1 //===- NVVMReflect.cpp - NVVM Emulate conditional compilation -------------===//
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 replaces occurrences of __nvvm_reflect("foo") and llvm.nvvm.reflect
10 // with an integer.
11 //
12 // We choose the value we use by looking at metadata in the module itself.  Note
13 // that we intentionally only have one way to choose these values, because other
14 // parts of LLVM (particularly, InstCombineCall) rely on being able to predict
15 // the values chosen by this pass.
16 //
17 // If we see an unknown string, we replace its call with 0.
18 //
19 //===----------------------------------------------------------------------===//
20 
21 #include "NVPTX.h"
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/StringMap.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DerivedTypes.h"
26 #include "llvm/IR/Function.h"
27 #include "llvm/IR/InstIterator.h"
28 #include "llvm/IR/Instructions.h"
29 #include "llvm/IR/Intrinsics.h"
30 #include "llvm/IR/IntrinsicsNVPTX.h"
31 #include "llvm/IR/Module.h"
32 #include "llvm/IR/PassManager.h"
33 #include "llvm/IR/Type.h"
34 #include "llvm/Pass.h"
35 #include "llvm/Support/CommandLine.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/raw_os_ostream.h"
38 #include "llvm/Support/raw_ostream.h"
39 #include "llvm/Transforms/Scalar.h"
40 #include <sstream>
41 #include <string>
42 #define NVVM_REFLECT_FUNCTION "__nvvm_reflect"
43 
44 using namespace llvm;
45 
46 #define DEBUG_TYPE "nvptx-reflect"
47 
48 namespace llvm { void initializeNVVMReflectPass(PassRegistry &); }
49 
50 namespace {
51 class NVVMReflect : public FunctionPass {
52 public:
53   static char ID;
54   unsigned int SmVersion;
55   NVVMReflect() : NVVMReflect(0) {}
56   explicit NVVMReflect(unsigned int Sm) : FunctionPass(ID), SmVersion(Sm) {
57     initializeNVVMReflectPass(*PassRegistry::getPassRegistry());
58   }
59 
60   bool runOnFunction(Function &) override;
61 };
62 }
63 
64 FunctionPass *llvm::createNVVMReflectPass(unsigned int SmVersion) {
65   return new NVVMReflect(SmVersion);
66 }
67 
68 static cl::opt<bool>
69 NVVMReflectEnabled("nvvm-reflect-enable", cl::init(true), cl::Hidden,
70                    cl::desc("NVVM reflection, enabled by default"));
71 
72 char NVVMReflect::ID = 0;
73 INITIALIZE_PASS(NVVMReflect, "nvvm-reflect",
74                 "Replace occurrences of __nvvm_reflect() calls with 0/1", false,
75                 false)
76 
77 static bool runNVVMReflect(Function &F, unsigned SmVersion) {
78   if (!NVVMReflectEnabled)
79     return false;
80 
81   if (F.getName() == NVVM_REFLECT_FUNCTION) {
82     assert(F.isDeclaration() && "_reflect function should not have a body");
83     assert(F.getReturnType()->isIntegerTy() &&
84            "_reflect's return type should be integer");
85     return false;
86   }
87 
88   SmallVector<Instruction *, 4> ToRemove;
89 
90   // Go through the calls in this function.  Each call to __nvvm_reflect or
91   // llvm.nvvm.reflect should be a CallInst with a ConstantArray argument.
92   // First validate that. If the c-string corresponding to the ConstantArray can
93   // be found successfully, see if it can be found in VarMap. If so, replace the
94   // uses of CallInst with the value found in VarMap. If not, replace the use
95   // with value 0.
96 
97   // The IR for __nvvm_reflect calls differs between CUDA versions.
98   //
99   // CUDA 6.5 and earlier uses this sequence:
100   //    %ptr = tail call i8* @llvm.nvvm.ptr.constant.to.gen.p0i8.p4i8
101   //        (i8 addrspace(4)* getelementptr inbounds
102   //           ([8 x i8], [8 x i8] addrspace(4)* @str, i32 0, i32 0))
103   //    %reflect = tail call i32 @__nvvm_reflect(i8* %ptr)
104   //
105   // The value returned by Sym->getOperand(0) is a Constant with a
106   // ConstantDataSequential operand which can be converted to string and used
107   // for lookup.
