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 //===- PlaceSafepoints.h - Place GC Safepoints ----------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Place garbage collection safepoints at appropriate locations in the IR. This
 // does not make relocation semantics or variable liveness explicit.  That's
 // done by RewriteStatepointsForGC.
 //
 // Terminology:
 // - A call is said to be "parseable" if there is a stack map generated for the
 // return PC of the call.  A runtime can determine where values listed in the
 // deopt arguments and (after RewriteStatepointsForGC) gc arguments are located
 // on the stack when the code is suspended inside such a call.  Every parse
 // point is represented by a call wrapped in an gc.statepoint intrinsic.
 // - A "poll" is an explicit check in the generated code to determine if the
 // runtime needs the generated code to cooperate by calling a helper routine
 // and thus suspending its execution at a known state. The call to the helper
 // routine will be parseable.  The (gc & runtime specific) logic of a poll is
 // assumed to be provided in a function of the name "gc.safepoint_poll".
 //
 // We aim to insert polls such that running code can quickly be brought to a
 // well defined state for inspection by the collector.  In the current
 // implementation, this is done via the insertion of poll sites at method entry
 // and the backedge of most loops.  We try to avoid inserting more polls than
 // are necessary to ensure a finite period between poll sites.  This is not
 // because the poll itself is expensive in the generated code; it's not.  Polls
 // do tend to impact the optimizer itself in negative ways; we'd like to avoid
 // perturbing the optimization of the method as much as we can.
 //
 // We also need to make most call sites parseable.  The callee might execute a
 // poll (or otherwise be inspected by the GC).  If so, the entire stack
 // (including the suspended frame of the current method) must be parseable.
 //
 // This pass will insert:
 // - Call parse points ("call safepoints") for any call which may need to
 // reach a safepoint during the execution of the callee function.
 // - Backedge safepoint polls and entry safepoint polls to ensure that
 // executing code reaches a safepoint poll in a finite amount of time.
 //
 // We do not currently support return statepoints, but adding them would not
 // be hard.  They are not required for correctness - entry safepoints are an
 // alternative - but some GCs may prefer them.  Patches welcome.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_TRANSFORMS_SCALAR_PLACESAFEPOINTS_H
 #define LLVM_TRANSFORMS_SCALAR_PLACESAFEPOINTS_H
 
 #include "llvm/IR/PassManager.h"
 
 namespace llvm {
 
 class TargetLibraryInfo;
 
 class PlaceSafepointsPass : public PassInfoMixin<PlaceSafepointsPass> {
 public:
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
 
   bool runImpl(Function &F, const TargetLibraryInfo &TLI);
 
   void cleanup() {}
 
 private:
 };
 } // namespace llvm
 
 #endif // LLVM_TRANSFORMS_SCALAR_PLACESAFEPOINTS_H

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