1 | //===- PlaceSafepoints.h - Place GC Safepoints ----------------------------===// |
2 | // |
3 | // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
4 | // See https://llvm.org/LICENSE.txt for license information. |
5 | // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
6 | // |
7 | //===----------------------------------------------------------------------===// |
8 | // |
9 | // Place garbage collection safepoints at appropriate locations in the IR. This |
10 | // does not make relocation semantics or variable liveness explicit. That's |
11 | // done by RewriteStatepointsForGC. |
12 | // |
13 | // Terminology: |
14 | // - A call is said to be "parseable" if there is a stack map generated for the |
15 | // return PC of the call. A runtime can determine where values listed in the |
16 | // deopt arguments and (after RewriteStatepointsForGC) gc arguments are located |
17 | // on the stack when the code is suspended inside such a call. Every parse |
18 | // point is represented by a call wrapped in an gc.statepoint intrinsic. |
19 | // - A "poll" is an explicit check in the generated code to determine if the |
20 | // runtime needs the generated code to cooperate by calling a helper routine |
21 | // and thus suspending its execution at a known state. The call to the helper |
22 | // routine will be parseable. The (gc & runtime specific) logic of a poll is |
23 | // assumed to be provided in a function of the name "gc.safepoint_poll". |
24 | // |
25 | // We aim to insert polls such that running code can quickly be brought to a |
26 | // well defined state for inspection by the collector. In the current |
27 | // implementation, this is done via the insertion of poll sites at method entry |
28 | // and the backedge of most loops. We try to avoid inserting more polls than |
29 | // are necessary to ensure a finite period between poll sites. This is not |
30 | // because the poll itself is expensive in the generated code; it's not. Polls |
31 | // do tend to impact the optimizer itself in negative ways; we'd like to avoid |
32 | // perturbing the optimization of the method as much as we can. |
33 | // |
34 | // We also need to make most call sites parseable. The callee might execute a |
35 | // poll (or otherwise be inspected by the GC). If so, the entire stack |
36 | // (including the suspended frame of the current method) must be parseable. |
37 | // |
38 | // This pass will insert: |
39 | // - Call parse points ("call safepoints") for any call which may need to |
40 | // reach a safepoint during the execution of the callee function. |
41 | // - Backedge safepoint polls and entry safepoint polls to ensure that |
42 | // executing code reaches a safepoint poll in a finite amount of time. |
43 | // |
44 | // We do not currently support return statepoints, but adding them would not |
45 | // be hard. They are not required for correctness - entry safepoints are an |
46 | // alternative - but some GCs may prefer them. Patches welcome. |
47 | // |
48 | //===----------------------------------------------------------------------===// |
49 | |
50 | #ifndef LLVM_TRANSFORMS_SCALAR_PLACESAFEPOINTS_H |
51 | #define LLVM_TRANSFORMS_SCALAR_PLACESAFEPOINTS_H |
52 | |
53 | #include "llvm/IR/PassManager.h" |
54 | |
55 | namespace llvm { |
56 | |
57 | class TargetLibraryInfo; |
58 | |
59 | class PlaceSafepointsPass : public PassInfoMixin<PlaceSafepointsPass> { |
60 | public: |
61 | PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); |
62 | |
63 | bool runImpl(Function &F, const TargetLibraryInfo &TLI); |
64 | |
65 | void cleanup() {} |
66 | |
67 | private: |
68 | }; |
69 | } // namespace llvm |
70 | |
71 | #endif // LLVM_TRANSFORMS_SCALAR_PLACESAFEPOINTS_H |
72 | |