1 | //===- SpeculativeExecution.h -----------------------------------*- C++ -*-===// |
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 | // This pass hoists instructions to enable speculative execution on |
10 | // targets where branches are expensive. This is aimed at GPUs. It |
11 | // currently works on simple if-then and if-then-else |
12 | // patterns. |
13 | // |
14 | // Removing branches is not the only motivation for this |
15 | // pass. E.g. consider this code and assume that there is no |
16 | // addressing mode for multiplying by sizeof(*a): |
17 | // |
18 | // if (b > 0) |
19 | // c = a[i + 1] |
20 | // if (d > 0) |
21 | // e = a[i + 2] |
22 | // |
23 | // turns into |
24 | // |
25 | // p = &a[i + 1]; |
26 | // if (b > 0) |
27 | // c = *p; |
28 | // q = &a[i + 2]; |
29 | // if (d > 0) |
30 | // e = *q; |
31 | // |
32 | // which could later be optimized to |
33 | // |
34 | // r = &a[i]; |
35 | // if (b > 0) |
36 | // c = r[1]; |
37 | // if (d > 0) |
38 | // e = r[2]; |
39 | // |
40 | // Later passes sink back much of the speculated code that did not enable |
41 | // further optimization. |
42 | // |
43 | // This pass is more aggressive than the function SpeculativeyExecuteBB in |
44 | // SimplifyCFG. SimplifyCFG will not speculate if no selects are introduced and |
45 | // it will speculate at most one instruction. It also will not speculate if |
46 | // there is a value defined in the if-block that is only used in the then-block. |
47 | // These restrictions make sense since the speculation in SimplifyCFG seems |
48 | // aimed at introducing cheap selects, while this pass is intended to do more |
49 | // aggressive speculation while counting on later passes to either capitalize on |
50 | // that or clean it up. |
51 | // |
52 | // If the pass was created by calling |
53 | // createSpeculativeExecutionIfHasBranchDivergencePass or the |
54 | // -spec-exec-only-if-divergent-target option is present, this pass only has an |
55 | // effect on targets where TargetTransformInfo::hasBranchDivergence() is true; |
56 | // on other targets, it is a nop. |
57 | // |
58 | // This lets you include this pass unconditionally in the IR pass pipeline, but |
59 | // only enable it for relevant targets. |
60 | // |
61 | //===----------------------------------------------------------------------===// |
62 | #ifndef LLVM_TRANSFORMS_SCALAR_SPECULATIVEEXECUTION_H |
63 | #define LLVM_TRANSFORMS_SCALAR_SPECULATIVEEXECUTION_H |
64 | |
65 | #include "llvm/IR/PassManager.h" |
66 | |
67 | namespace llvm { |
68 | class TargetTransformInfo; |
69 | class SpeculativeExecutionPass |
70 | : public PassInfoMixin<SpeculativeExecutionPass> { |
71 | public: |
72 | SpeculativeExecutionPass(bool OnlyIfDivergentTarget = false); |
73 | |
74 | PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); |
75 | |
76 | void printPipeline(raw_ostream &OS, |
77 | function_ref<StringRef(StringRef)> MapClassName2PassName); |
78 | |
79 | // Glue for old PM |
80 | bool runImpl(Function &F, TargetTransformInfo *TTI); |
81 | |
82 | private: |
83 | bool runOnBasicBlock(BasicBlock &B); |
84 | bool considerHoistingFromTo(BasicBlock &FromBlock, BasicBlock &ToBlock); |
85 | |
86 | // If true, this pass is a nop unless the target architecture has branch |
87 | // divergence. |
88 | const bool OnlyIfDivergentTarget = false; |
89 | |
90 | TargetTransformInfo *TTI = nullptr; |
91 | }; |
92 | } |
93 | |
94 | #endif // LLVM_TRANSFORMS_SCALAR_SPECULATIVEEXECUTION_H |
95 | |