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.
