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| author | Noah Goldstein <goldstein.w.n@gmail.com> | 2022-09-30 21:13:27 -0700 |
|---|---|---|
| committer | Noah Goldstein <goldstein.w.n@gmail.com> | 2022-10-03 14:13:49 -0700 |
| commit | 653c12c7d880340462bd963752619a7a61bcb4e3 (patch) | |
| tree | 67c128963342987f4023beba5f752f20c59c8863 | |
| parent | 10c779f44ab3e9525f2d2a3c9a0aa9dedea5f1ec (diff) | |
| download | glibc-653c12c7d880340462bd963752619a7a61bcb4e3.tar.gz glibc-653c12c7d880340462bd963752619a7a61bcb4e3.tar.xz glibc-653c12c7d880340462bd963752619a7a61bcb4e3.zip | |
x86: Cleanup pthread_spin_{try}lock.S
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.
| -rw-r--r-- | sysdeps/x86_64/nptl/pthread_spin_lock.S | 23 | ||||
| -rw-r--r-- | 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) |