[PATCH v3 1/4] x86: Simplify TCB access expressions
H.J. Lu
hjl.tools@gmail.com
Wed Oct 8 02:59:00 GMT 2025
On Wed, Oct 1, 2025 at 8:31 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> Replace manual cast with a direct `(__typeof(*descr) __seg_gs *)0`
> dereference to access `member`. This avoids redundant
> `offsetof(struct pthread, member)` and improves readability while
> preserving semantics.
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> ---
> v2: Add volatile to the inner cast and drop the outer cast.
> ---
> sysdeps/i386/nptl/tcb-access.h | 15 +++++----------
> sysdeps/i386/nptl/tls.h | 5 ++---
> sysdeps/x86_64/nptl/tcb-access.h | 15 +++++----------
> sysdeps/x86_64/nptl/tls.h | 5 ++---
> 4 files changed, 14 insertions(+), 26 deletions(-)
>
> diff --git a/sysdeps/i386/nptl/tcb-access.h b/sysdeps/i386/nptl/tcb-access.h
> index 2a7f9d2a57..5bb58c37fa 100644
> --- a/sysdeps/i386/nptl/tcb-access.h
> +++ b/sysdeps/i386/nptl/tcb-access.h
> @@ -23,8 +23,7 @@
> || sizeof (descr->member) == 4 \
> || sizeof (descr->member) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member) __seg_gs *) \
> - offsetof (struct pthread, member)); \
> + ((__typeof (*descr) __seg_gs *)0)->member; \
> })
>
> #define THREAD_GETMEM_VOLATILE(descr, member) \
> @@ -33,8 +32,7 @@
> || sizeof (descr->member) == 4 \
> || sizeof (descr->member) == 8, \
> "size of per-thread data"); \
> - (*(volatile __typeof (descr->member) __seg_gs *) \
> - offsetof (struct pthread, member)); \
> + ((volatile __typeof (*descr) __seg_gs *)0)->member; \
> })
>
> /* Same as THREAD_GETMEM, but the member offset can be non-constant. */
> @@ -44,8 +42,7 @@
> || sizeof (descr->member[0]) == 4 \
> || sizeof (descr->member[0]) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member[0]) __seg_gs *) \
> - offsetof (struct pthread, member[idx])); \
> + ((__typeof (*descr) __seg_gs *)0)->member[idx]; \
> })
>
> /* Set member of the thread descriptor directly. */
> @@ -55,8 +52,7 @@
> || sizeof (descr->member) == 4 \
> || sizeof (descr->member) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member) __seg_gs *) \
> - offsetof (struct pthread, member) = (value)); \
> + ((__typeof (*descr) __seg_gs *)0)->member = (value); \
> })
>
> /* Same as THREAD_SETMEM, but the member offset can be non-constant. */
> @@ -66,6 +62,5 @@
> || sizeof (descr->member[0]) == 4 \
> || sizeof (descr->member[0]) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member[0]) __seg_gs *) \
> - offsetof (struct pthread, member[idx]) = (value)); \
> + ((__typeof (*descr) __seg_gs *)0)->member[idx] = (value); \
> })
> diff --git a/sysdeps/i386/nptl/tls.h b/sysdeps/i386/nptl/tls.h
> index 5c266c7191..7b297d4043 100644
> --- a/sysdeps/i386/nptl/tls.h
> +++ b/sysdeps/i386/nptl/tls.h
> @@ -223,7 +223,7 @@ tls_fill_user_desc (union user_desc_init *desc,
>
> /* Return the thread descriptor for the current thread. */
> # define THREAD_SELF \
> - (*(struct pthread *__seg_gs *) offsetof (struct pthread, header.self))
> + (*(struct pthread *__seg_gs *)&((struct pthread __seg_gs *)0)->header.self)
>
> /* Magic for libthread_db to know how to do THREAD_SELF. */
> # define DB_THREAD_SELF \
> @@ -257,8 +257,7 @@ tls_fill_user_desc (union user_desc_init *desc,
> { int __res; \
> asm volatile ("xchgl %1, %0" \
> : "=r" (__res) \
> - : "m" (*(int __seg_gs *) \
