates the same GIMPLE code as say
long double func1 (long double x);

long double
func2 (long double x)
{
  return func1 (x);
}
on both mingw and x86_64-linux) doesn't really work, because calls.c treats
calls in thunks differently from other calls - only in the thunks it takes
ADDR_EXPR of the parameter/result.  Thus your patch is ok
for trunk, but please add the testcase into the testsuite.

> 	PR ipa/65540
> 	* calls.c (initialize_argument_information): When producing tail
> 	call also turn SSA_NAMES passed by references to original PARM_DECLs
> Index: calls.c
> ===================================================================
> --- calls.c	(revision 221805)
> +++ calls.c	(working copy)
> @@ -1321,6 +1321,15 @@ initialize_argument_information (int num
>  		  && TREE_CODE (base) != SSA_NAME
>  		  && (!DECL_P (base) || MEM_P (DECL_RTL (base)))))
>  	    {
> +	      /* We may have turned the parameter value into an SSA name.
> +		 Go back to the original parameter so we can take the
> +		 address.  */
> +	      if (TREE_CODE (args[i].tree_value) == SSA_NAME)
> +		{
> +		  gcc_assert (SSA_NAME_IS_DEFAULT_DEF (args[i].tree_value));
> +		  args[i].tree_value = SSA_NAME_VAR (args[i].tree_value);
> +		  gcc_assert (TREE_CODE (args[i].tree_value) == PARM_DECL);
> +		}
>  	      /* Argument setup code may have copied the value to register.  We
>  		 revert that optimization now because the tail call code must
>  		 use the original location.  */

	Jakub