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| 14 | pmbaty | 1 | //===- SpeculativeExecution.h -----------------------------------*- C++ -*-===// |
| 2 | // |
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| 3 | // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
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| 4 | // See https://llvm.org/LICENSE.txt for license information. |
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| 5 | // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
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| 6 | // |
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| 7 | //===----------------------------------------------------------------------===// |
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| 8 | // |
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| 9 | // This pass hoists instructions to enable speculative execution on |
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| 10 | // targets where branches are expensive. This is aimed at GPUs. It |
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| 11 | // currently works on simple if-then and if-then-else |
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| 12 | // patterns. |
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| 13 | // |
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| 14 | // Removing branches is not the only motivation for this |
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| 15 | // pass. E.g. consider this code and assume that there is no |
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| 16 | // addressing mode for multiplying by sizeof(*a): |
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| 17 | // |
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| 18 | // if (b > 0) |
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| 19 | // c = a[i + 1] |
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| 20 | // if (d > 0) |
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| 21 | // e = a[i + 2] |
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| 22 | // |
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| 23 | // turns into |
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| 24 | // |
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| 25 | // p = &a[i + 1]; |
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| 26 | // if (b > 0) |
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| 27 | // c = *p; |
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| 28 | // q = &a[i + 2]; |
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| 29 | // if (d > 0) |
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| 30 | // e = *q; |
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| 31 | // |
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| 32 | // which could later be optimized to |
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| 33 | // |
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| 34 | // r = &a[i]; |
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| 35 | // if (b > 0) |
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| 36 | // c = r[1]; |
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| 37 | // if (d > 0) |
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| 38 | // e = r[2]; |
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| 39 | // |
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| 40 | // Later passes sink back much of the speculated code that did not enable |
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| 41 | // further optimization. |
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| 42 | // |
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| 43 | // This pass is more aggressive than the function SpeculativeyExecuteBB in |
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| 44 | // SimplifyCFG. SimplifyCFG will not speculate if no selects are introduced and |
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| 45 | // it will speculate at most one instruction. It also will not speculate if |
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| 46 | // there is a value defined in the if-block that is only used in the then-block. |
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| 47 | // These restrictions make sense since the speculation in SimplifyCFG seems |
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| 48 | // aimed at introducing cheap selects, while this pass is intended to do more |
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| 49 | // aggressive speculation while counting on later passes to either capitalize on |
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| 50 | // that or clean it up. |
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| 51 | // |
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| 52 | // If the pass was created by calling |
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| 53 | // createSpeculativeExecutionIfHasBranchDivergencePass or the |
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| 54 | // -spec-exec-only-if-divergent-target option is present, this pass only has an |
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| 55 | // effect on targets where TargetTransformInfo::hasBranchDivergence() is true; |
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| 56 | // on other targets, it is a nop. |
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| 57 | // |
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| 58 | // This lets you include this pass unconditionally in the IR pass pipeline, but |
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| 59 | // only enable it for relevant targets. |
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| 60 | // |
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| 61 | //===----------------------------------------------------------------------===// |
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| 62 | #ifndef LLVM_TRANSFORMS_SCALAR_SPECULATIVEEXECUTION_H |
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| 63 | #define LLVM_TRANSFORMS_SCALAR_SPECULATIVEEXECUTION_H |
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| 64 | |||
| 65 | #include "llvm/IR/PassManager.h" |
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| 66 | |||
| 67 | namespace llvm { |
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| 68 | class TargetTransformInfo; |
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| 69 | class SpeculativeExecutionPass |
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| 70 | : public PassInfoMixin<SpeculativeExecutionPass> { |
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| 71 | public: |
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| 72 | SpeculativeExecutionPass(bool OnlyIfDivergentTarget = false); |
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| 73 | |||
| 74 | PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); |
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| 75 | |||
| 76 | // Glue for old PM |
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| 77 | bool runImpl(Function &F, TargetTransformInfo *TTI); |
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| 78 | |||
| 79 | private: |
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| 80 | bool runOnBasicBlock(BasicBlock &B); |
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| 81 | bool considerHoistingFromTo(BasicBlock &FromBlock, BasicBlock &ToBlock); |
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| 82 | |||
| 83 | // If true, this pass is a nop unless the target architecture has branch |
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| 84 | // divergence. |
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| 85 | const bool OnlyIfDivergentTarget = false; |
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| 86 | |||
| 87 | TargetTransformInfo *TTI = nullptr; |
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| 88 | }; |
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| 89 | } |
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| 90 | |||
| 91 | #endif // LLVM_TRANSFORMS_SCALAR_SPECULATIVEEXECUTION_H |