https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124660
Bug ID: 124660
Summary: Missed optimization for reusing stack space of
temporary objects in C
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: star-code at tuta dot com
Target Milestone: ---
It seems that, in C, GCC currently does not reuse stack space for temporary
objects when their lifetimes do not overlap [1]. GCC does perform this
optimization for named variables, though. For example [2]:
// -O2
struct S {
char a[256];
};
struct S f();
void g(char *);
void test1() {
g(f().a);
g(f().a);
}
void test2() {
{
struct S s = f();
g(s.a);
}
{
struct S s = f();
g(s.a);
}
}
GCC compiles this code into:
test1:
sub rsp, 520
mov rdi, rsp
call f
mov rdi, rsp
call g
lea rdi, [rsp+256]
call f
lea rdi, [rsp+256]
call g
add rsp, 520
ret
test2:
sub rsp, 264
mov rdi, rsp
call f
mov rdi, rsp
call g
mov rdi, rsp
call f
mov rdi, rsp
call g
add rsp, 264
ret
Observe that the stack usage in `test1` is not optimized. However, when
compiling in C++ mode, this optimization does occur:
test1():
sub rsp, 264
mov rdi, rsp
call f()
mov rdi, rsp
call g(char*)
mov rdi, rsp
call f()
mov rdi, rsp
call g(char*)
add rsp, 264
ret
test2():
jmp test1()
So I think this optimization should be implemented for C as well, and the
option `-fstack-reuse` [3] could be used to enable or disable the behavior.
Currently, the values `all` and `named_vars` for that option appear to make no
difference in C mode.
[1] https://www.reddit.com/r/C_Programming/comments/1s3cohq/comment/ocfjnuk/
[2] https://godbolt.org/z/87vqqeTTd
[3]
https://gcc.gnu.org/onlinedocs/gcc-15.2.0/gcc/Code-Gen-Options.html#index-fstack_005freuse
