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Example of what can be done with the vcperf code base.
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| #pragma once | ||
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| #include <unordered_set> | ||
| #include <cassert> | ||
| #include "VcperfBuildInsights.h" | ||
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| namespace vcperf | ||
| { | ||
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| // Linker invocations sometimes restart themselves. This can happen in the | ||
| // following cases: | ||
| // | ||
| // 1. The /LTCG switch was not used, and an object compiled with /GL was found. | ||
| // The linker is restarted with /LTCG. | ||
| // 2. A 32-bit linker runs out of memory address space. The linker is restarted | ||
| // in 64-bit. | ||
| // | ||
| // These restarts may have a non-negligible impact on throughput so we would | ||
| // like to identify them automatically. The following C++ Build Insights | ||
| // analyzer implements this functionality. | ||
| class RestartedLinkerDetector : public BI::IAnalyzer | ||
| { | ||
| public: | ||
| RestartedLinkerDetector(): | ||
| pass_{0}, | ||
| restartedLinkers_{} | ||
| {} | ||
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| // Some analyses are done in multiple passes. The OnBeginAnalysisPass | ||
| // function comes from the IAnalyzer interface, and will be called every | ||
| // time a pass begins. | ||
| BI::AnalysisControl OnBeginAnalysisPass() override | ||
| { | ||
| // Let's keep track of the pass we are in. | ||
| ++pass_; | ||
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| // We return CONTINUE to pass control over to the next | ||
| // analyzer in the analysis chain. | ||
| return BI::AnalysisControl::CONTINUE; | ||
| } | ||
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| // The OnStartActivity function will be called every time an activity start | ||
| // event is seen. An activity is something happening in MSVC that has a | ||
| // start and a stop time. The list of activities that generate start | ||
| // events can be found in the Microsoft::Cpp::BuildInsights::Activities | ||
| // namespace. | ||
| BI::AnalysisControl OnStartActivity(const BI::EventStack& eventStack) override | ||
| { | ||
| // This analyzer is only active in the first pass. During this pass, | ||
| // it will remember which linkers have been restarted by storing the | ||
| // information in a cache. | ||
| if (pass_ > 1) return BI::AnalysisControl::CONTINUE; | ||
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| // BACKGROUND: | ||
| // | ||
| // C++ Build Insights events are related to each other in the | ||
| // form of a graph. Nodes are either 'activities' which have a start | ||
| // and stop time, or 'simple events' which are punctual. Activity nodes | ||
| // may have children nodes. Simple events are always leaves. When an | ||
| // activity has a child node, it means that the child event occurred | ||
| // while the parent was still ongoing. For example, a Linker activity | ||
| // may encapsulate the LinkerPass2 activity, which itself may | ||
| // encapsulate a LinkerLTCG activity. In this case, the graph would have | ||
| // a branch that looks like: Linker --> LinkerPass2 --> LinkerLTCG. | ||
| // | ||
| // EVENT STACKS: | ||
| // | ||
| // The 'eventStack' parameter contains the path in the graph for the | ||
| // current event. Using the example above, if the current event is the | ||
| // start of LinkerPass2, the event stack would contain: | ||
| // [Linker, LinkerPass2], where LinkerPass2 is at the top of the stack. | ||
| // You can think of the event stack as a call stack, because it tells | ||
| // you what MSVC was currently executing, as well as how it got there. | ||
| // | ||
| // IDENTIFY RELEVANT BRANCHES: | ||
| // | ||
| // When writing a C++ Build Insights analyzer, the first thing you will | ||
| // typically want to do is identify the branches in the graph that are | ||
| // relevant to your analysis. In our case, we want to identify linkers | ||
| // that are being restarted (i.e. linkers that are encapsulated by | ||
| // another linker). This can be represented by a branch of this | ||
| // form: X --> Linker --> X --> Linker, where the X's are activities we | ||
| // don't care about. | ||
| // | ||
| // MATCHING A BRANCH | ||
| // | ||
| // Once the branch of interest has been identified, the next step is to | ||
| // match the event stack against it. Here, this is done using | ||
| // MatchEventStackInMemberFunction, which looks at a member function's | ||
| // parameters to determine the branch to match. The last parameter in | ||
| // the list is always the leaf, while others can be anywhere along the | ||
| // path from the root. In this case we are matching the stack in the | ||
| // FlagRestartedLinker member function. Its parameters describe a branch | ||
| // that ends in a Linker and that has a parent Linker somewhere along | ||
| // the path. This is exactly what our analysis needs. | ||
| BI::MatchEventStackInMemberFunction(eventStack, this, | ||
| &RestartedLinkerDetector::FlagRestartedLinker); | ||
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| return BI::AnalysisControl::CONTINUE; | ||
| } | ||
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| void FlagRestartedLinker(const A::Linker& parent, const A::Linker& child) | ||
| { | ||
| // Flag the parent linker as having been restarted | ||
| // by inserting its instance ID into our cache. | ||
| // In a given trace, each C++ Build Insights activity | ||
| // or simple event has a unique instance ID. This ID is | ||
| // useful to correlate events between different analysis | ||
| // passes for the same trace. | ||
| restartedLinkers_.insert(parent.EventInstanceId()); | ||
| } | ||
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| // This function is intended to be called by other analyzers | ||
| // in the chain if they need to know whether a linker was | ||
| // restarted. Restarted linkers have already been flagged in pass 1, | ||
| // so we look into the cache to gather the information. | ||
| bool WasLinkerRestarted(const A::Invocation& linker) const | ||
| { | ||
| return restartedLinkers_.find(linker.EventInstanceId()) != | ||
| restartedLinkers_.end(); | ||
| } | ||
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| private: | ||
| unsigned pass_; | ||
| std::unordered_set<unsigned long long> restartedLinkers_; | ||
| }; | ||
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| } // namespace vcperf |
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