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Use atomics instead of volatile

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Rely on well defined behaviour for message passing, instead of volatile. Three
versions have been tested, to make sure this wouldn't cause a slowdown on any
platform.

v1: Sequentially consistent atomics

No mesurable regression, despite the extra memory barriers on x86. Even with 15
threads and extreme time pressure, both acting as a magnifying glass:

threads=15, tc=2+0.02
ELO: 2.59 +-3.4 (95%) LOS: 93.3%
Total: 18132 W: 4113 L: 3978 D: 10041

threads=7, tc=2+0.02
ELO: -1.64 +-3.6 (95%) LOS: 18.8%
Total: 16914 W: 4053 L: 4133 D: 8728

v2: Acquire/Release semantics

This version generates no extra barriers for x86 (on the hot path). As expected,
no regression either, under the same conditions:

threads=15, tc=2+0.02
ELO: 2.85 +-3.3 (95%) LOS: 95.4%
Total: 19661 W: 4640 L: 4479 D: 10542

threads=7, tc=2+0.02
ELO: 0.23 +-3.5 (95%) LOS: 55.1%
Total: 18108 W: 4326 L: 4314 D: 9468

As suggested by Joona, another test at LTC:

threads=15, tc=20+0.05
ELO: 0.64 +-2.6 (95%) LOS: 68.3%
Total: 20000 W: 3053 L: 3016 D: 13931

v3: Final version: SeqCst/Relaxed

threads=15, tc=10+0.1
ELO: 0.87 +-3.9 (95%) LOS: 67.1%
Total: 9541 W: 1478 L: 1454 D: 6609

Resolves #474
This commit is contained in:
lucasart
2015-10-24 22:50:51 +01:00
committed by Joona Kiiski
parent 3428a28751
commit 00d9e9fd28
4 changed files with 22 additions and 20 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
src/search.h
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@@ -20,7 +20,8 @@
#ifndef SEARCH_H_INCLUDED
#define SEARCH_H_INCLUDED
#include <memory> // For std::auto_ptr
#include <atomic>
#include <memory> // For std::unique_ptr
#include <stack>
#include <vector>
@@ -91,16 +92,16 @@ struct LimitsType {
TimePoint startTime;
};
/// The SignalsType struct stores volatile flags updated during the search
/// The SignalsType struct stores atomic flags updated during the search
/// typically in an async fashion e.g. to stop the search by the GUI.
struct SignalsType {
bool stop, stopOnPonderhit, firstRootMove, failedLowAtRoot;
std::atomic<bool> stop, stopOnPonderhit, firstRootMove, failedLowAtRoot;
};
typedef std::unique_ptr<std::stack<StateInfo>> StateStackPtr;
extern volatile SignalsType Signals;
extern SignalsType Signals;
extern LimitsType Limits;
extern StateStackPtr SetupStates;