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1 //===-- NVPTXLowerArgs.cpp - Lower arguments ------------------------------===//
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2 //
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3 // The LLVM Compiler Infrastructure
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4 //
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5 // This file is distributed under the University of Illinois Open Source
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6 // License. See LICENSE.TXT for details.
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7 //
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8 //===----------------------------------------------------------------------===//
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9 //
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10 //
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11 // Arguments to kernel and device functions are passed via param space,
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12 // which imposes certain restrictions:
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13 // http://docs.nvidia.com/cuda/parallel-thread-execution/#state-spaces
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14 //
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15 // Kernel parameters are read-only and accessible only via ld.param
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16 // instruction, directly or via a pointer. Pointers to kernel
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17 // arguments can't be converted to generic address space.
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18 //
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19 // Device function parameters are directly accessible via
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20 // ld.param/st.param, but taking the address of one returns a pointer
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21 // to a copy created in local space which *can't* be used with
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22 // ld.param/st.param.
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23 //
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24 // Copying a byval struct into local memory in IR allows us to enforce
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25 // the param space restrictions, gives the rest of IR a pointer w/o
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26 // param space restrictions, and gives us an opportunity to eliminate
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27 // the copy.
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28 //
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29 // Pointer arguments to kernel functions need more work to be lowered:
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30 //
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31 // 1. Convert non-byval pointer arguments of CUDA kernels to pointers in the
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32 // global address space. This allows later optimizations to emit
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33 // ld.global.*/st.global.* for accessing these pointer arguments. For
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34 // example,
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35 //
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36 // define void @foo(float* %input) {
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37 // %v = load float, float* %input, align 4
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38 // ...
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39 // }
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40 //
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41 // becomes
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42 //
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43 // define void @foo(float* %input) {
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44 // %input2 = addrspacecast float* %input to float addrspace(1)*
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45 // %input3 = addrspacecast float addrspace(1)* %input2 to float*
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46 // %v = load float, float* %input3, align 4
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47 // ...
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48 // }
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49 //
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50 // Later, NVPTXInferAddressSpaces will optimize it to
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51 //
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52 // define void @foo(float* %input) {
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53 // %input2 = addrspacecast float* %input to float addrspace(1)*
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54 // %v = load float, float addrspace(1)* %input2, align 4
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55 // ...
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56 // }
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57 //
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58 // 2. Convert pointers in a byval kernel parameter to pointers in the global
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59 // address space. As #2, it allows NVPTX to emit more ld/st.global. E.g.,
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60 //
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61 // struct S {
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62 // int *x;
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63 // int *y;
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64 // };
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65 // __global__ void foo(S s) {
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66 // int *b = s.y;
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67 // // use b
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68 // }
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69 //
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70 // "b" points to the global address space. In the IR level,
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71 //
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72 // define void @foo({i32*, i32*}* byval %input) {
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73 // %b_ptr = getelementptr {i32*, i32*}, {i32*, i32*}* %input, i64 0, i32 1
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74 // %b = load i32*, i32** %b_ptr
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75 // ; use %b
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76 // }
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77 //
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78 // becomes
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79 //
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80 // define void @foo({i32*, i32*}* byval %input) {
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81 // %b_ptr = getelementptr {i32*, i32*}, {i32*, i32*}* %input, i64 0, i32 1
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82 // %b = load i32*, i32** %b_ptr
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83 // %b_global = addrspacecast i32* %b to i32 addrspace(1)*
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84 // %b_generic = addrspacecast i32 addrspace(1)* %b_global to i32*
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85 // ; use %b_generic
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86 // }
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87 //
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88 // TODO: merge this pass with NVPTXInferAddressSpaces so that other passes don't
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89 // cancel the addrspacecast pair this pass emits.
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90 //===----------------------------------------------------------------------===//
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91
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92 #include "NVPTX.h"
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93 #include "NVPTXUtilities.h"
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94 #include "NVPTXTargetMachine.h"
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95 #include "llvm/Analysis/ValueTracking.h"
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96 #include "llvm/IR/Function.h"
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97 #include "llvm/IR/Instructions.h"
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98 #include "llvm/IR/Module.h"
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99 #include "llvm/IR/Type.h"
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100 #include "llvm/Pass.h"
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101
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102 using namespace llvm;
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103
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104 namespace llvm {
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105 void initializeNVPTXLowerArgsPass(PassRegistry &);
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106 }
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107
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108 namespace {
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109 class NVPTXLowerArgs : public FunctionPass {
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110 bool runOnFunction(Function &F) override;
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111
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112 bool runOnKernelFunction(Function &F);
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113 bool runOnDeviceFunction(Function &F);
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114
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115 // handle byval parameters
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116 void handleByValParam(Argument *Arg);
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117 // Knowing Ptr must point to the global address space, this function
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118 // addrspacecasts Ptr to global and then back to generic. This allows
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119 // NVPTXInferAddressSpaces to fold the global-to-generic cast into
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120 // loads/stores that appear later.
