1 //===-- PPCTOCRegDeps.cpp - Add Extra TOC Register Dependencies -----------===// 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 // When resolving an address using the ELF ABI TOC pointer, two relocations are 10 // generally required: one for the high part and one for the low part. Only 11 // the high part generally explicitly depends on r2 (the TOC pointer). And, so, 12 // we might produce code like this: 13 // 14 // .Ltmp526: 15 // addis 3, 2, .LC12@toc@ha 16 // .Ltmp1628: 17 // std 2, 40(1) 18 // ld 5, 0(27) 19 // ld 2, 8(27) 20 // ld 11, 16(27) 21 // ld 3, .LC12@toc@l(3) 22 // rldicl 4, 4, 0, 32 23 // mtctr 5 24 // bctrl 25 // ld 2, 40(1) 26 // 27 // And there is nothing wrong with this code, as such, but there is a linker bug 28 // in binutils (https://sourceware.org/bugzilla/show_bug.cgi?id=18414) that will 29 // misoptimize this code sequence to this: 30 // nop 31 // std r2,40(r1) 32 // ld r5,0(r27) 33 // ld r2,8(r27) 34 // ld r11,16(r27) 35 // ld r3,-32472(r2) 36 // clrldi r4,r4,32 37 // mtctr r5 38 // bctrl 39 // ld r2,40(r1) 40 // because the linker does not know (and does not check) that the value in r2 41 // changed in between the instruction using the .LC12@toc@ha (TOC-relative) 42 // relocation and the instruction using the .LC12@toc@l(3) relocation. 43 // Because it finds these instructions using the relocations (and not by 44 // scanning the instructions), it has been asserted that there is no good way 45 // to detect the change of r2 in between. As a result, this bug may never be 46 // fixed (i.e. it may become part of the definition of the ABI). GCC was 47 // updated to add extra dependencies on r2 to instructions using the @toc@l 48 // relocations to avoid this problem, and we'll do the same here. 49 // 50 // This is done as a separate pass because: 51 // 1. These extra r2 dependencies are not really properties of the 52 // instructions, but rather due to a linker bug, and maybe one day we'll be 53 // able to get rid of them when targeting linkers without this bug (and, 54 // thus, keeping the logic centralized here will make that 55 // straightforward). 56 // 2. There are ISel-level peephole optimizations that propagate the @toc@l 57 // relocations to some user instructions, and so the exta dependencies do 58 // not apply only to a fixed set of instructions (without undesirable 59 // definition replication). 60 // 61 //===----------------------------------------------------------------------===// 62 63 #include "PPC.h" 64 #include "PPCInstrInfo.h" 65 #include "PPCTargetMachine.h" 66 #include "llvm/ADT/STLExtras.h" 67 #include "llvm/ADT/Statistic.h" 68 #include "llvm/CodeGen/MachineFrameInfo.h" 69 #include "llvm/CodeGen/MachineFunctionPass.h" 70 #include "llvm/CodeGen/MachineInstr.h" 71 #include "llvm/Support/ErrorHandling.h" 72 73 using namespace llvm; 74 75 #define DEBUG_TYPE "ppc-toc-reg-deps" 76 77 namespace { 78 // PPCTOCRegDeps pass - For simple functions without epilogue code, move 79 // returns up, and create conditional returns, to avoid unnecessary 80 // branch-to-blr sequences. 81 struct PPCTOCRegDeps : public MachineFunctionPass { 82 static char ID; 83 PPCTOCRegDeps() : MachineFunctionPass(ID) { 84 initializePPCTOCRegDepsPass(*PassRegistry::getPassRegistry()); 85 } 86 87 protected: 88 bool hasTOCLoReloc(const MachineInstr &MI) { 89 if (MI.getOpcode() == PPC::LDtocL || MI.getOpcode() == PPC::ADDItocL8 || 90 MI.getOpcode() == PPC::LWZtocL) 91 return true; 92 93 for (const MachineOperand &MO : MI.operands()) { 94 if (MO.getTargetFlags() == PPCII::MO_TOC_LO) 95 return true; 96 } 97 98 return false; 99 } 100 101 bool processBlock(MachineBasicBlock &MBB) { 102 bool Changed = false; 103 104 const bool isPPC64 = 105 MBB.getParent()->getSubtarget<PPCSubtarget>().isPPC64(); 106 const unsigned TOCReg = isPPC64 ? PPC::X2 : PPC::R2; 107 108 for (auto &MI : MBB) { 109 if (!hasTOCLoReloc(MI)) 110 continue; 111 112 MI.addOperand(MachineOperand::CreateReg(TOCReg, 113 false /*IsDef*/, 114 true /*IsImp*/)); 115 Changed = true; 116 } 117 118 return Changed; 119 } 120 121 public: 122 bool runOnMachineFunction(MachineFunction &MF) override { 123 bool Changed = false; 124 125 for (MachineBasicBlock &B : llvm::make_early_inc_range(MF)) 126 if (processBlock(B)) 127 Changed = true; 128 129 return Changed; 130 } 131 132 void getAnalysisUsage(AnalysisUsage &AU) const override { 133 MachineFunctionPass::getAnalysisUsage(AU); 134 } 135 }; 136 } 137 138 INITIALIZE_PASS(PPCTOCRegDeps, DEBUG_TYPE, 139 "PowerPC TOC Register Dependencies", false, false) 140 141 char PPCTOCRegDeps::ID = 0; 142 FunctionPass* 143 llvm::createPPCTOCRegDepsPass() { return new PPCTOCRegDeps(); } 144 145