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From: "lh_mouse at 126 dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/105495] `__atomic_compare_exchange` prevents tail-call optimization Date: Fri, 06 May 2022 07:40:33 +0000 [thread overview] Message-ID: <bug-105495-4-PSFkeqhpkP@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-105495-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105495 --- Comment #5 from LIU Hao <lh_mouse at 126 dot com> --- This does not trigger the issue: ```c #define __atomic_compare_exchange(p,c,n,w,ms,mf) \ ({ int __temp; \ __builtin_memcpy(&__temp, c, sizeof(*c)); \ _Bool __r = __atomic_compare_exchange(p, (__typeof__(*(c))*) &__temp, n, w, ms, mf); \ __builtin_memcpy(c, &__temp, sizeof(*c)); \ __r; }); typedef struct { int b; } cond; int __MCF_batch_release_common(cond* p, int c); int _MCF_cond_signal_some(cond* p, int x) { cond c = {x}, n = {2}; __atomic_compare_exchange(p, &c, &n, 1, 0, 0); return __MCF_batch_release_common(p, x); } ``` which results in (godbolt https://gcc.godbolt.org/z/n68T1c6oP): ```asm _MCF_cond_signal_some: mov edx, 2 mov eax, esi lock cmpxchg DWORD PTR [rdi], edx jmp __MCF_batch_release_common ``` Effectively, we are using a `int` to provide storage for a struct of `sizeof(int)`. But if we use a `long` to provide storage for the struct, such issue reappears: ```c #define __atomic_compare_exchange(p,c,n,w,ms,mf) \ ({ long __temp; \ __builtin_memcpy(&__temp, c, sizeof(*c)); \ _Bool __r = __atomic_compare_exchange(p, (__typeof__(*(c))*) &__temp, n, w, ms, mf); \ __builtin_memcpy(c, &__temp, sizeof(*c)); \ __r; }); typedef struct { int b; } cond; int __MCF_batch_release_common(cond* p, int c); int _MCF_cond_signal_some(cond* p, int x) { cond c = {x}, n = {2}; __atomic_compare_exchange(p, &c, &n, 1, 0, 0); return __MCF_batch_release_common(p, x); } ``` which results in (https://gcc.godbolt.org/z/PGof8nGd7) ```asm _MCF_cond_signal_some: mov edx, 2 mov eax, esi mov DWORD PTR [rsp-16], esi lock cmpxchg DWORD PTR [rdi], edx je .L2 mov DWORD PTR [rsp-16], eax .L2: jmp __MCF_batch_release_common ``` It is also notable that with this kind of hacks, GCC is finally able to perform TCO on the return statement.
next prev parent reply other threads:[~2022-05-06 7:40 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-05-05 14:00 [Bug c/105495] New: " lh_mouse at 126 dot com 2022-05-05 14:10 ` [Bug c/105495] " lh_mouse at 126 dot com 2022-05-05 14:17 ` rguenth at gcc dot gnu.org 2022-05-05 14:30 ` lh_mouse at 126 dot com 2022-05-05 14:33 ` jakub at gcc dot gnu.org 2022-05-05 15:28 ` [Bug middle-end/105495] " pinskia at gcc dot gnu.org 2022-05-06 7:40 ` lh_mouse at 126 dot com [this message] 2022-10-31 7:55 ` [Bug tree-optimization/105495] " pinskia at gcc dot gnu.org
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