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121 void markPointerAsGlobal(Value *Ptr);
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122
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123 public:
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124 static char ID; // Pass identification, replacement for typeid
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125 NVPTXLowerArgs(const NVPTXTargetMachine *TM = nullptr)
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126 : FunctionPass(ID), TM(TM) {}
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127 StringRef getPassName() const override {
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128 return "Lower pointer arguments of CUDA kernels";
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129 }
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130
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131 private:
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132 const NVPTXTargetMachine *TM;
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133 };
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134 } // namespace
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135
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136 char NVPTXLowerArgs::ID = 1;
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137
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138 INITIALIZE_PASS(NVPTXLowerArgs, "nvptx-lower-args",
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139 "Lower arguments (NVPTX)", false, false)
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140
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141 // =============================================================================
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142 // If the function had a byval struct ptr arg, say foo(%struct.x* byval %d),
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143 // then add the following instructions to the first basic block:
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144 //
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145 // %temp = alloca %struct.x, align 8
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146 // %tempd = addrspacecast %struct.x* %d to %struct.x addrspace(101)*
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147 // %tv = load %struct.x addrspace(101)* %tempd
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148 // store %struct.x %tv, %struct.x* %temp, align 8
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149 //
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150 // The above code allocates some space in the stack and copies the incoming
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151 // struct from param space to local space.
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152 // Then replace all occurrences of %d by %temp.
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153 // =============================================================================
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154 void NVPTXLowerArgs::handleByValParam(Argument *Arg) {
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155 Function *Func = Arg->getParent();
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156 Instruction *FirstInst = &(Func->getEntryBlock().front());
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157 PointerType *PType = dyn_cast<PointerType>(Arg->getType());
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158
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159 assert(PType && "Expecting pointer type in handleByValParam");
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160
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161 Type *StructType = PType->getElementType();
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162 AllocaInst *AllocA = new AllocaInst(StructType, Arg->getName(), FirstInst);
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163 // Set the alignment to alignment of the byval parameter. This is because,
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164 // later load/stores assume that alignment, and we are going to replace
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165 // the use of the byval parameter with this alloca instruction.
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166 AllocA->setAlignment(Func->getParamAlignment(Arg->getArgNo() + 1));
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167 Arg->replaceAllUsesWith(AllocA);
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168
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169 Value *ArgInParam = new AddrSpaceCastInst(
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170 Arg, PointerType::get(StructType, ADDRESS_SPACE_PARAM), Arg->getName(),
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171 FirstInst);
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172 LoadInst *LI = new LoadInst(ArgInParam, Arg->getName(), FirstInst);
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173 new StoreInst(LI, AllocA, FirstInst);
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174 }
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175
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176 void NVPTXLowerArgs::markPointerAsGlobal(Value *Ptr) {
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177 if (Ptr->getType()->getPointerAddressSpace() == ADDRESS_SPACE_GLOBAL)
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178 return;
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179
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180 // Deciding where to emit the addrspacecast pair.
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181 BasicBlock::iterator InsertPt;
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182 if (Argument *Arg = dyn_cast<Argument>(Ptr)) {
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183 // Insert at the functon entry if Ptr is an argument.
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184 InsertPt = Arg->getParent()->getEntryBlock().begin();
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185 } else {
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186 // Insert right after Ptr if Ptr is an instruction.
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187 InsertPt = ++cast<Instruction>(Ptr)->getIterator();
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188 assert(InsertPt != InsertPt->getParent()->end() &&
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189 "We don't call this function with Ptr being a terminator.");
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190 }
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191
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192 Instruction *PtrInGlobal = new AddrSpaceCastInst(
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193 Ptr, PointerType::get(Ptr->getType()->getPointerElementType(),
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194 ADDRESS_SPACE_GLOBAL),
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195 Ptr->getName(), &*InsertPt);
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196 Value *PtrInGeneric = new AddrSpaceCastInst(PtrInGlobal, Ptr->getType(),
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197 Ptr->getName(), &*InsertPt);
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198 // Replace with PtrInGeneric all uses of Ptr except PtrInGlobal.
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199 Ptr->replaceAllUsesWith(PtrInGeneric);
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200 PtrInGlobal->setOperand(0, Ptr);
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201 }
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202
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203 // =============================================================================
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204 // Main function for this pass.
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205 // =============================================================================
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206 bool NVPTXLowerArgs::runOnKernelFunction(Function &F) {
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207 if (TM && TM->getDrvInterface() == NVPTX::CUDA) {
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208 // Mark pointers in byval structs as global.
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209 for (auto &B : F) {
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210 for (auto &I : B) {
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211 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
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212 if (LI->getType()->isPointerTy()) {
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213 Value *UO = GetUnderlyingObject(LI->getPointerOperand(),
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214 F.getParent()->getDataLayout());
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215 if (Argument *Arg = dyn_cast<Argument>(UO)) {
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216 if (Arg->hasByValAttr()) {
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217 // LI is a load from a pointer within a byval kernel parameter.
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218 markPointerAsGlobal(LI);
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219 }
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220 }
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221 }
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222 }
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223 }
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224 }
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225 }
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226
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227 for (Argument &Arg : F.args()) {
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228 if (Arg.getType()->isPointerTy()) {
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229 if (Arg.hasByValAttr())
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230 handleByValParam(&Arg);
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231 else if (TM && TM->getDrvInterface() == NVPTX::CUDA)
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232 markPointerAsGlobal(&Arg);
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233 }
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234 }
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235 return true;
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236 }
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237
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238 // Device functions only need to copy byval args into local memory.
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239 bool NVPTXLowerArgs::runOnDeviceFunction(Function &F) {
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240 for (Argument &Arg : F.args())
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241 if (Arg.getType()->isPointerTy() && Arg.hasByValAttr())
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242 handleByValParam(&Arg);
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243 return true;
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244 }
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245
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246 bool NVPTXLowerArgs::runOnFunction(Function &F) {
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247 return isKernelFunction(F) ? runOnKernelFunction(F) : runOnDeviceFunction(F);
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248 }
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249
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250 FunctionPass *
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251 llvm::createNVPTXLowerArgsPass(const NVPTXTargetMachine *TM) {
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252 return new NVPTXLowerArgs(TM);
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253 }
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