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shared_ptr's increment/decrement use sequential consistency. #23177

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horenmar opened this issue Mar 5, 2015 · 5 comments
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shared_ptr's increment/decrement use sequential consistency. #23177

horenmar opened this issue Mar 5, 2015 · 5 comments
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@mclow
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bugzilla Issues migrated from bugzilla libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi.

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@horenmar
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horenmar commented Mar 5, 2015

Bugzilla Link 22803
Resolution FIXED
Resolved on Jul 08, 2016 03:03
Version unspecified
OS All
CC @mclow

Extended Description

Current shared_ptr implementation uses sequential consistency for reference counting, even though less strong atomics are good enough. This leads to suboptimal performance for platforms other than x86 (x64).

Relaxed and acq/rel atomics (for increment and decrement respectively) should be sufficient for thread safety. The reason it is currently implemented like this was that Clang at the time didn't support full range of C++11 atomics and noone ever got back to cleaning it up.
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horenmar commented Mar 5, 2015

assigned to @mclow

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mclow commented Jun 22, 2015

Reference: http://stackoverflow.com/questions/28199212/why-does-libcs-implementation-of-shared-ptr-use-full-memory-barriers-instead

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llvmbot commented Jun 22, 2015

Related Patch (that I should probably change to fix this):

http://reviews.llvm.org/D10406

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llvmbot commented Jul 7, 2015

Fixed in r241532.

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llvmbot commented Jul 8, 2016

Current shared_ptr implementation uses sequential consistency for reference
counting, even though less strong atomics are good enough. This leads to
suboptimal performance for platforms other than x86 (x64).

Relaxed and acq/rel atomics (for increment and decrement respectively)
should be sufficient for thread safety. The reason it is currently
implemented like this was that Clang at the time didn't support full range
of C++11 atomics and noone ever got back to cleaning it up.

Hi;candecrement() use relaxed atomics?

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 9, 2021
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bugzilla Issues migrated from bugzilla libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi.
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