[Bug middle-end/84877] New: Local stack copy of BLKmode parameter on the stack is not aligned when the requested alignment exceeds MAX_SUPPORTED_STACK_ALIGNMENT

["renlin at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84877

            Bug ID: 84877
           Summary: Local stack copy of BLKmode parameter on the stack is
                    not aligned when the requested alignment exceeds
                    MAX_SUPPORTED_STACK_ALIGNMENT
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: renlin at gcc dot gnu.org
  Target Milestone: ---

For a test case like this,

#include <stdint.h>
struct U {
    uint32_t M0;
    uint32_t M1;
} __attribute((aligned(16)));

void tmp (struct U *);
void foo(struct U P0)
{
  struct U P1 = P0;
  tmp (&P1);
}

void bar(struct U P0)
{
  tmp (&P0);
}

The required alignment of a BLKmode parameter is truncated to
MAX_SUPPORTED_STACK_ALIGNMENT when it exceeds.
On the other hand, the compiler will try to dynamically align
the stack slot for local variable.

For example, on arm-gcc toolchain,
The function foo () will return a 16-byte aligned address.
However, P0 is temporarily stored on stack in an unaligned address.
Function bar () will return an unaligned address which is the address
of local stack copy of P0.

a warning could be emitted when the alignment could not be fulfilled or
dynamically align it thought it will waste stack space.