https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101786
Bug ID: 101786
Summary: P1143R2 constinit implementation is incomplete
(joining with thread_local)
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jpegqs at gmail dot com
CC: mpolacek at gcc dot gnu.org
Target Milestone: ---
The paper says:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1143r2.html
> constinit can also be useful to compilers for non-initializing declarations
> of thread_local variables:
>
> extern thread_local constinit x;
> int f() { return x; }
>
> Without constinit, runtime code must be executed to perform a check of a
> guard variable and conditionally initialize x each time it is used. (Other
> techniques exist, but this approach is common.) If the variable is known to
> have constant initialization, this can be avoided.
Let's fix the missing type for x and try:
extern thread_local constinit int x;
int f() { return x; }
In case of compilation, GCC does not remove the TLS wrapper function as it
should according to this paper:
_ZTW1x:
push rbp
mov rbp, rsp
mov eax, OFFSET FLAT:_ZTH1x
test rax, rax
je .L2
call _ZTH1x
.L2:
mov rdx, QWORD PTR fs:0
mov rax, QWORD PTR x@gottpoff[rip]
add rax, rdx
pop rbp
ret
_Z1fv:
push rbp
mov rbp, rsp
call _ZTW1x
mov eax, DWORD PTR [rax]
pop rbp
ret
The code it should produce should look like this:
_Z1fv:
push rbp
mov rbp, rsp
mov rax, QWORD PTR x@gottpoff[rip]
mov eax, DWORD PTR fs:[rax]
pop rbp
ret
What I can get now is only by replacing "thread_local constinit" with
"__thread".
Clang implements this feature.
