Possible bug in PowerPC structure passing

[Turly O'Connor]

On 4/4/2000 17:31, David Edelsohn (dje@watson.ibm.com) wrote:

>	The failure does not involve string instructions.  I actually
>tried using -mstring to see if that helped, but it did not.

This problem is connected with the notion of REG_PARM_STACK_SPACE,
which for PPC is a 32-byte area on the stack.  REG_PARM_STACK_SPACE
is the amount of stack space allocated for function arguments even
when their values are passed in registers.

When we pass a struct which is bigger than 32 bytes by value, the 
compiler generates a call to memcpy() which is supposed to copy
the struct to this area.

To stop this memcpy() libcall from furtling with the 32-byte area on 
the stack, code in store_one_arg() in calls.c marks the 32-byte area 
as being "dirty".  emit_library_call() will then save and restore 
the area in use around the call to memcpy(), which is bad news as 
the destination of the memcpy() *IS* the 32-byte area.

--turly