> - offsetof (struct pthread, header.gscope_flag)), \
> + : "m" (((struct pthread __seg_gs *)0)->header.gscope_flag), \
> "0" (THREAD_GSCOPE_FLAG_UNUSED)); \
> if (__res == THREAD_GSCOPE_FLAG_WAIT) \
> lll_futex_wake (&THREAD_SELF->header.gscope_flag, 1, LLL_PRIVATE); \
> diff --git a/sysdeps/x86_64/nptl/tcb-access.h b/sysdeps/x86_64/nptl/tcb-access.h
> index 445f328ac2..7a418f945f 100644
> --- a/sysdeps/x86_64/nptl/tcb-access.h
> +++ b/sysdeps/x86_64/nptl/tcb-access.h
> @@ -23,8 +23,7 @@
> || sizeof (descr->member) == 4 \
> || sizeof (descr->member) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member) __seg_fs *) \
> - offsetof (struct pthread, member)); \
> + ((__typeof (*descr) __seg_fs *)0)->member; \
> })
>
> #define THREAD_GETMEM_VOLATILE(descr, member) \
> @@ -33,8 +32,7 @@
> || sizeof (descr->member) == 4 \
> || sizeof (descr->member) == 8, \
> "size of per-thread data"); \
> - (*(volatile __typeof (descr->member) __seg_fs *) \
> - offsetof (struct pthread, member)); \
> + ((volatile __typeof (*descr) __seg_fs *)0)->member; \
> })
>
> /* Same as THREAD_GETMEM, but the member offset can be non-constant. */
> @@ -44,8 +42,7 @@
> || sizeof (descr->member[0]) == 4 \
> || sizeof (descr->member[0]) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member[0]) __seg_fs *) \
> - offsetof (struct pthread, member[idx])); \
> + ((__typeof (*descr) __seg_fs *)0)->member[idx]; \
> })
>
> /* Set member of the thread descriptor directly. */
> @@ -55,8 +52,7 @@
> || sizeof (descr->member) == 4 \
> || sizeof (descr->member) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member) __seg_fs *) \
> - offsetof (struct pthread, member) = (value)); \
> + ((__typeof (*descr) __seg_fs *)0)->member = (value); \
> })
>
> /* Same as THREAD_SETMEM, but the member offset can be non-constant. */
> @@ -66,6 +62,5 @@
> || sizeof (descr->member[0]) == 4 \
> || sizeof (descr->member[0]) == 8, \
> "size of per-thread data"); \
> - (*(__typeof (descr->member[0]) __seg_fs *) \
> - offsetof (struct pthread, member[idx]) = (value)); \
> + ((__typeof (*descr) __seg_fs *)0)->member[idx] = (value); \
> })
> diff --git a/sysdeps/x86_64/nptl/tls.h b/sysdeps/x86_64/nptl/tls.h
> index cf41430050..abfeb88054 100644
> --- a/sysdeps/x86_64/nptl/tls.h
> +++ b/sysdeps/x86_64/nptl/tls.h
> @@ -170,7 +170,7 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
>
> /* Return the thread descriptor for the current thread. */
> # define THREAD_SELF \
> - (*(struct pthread *__seg_fs *) offsetof (struct pthread, header.self))
> + (*(struct pthread *__seg_fs *)&((struct pthread __seg_fs *)0)->header.self)
>
> /* Magic for libthread_db to know how to do THREAD_SELF. */
> # define DB_THREAD_SELF_INCLUDE <sys/reg.h> /* For the FS constant. */
> @@ -203,8 +203,7 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
> { int __res; \
> asm volatile ("xchgl %1, %0" \
> : "=r" (__res) \
> - : "m" (*(int __seg_fs *) \
> - offsetof (struct pthread, header.gscope_flag)), \
> + : "m" (((struct pthread __seg_fs *)0)->header.gscope_flag), \
> "0" (THREAD_GSCOPE_FLAG_UNUSED)); \
> if (__res == THREAD_GSCOPE_FLAG_WAIT) \
> lll_futex_wake (&THREAD_SELF->header.gscope_flag, 1, LLL_PRIVATE); \
> --
> 2.51.0
>
LGTM.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
Thanks.
--
H.J.
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