https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102355
Bug ID: 102355
Summary: excessive stack usage
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: vanyacpp at gmail dot com
Target Milestone: ---
void escape(unsigned long long& a);
void foobar()
{
unsigned long long local;
escape(local);
}
For the function "foobar" GCC allocates excessive stack space:
foobar():
sub rsp, 24
lea rdi, [rsp+8]
call escape(unsigned long long&)
add rsp, 24
ret
The function "foobar" only needs 8 bytes of stack space, but GCC allocates 24.
Please note, that this excessive allocation isn't needed for stack alignment: 8
bytes of local variables are enough to keep the stack aligned. I also tested
Clang and it allocates 8 bytes.
GCC makes this stack layout:
8 bytes padding
8 bytes variable "local"
8 bytes padding
8 bytes return address
I believe the problem is related to the fact that GCC aligns the stack twice:
the first time after the return address placement and the second time after the
local variables are placed. Playing with -mpreferred-stack-boundary confirms
this:
-mpreferred-stack-boundary | stack usage
3 8
4 (default) 24
5 56
6 120
https://godbolt.org/z/h56aoKvvh
In all cases the stack usage is twice as much (minus 8 bytes for return
address) as the required alignment. I believe stack space can be conserved by
doing alignment only once.