108   //
109   // CUDA 7.0 does it slightly differently:
110   //   %reflect = call i32 @__nvvm_reflect(i8* addrspacecast
111   //        (i8 addrspace(1)* getelementptr inbounds
112   //           ([8 x i8], [8 x i8] addrspace(1)* @str, i32 0, i32 0) to i8*))
113   //
114   // In this case, we get a Constant with a GlobalVariable operand and we need
115   // to dig deeper to find its initializer with the string we'll use for lookup.
116   for (Instruction &I : instructions(F)) {
117     CallInst *Call = dyn_cast<CallInst>(&I);
118     if (!Call)
119       continue;
120     Function *Callee = Call->getCalledFunction();
121     if (!Callee || (Callee->getName() != NVVM_REFLECT_FUNCTION &&
122                     Callee->getIntrinsicID() != Intrinsic::nvvm_reflect))
123       continue;
124 
125     // FIXME: Improve error handling here and elsewhere in this pass.
126     assert(Call->getNumOperands() == 2 &&
127            "Wrong number of operands to __nvvm_reflect function");
128 
129     // In cuda 6.5 and earlier, we will have an extra constant-to-generic
130     // conversion of the string.
131     const Value *Str = Call->getArgOperand(0);
132     if (const CallInst *ConvCall = dyn_cast<CallInst>(Str)) {
133       // FIXME: Add assertions about ConvCall.
134       Str = ConvCall->getArgOperand(0);
135     }
136     assert(isa<ConstantExpr>(Str) &&
137            "Format of __nvvm__reflect function not recognized");
138     const ConstantExpr *GEP = cast<ConstantExpr>(Str);
139 
140     const Value *Sym = GEP->getOperand(0);
141     assert(isa<Constant>(Sym) &&
142            "Format of __nvvm_reflect function not recognized");
143 
144     const Value *Operand = cast<Constant>(Sym)->getOperand(0);
145     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Operand)) {
146       // For CUDA-7.0 style __nvvm_reflect calls, we need to find the operand's
147       // initializer.
148       assert(GV->hasInitializer() &&
149              "Format of _reflect function not recognized");
150       const Constant *Initializer = GV->getInitializer();
151       Operand = Initializer;
152     }
153 
154     assert(isa<ConstantDataSequential>(Operand) &&
155            "Format of _reflect function not recognized");
156     assert(cast<ConstantDataSequential>(Operand)->isCString() &&
157            "Format of _reflect function not recognized");
158 
159     StringRef ReflectArg = cast<ConstantDataSequential>(Operand)->getAsString();
160     ReflectArg = ReflectArg.substr(0, ReflectArg.size() - 1);
161     LLVM_DEBUG(dbgs() << "Arg of _reflect : " << ReflectArg << "\n");
162 
163     int ReflectVal = 0; // The default value is 0
164     if (ReflectArg == "__CUDA_FTZ") {
165       // Try to pull __CUDA_FTZ from the nvvm-reflect-ftz module flag.  Our
166       // choice here must be kept in sync with AutoUpgrade, which uses the same
167       // technique to detect whether ftz is enabled.
168       if (auto *Flag = mdconst::extract_or_null<ConstantInt>(
169               F.getParent()->getModuleFlag("nvvm-reflect-ftz")))
170         ReflectVal = Flag->getSExtValue();
171     } else if (ReflectArg == "__CUDA_ARCH") {
172       ReflectVal = SmVersion * 10;
173     }
174     Call->replaceAllUsesWith(ConstantInt::get(Call->getType(), ReflectVal));
175     ToRemove.push_back(Call);
176   }
177 
178   for (Instruction *I : ToRemove)
179     I->eraseFromParent();
180 
181   return ToRemove.size() > 0;
182 }
183 
184 bool NVVMReflect::runOnFunction(Function &F) {
185   return runNVVMReflect(F, SmVersion);
186 }
187 
188 NVVMReflectPass::NVVMReflectPass() : NVVMReflectPass(0) {}
189 
190 PreservedAnalyses NVVMReflectPass::run(Function &F,
191                                        FunctionAnalysisManager &AM) {
192   return runNVVMReflect(F, SmVersion) ? PreservedAnalyses::none()
193                                       : PreservedAnalyses::all();
194 }
195