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| 1 | +//===-- NVPTXLowerUnreachable.cpp - Lower unreachables to exit =====--===// |
| 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 | +// PTX does not have a notion of `unreachable`, which results in emitted basic |
| 10 | +// blocks having an edge to the next block: |
| 11 | +// |
| 12 | +// block1: |
| 13 | +// call @does_not_return(); |
| 14 | +// // unreachable |
| 15 | +// block2: |
| 16 | +// // ptxas will create a CFG edge from block1 to block2 |
| 17 | +// |
| 18 | +// This may result in significant changes to the control flow graph, e.g., when |
| 19 | +// LLVM moves unreachable blocks to the end of the function. That's a problem |
| 20 | +// in the context of divergent control flow, as `ptxas` uses the CFG to |
| 21 | +// determine divergent regions, and some intructions may not be executed |
| 22 | +// divergently. |
| 23 | +// |
| 24 | +// For example, `bar.sync` is not allowed to be executed divergently on Pascal |
| 25 | +// or earlier. If we start with the following: |
| 26 | +// |
| 27 | +// entry: |
| 28 | +// // start of divergent region |
| 29 | +// @%p0 bra cont; |
| 30 | +// @%p1 bra unlikely; |
| 31 | +// ... |
| 32 | +// bra.uni cont; |
| 33 | +// unlikely: |
| 34 | +// ... |
| 35 | +// // unreachable |
| 36 | +// cont: |
| 37 | +// // end of divergent region |
| 38 | +// bar.sync 0; |
| 39 | +// bra.uni exit; |
| 40 | +// exit: |
| 41 | +// ret; |
| 42 | +// |
| 43 | +// it is transformed by the branch-folder and block-placement passes to: |
| 44 | +// |
| 45 | +// entry: |
| 46 | +// // start of divergent region |
| 47 | +// @%p0 bra cont; |
| 48 | +// @%p1 bra unlikely; |
| 49 | +// ... |
| 50 | +// bra.uni cont; |
| 51 | +// cont: |
| 52 | +// bar.sync 0; |
| 53 | +// bra.uni exit; |
| 54 | +// unlikely: |
| 55 | +// ... |
| 56 | +// // unreachable |
| 57 | +// exit: |
| 58 | +// // end of divergent region |
| 59 | +// ret; |
| 60 | +// |
| 61 | +// After moving the `unlikely` block to the end of the function, it has an edge |
| 62 | +// to the `exit` block, which widens the divergent region and makes the |
| 63 | +// `bar.sync` instruction happen divergently. |
| 64 | +// |
| 65 | +// To work around this, we add an `exit` instruction before every `unreachable`, |
| 66 | +// as `ptxas` understands that exit terminates the CFG. Note that `trap` is not |
| 67 | +// equivalent, and only future versions of `ptxas` will model it like `exit`. |
| 68 | +// |
| 69 | +//===----------------------------------------------------------------------===// |
| 70 | + |
| 71 | +#include "NVPTX.h" |
| 72 | +#include "llvm/IR/Function.h" |
| 73 | +#include "llvm/IR/InlineAsm.h" |
| 74 | +#include "llvm/IR/Instructions.h" |
| 75 | +#include "llvm/IR/Type.h" |
| 76 | +#include "llvm/Pass.h" |
| 77 | + |
| 78 | +using namespace llvm; |
| 79 | + |
| 80 | +namespace llvm { |
| 81 | +void initializeNVPTXLowerUnreachablePass(PassRegistry &); |
| 82 | +} |
| 83 | + |
| 84 | +namespace { |
| 85 | +class NVPTXLowerUnreachable : public FunctionPass { |
| 86 | + bool runOnFunction(Function &F) override; |
| 87 | + |
| 88 | +public: |
| 89 | + static char ID; // Pass identification, replacement for typeid |
| 90 | + NVPTXLowerUnreachable() : FunctionPass(ID) {} |
| 91 | + StringRef getPassName() const override { |
| 92 | + return "add an exit instruction before every unreachable"; |
| 93 | + } |
| 94 | +}; |
| 95 | +} // namespace |
| 96 | + |
| 97 | +char NVPTXLowerUnreachable::ID = 1; |
| 98 | + |
| 99 | +INITIALIZE_PASS(NVPTXLowerUnreachable, "nvptx-lower-unreachable", |
| 100 | + "Lower Unreachable", false, false) |
| 101 | + |
| 102 | +// ============================================================================= |
| 103 | +// Main function for this pass. |
| 104 | +// ============================================================================= |
| 105 | +bool NVPTXLowerUnreachable::runOnFunction(Function &F) { |
| 106 | + if (skipFunction(F)) |
| 107 | + return false; |
| 108 | + |
| 109 | + LLVMContext &C = F.getContext(); |
| 110 | + FunctionType *ExitFTy = FunctionType::get(Type::getVoidTy(C), false); |
| 111 | + InlineAsm *Exit = InlineAsm::get(ExitFTy, "exit;", "", true); |
| 112 | + |
| 113 | + bool Changed = false; |
| 114 | + for (auto &BB : F) |
| 115 | + for (auto &I : BB) { |
| 116 | + if (auto unreachableInst = dyn_cast<UnreachableInst>(&I)) { |
| 117 | + Changed = true; |
| 118 | + CallInst::Create(ExitFTy, Exit, "", unreachableInst); |
| 119 | + } |
| 120 | + } |
| 121 | + return Changed; |
| 122 | +} |
| 123 | + |
| 124 | +FunctionPass *llvm::createNVPTXLowerUnreachablePass() { |
| 125 | + return new NVPTXLowerUnreachable(); |
| 126 | +} |
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