This patch updates assign_parm_find_data_types to assign passed_mode and 
nominal_mode with the mode of the built pointer type instead of the 
hard-coded Pmode in the case of pass-by-reference.  This is in line with 
the assignment to passed_mode and nominal_mode in other cases inside the 
function.

assign_parm_find_data_types generally uses TYPE_MODE to calculate 
passed_mode and nominal_mode:

   /* Find mode of arg as it is passed, and mode of arg as it should be
      during execution of this function.  */
   passed_mode = TYPE_MODE (passed_type);
   nominal_mode = TYPE_MODE (nominal_type);

this includes the case when the passed argument is a pointer by itself.

However there is a discrepancy when it deals with argument passed by 
invisible reference; it builds the argument's corresponding pointer 
type, but sets passed_mode and nominal_mode with Pmode directly.

This is OK for targets where Pmode == ptr_mode, but on AArch64 with 
ILP32 they are different with Pmode as DImode and ptr_mode as SImode. 
When such a reference is passed on stack, the reference is prepared by 
the caller in the lower 4 bytes of an 8-byte slot but is fetched by the 
callee as an 8-byte datum, of which the higher 4 bytes may contain junk. 
  It is probably the combination of Pmode != ptr_mode and the particular 
ABI specification that make the AArch64 ILP32 the first target on which 
the issue manifests itself.

Bootstrapped on x86_64-none-linux-gnu.

OK for the trunk?

Thanks,
Yufeng


gcc/
	* function.c (assign_parm_find_data_types): Set passed_mode and
	nominal_mode to the TYPE_MODE of nominal_type for the built
	pointer type in case of the struct-pass-by-reference.