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Simplify the ReusableObjectHolder #36068

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merged 6 commits into from Nov 12, 2021
Merged

Simplify the ReusableObjectHolder #36068

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7d5d8dd
ReusableObjectHolder: remove unused header
fwyzard Nov 10, 2021
a902d15
ReusableObjectHolder: reduce code duplication
fwyzard Nov 10, 2021
28cd464
ReusableObjectHolder: do not add new objects to the cache before retu…
fwyzard Nov 10, 2021
ca0e6ab
ReusableObjectHolder: check the result of try_pop instead of the valu…
fwyzard Nov 10, 2021
8061379
ReusableObjectHolder: avoid potential copies of the functors
fwyzard Nov 11, 2021
35efc5c
ReusableObjectHolder: update class documentation
fwyzard Nov 12, 2021
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51 changes: 26 additions & 25 deletions FWCore/Utilities/interface/ReusableObjectHolder.h
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Expand Up @@ -69,11 +69,11 @@
// Created: Fri, 31 July 2014 14:29:41 GMT
//

#include <memory>
#include <cassert>
#include <atomic>
#include "tbb/task.h"
#include "tbb/concurrent_queue.h"
#include <cassert>
#include <memory>

#include <tbb/concurrent_queue.h>

namespace edm {
template <class T, class Deleter = std::default_delete<T>>
Expand Down Expand Up @@ -108,43 +108,44 @@ namespace edm {
/// Use this function in conjunction with add()
std::shared_ptr<T> tryToGet() {
std::unique_ptr<T, Deleter> item;
m_availableQueue.try_pop(item);
if (nullptr == item) {
if (m_availableQueue.try_pop(item)) {
return wrapCustomDeleter(std::move(item));
} else {
return std::shared_ptr<T>{};
}
//instead of deleting, hand back to queue
auto pHolder = this;
auto deleter = item.get_deleter();
++m_outstandingObjects;
return std::shared_ptr<T>{item.release(), [pHolder, deleter](T* iItem) {
pHolder->addBack(std::unique_ptr<T, Deleter>{iItem, deleter});
}};
}

///If there isn't an object already available, creates a new one using iFunc
///Takes an object from the queue if one is available, or creates one using iFunc.
template <typename F>
std::shared_ptr<T> makeOrGet(F iFunc) {
std::shared_ptr<T> returnValue;
while (!(returnValue = tryToGet())) {
add(makeUnique(iFunc()));
std::unique_ptr<T, Deleter> item;
if (m_availableQueue.try_pop(item)) {
return wrapCustomDeleter(std::move(item));
} else {
return wrapCustomDeleter(makeUnique(iFunc()));
}
return returnValue;
}

///If there is an object already available, passes the object to iClearFunc and then
/// returns the object.
///If there is not an object already available, creates a new one using iMakeFunc
///Takes an object from the queue if one is available, or creates one using iMakeFunc.
///Then, passes the object to iClearFunc, and returns it.
template <typename FM, typename FC>
std::shared_ptr<T> makeOrGetAndClear(FM iMakeFunc, FC iClearFunc) {
std::shared_ptr<T> returnValue;
while (!(returnValue = tryToGet())) {
add(makeUnique(iMakeFunc()));
}
std::shared_ptr<T> returnValue = makeOrGet(iMakeFunc);
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I believe doing

Suggested change
std::shared_ptr<T> returnValue = makeOrGet(iMakeFunc);
std::shared_ptr<T> returnValue = makeOrGet(std::move(iMakeFunc));

would avoid a possible copy of the functor.

Possibly an even better solution would be to change the function signature to

makeOrGetAndClear(FM&& iMakeFunc, FC&& iClearFunc)

and then do

Suggested change
std::shared_ptr<T> returnValue = makeOrGet(iMakeFunc);
std::shared_ptr<T> returnValue = makeOrGet(std::forward<FM>(iMakeFunc));

A similar function signature change to makeOrGet would probably be required to get the most benefit of such a change.

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Good catch, thanks.

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I think this change

    template <typename FM, typename FC>
    std::shared_ptr<T> makeOrGetAndClear(FM&& iMakeFunc, FC iClearFunc) {
      std::shared_ptr<T> returnValue = makeOrGet(std::forward<FM>(iMakeFunc));
      iClearFunc(returnValue.get());
      return returnValue;
    }

should be enough, since iMakeFunc is the only one that is copied.

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@fwyzard fwyzard Nov 11, 2021 •
edited

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At least, my confused understanding is that FC iClearFunc (in makeOrGetAndClear) and F iFunc (in makeOrGet) should already take any of the functors by reference if they are lvalues.

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I think using FC instead of FC&& means the compiler (before optimization) will call the move constructor of FC when passed an lvalue. I'd bet that all compilers will remove that call when optimation is turned on. However, using FC&& does make the requirements of the arguments more explicit.

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I was wrong. I tested in godbolt and for the case where the object is created directly in the argument list they are identical.

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It does appear that if the argument is created outside of the function argument list, then FC&& is the better choice as it does avoid an extra copy.

https://godbolt.org/z/7fYG641qq

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OK, done - let me know if I missed anything (here or elsewhere).

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Sorry, I missed the last comment...

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...done, now.

iClearFunc(returnValue.get());
return returnValue;
}

private:
///Wraps an object in a shared_ptr<T> with a custom deleter, that hands the wrapped object
// back to the queue instead of deleting it
std::shared_ptr<T> wrapCustomDeleter(std::unique_ptr<T, Deleter> item) {
auto deleter = item.get_deleter();
++m_outstandingObjects;
return std::shared_ptr<T>{item.release(), [this, deleter](T* iItem) {
this->addBack(std::unique_ptr<T, Deleter>{iItem, deleter});
}};
}

std::unique_ptr<T> makeUnique(T* ptr) {
static_assert(std::is_same_v<Deleter, std::default_delete<T>>,
"Generating functions returning raw pointers are supported only with std::default_delete<T>");
Expand Down