[v2,2/2] x86: Cleanup pthread_spin_{try}lock.S
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| dj/TryBot-apply_patch |
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Patch applied to master at the time it was sent
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| dj/TryBot-32bit |
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Build for i686
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Commit Message
Save a jmp on the lock path coming from an initial failure in
pthread_spin_lock.S. This costs 4-bytes of code but since the
function still fits in the same number of 16-byte blocks (default
function alignment) it does not have affect on the total binary size
of libc.so (unchanged after this commit).
pthread_spin_trylock was using a CAS when a simple xchg works which
is often more expensive.
Full check passes on x86-64.
---
sysdeps/x86_64/nptl/pthread_spin_lock.S | 23 +++++++++++++++-------
sysdeps/x86_64/nptl/pthread_spin_trylock.S | 18 ++++++++++++-----
2 files changed, 29 insertions(+), 12 deletions(-)
Comments
On Fri, Sep 30, 2022 at 9:13 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> Save a jmp on the lock path coming from an initial failure in
> pthread_spin_lock.S. This costs 4-bytes of code but since the
> function still fits in the same number of 16-byte blocks (default
> function alignment) it does not have affect on the total binary size
> of libc.so (unchanged after this commit).
>
> pthread_spin_trylock was using a CAS when a simple xchg works which
> is often more expensive.
>
> Full check passes on x86-64.
> ---
> sysdeps/x86_64/nptl/pthread_spin_lock.S | 23 +++++++++++++++-------
> sysdeps/x86_64/nptl/pthread_spin_trylock.S | 18 ++++++++++++-----
> 2 files changed, 29 insertions(+), 12 deletions(-)
>
> diff --git a/sysdeps/x86_64/nptl/pthread_spin_lock.S b/sysdeps/x86_64/nptl/pthread_spin_lock.S
> index 44b837d9db..1e09e59b10 100644
> --- a/sysdeps/x86_64/nptl/pthread_spin_lock.S
> +++ b/sysdeps/x86_64/nptl/pthread_spin_lock.S
> @@ -19,18 +19,27 @@
> #include <shlib-compat.h>
>
> ENTRY(__pthread_spin_lock)
> -1: LOCK
> - decl 0(%rdi)
> - jne 2f
> + /* Always return zero. */
> xor %eax, %eax
> + LOCK
> + decl 0(%rdi)
> + jne 1f
> ret
>
> .align 16
> -2: rep
> +1:
> + /* `rep nop` == `pause`. */
> + rep
> nop
> - cmpl $0, 0(%rdi)
> - jg 1b
> - jmp 2b
> + cmpl %eax, 0(%rdi)
> + jle 1b
> + /* Just repeat the `lock decl` logic here. The code size save
> + of jumping back to entry doesn't change how many 16-byte
> + chunks (default function alignment) that the code fits in. */
> + LOCK
> + decl 0(%rdi)
> + jne 1b
> + ret
> END(__pthread_spin_lock)
> versioned_symbol (libc, __pthread_spin_lock, pthread_spin_lock, GLIBC_2_34)
>
> diff --git a/sysdeps/x86_64/nptl/pthread_spin_trylock.S b/sysdeps/x86_64/nptl/pthread_spin_trylock.S
> index fffdb27dd9..a1f97cb420 100644
> --- a/sysdeps/x86_64/nptl/pthread_spin_trylock.S
> +++ b/sysdeps/x86_64/nptl/pthread_spin_trylock.S
> @@ -20,13 +20,21 @@
> #include <shlib-compat.h>
>
> ENTRY(__pthread_spin_trylock)
> - movl $1, %eax
> xorl %ecx, %ecx
> - lock
> - cmpxchgl %ecx, (%rdi)
> + /* xchg has implicit LOCK prefix. */
> + xchgl %ecx, (%rdi)
> +
> + /* Branch on result. Expectation is the use of trylock will be
> + branching on success/failure so this branch can be used to
> + to predict the coming branch. It has the benefit of
> + breaking the likely expensive memory dependency on (%rdi). */
> + cmpl $1, %ecx
> + jnz 1f
> + xorl %eax, %eax
> + ret
> +1:
> movl $EBUSY, %eax
> - cmovel %ecx, %eax
> - retq
> + ret
> END(__pthread_spin_trylock)
> versioned_symbol (libc, __pthread_spin_trylock, pthread_spin_trylock,
> GLIBC_2_34)
> --
> 2.34.1
>
LGTM.
Thanks.
@@ -19,18 +19,27 @@
#include <shlib-compat.h>
ENTRY(__pthread_spin_lock)
-1: LOCK
- decl 0(%rdi)
- jne 2f
+ /* Always return zero. */
xor %eax, %eax
+ LOCK
+ decl 0(%rdi)
+ jne 1f
ret
.align 16
-2: rep
+1:
+ /* `rep nop` == `pause`. */
+ rep
nop
- cmpl $0, 0(%rdi)
- jg 1b
- jmp 2b
+ cmpl %eax, 0(%rdi)
+ jle 1b
+ /* Just repeat the `lock decl` logic here. The code size save
+ of jumping back to entry doesn't change how many 16-byte
+ chunks (default function alignment) that the code fits in. */
+ LOCK
+ decl 0(%rdi)
+ jne 1b
+ ret
END(__pthread_spin_lock)
versioned_symbol (libc, __pthread_spin_lock, pthread_spin_lock, GLIBC_2_34)
@@ -20,13 +20,21 @@
#include <shlib-compat.h>
ENTRY(__pthread_spin_trylock)
- movl $1, %eax
xorl %ecx, %ecx
- lock
- cmpxchgl %ecx, (%rdi)
+ /* xchg has implicit LOCK prefix. */
+ xchgl %ecx, (%rdi)
+
+ /* Branch on result. Expectation is the use of trylock will be
+ branching on success/failure so this branch can be used to
+ to predict the coming branch. It has the benefit of
+ breaking the likely expensive memory dependency on (%rdi). */
+ cmpl $1, %ecx
+ jnz 1f
+ xorl %eax, %eax
+ ret
+1:
movl $EBUSY, %eax
- cmovel %ecx, %eax
- retq
+ ret
END(__pthread_spin_trylock)
versioned_symbol (libc, __pthread_spin_trylock, pthread_spin_trylock,
GLIBC_2_34)