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 #define NVVM_REFLECT_OCL_FUNCTION "__nvvm_reflect_ocl"
44 
45 using namespace llvm;
46 
47 #define DEBUG_TYPE "nvptx-reflect"
48 
49 namespace llvm { void initializeNVVMReflectPass(PassRegistry &); }
50 
51 namespace {
52 class NVVMReflect : public FunctionPass {
53 public:
54   static char ID;
55   unsigned int SmVersion;
56   NVVMReflect() : NVVMReflect(0) {}
57   explicit NVVMReflect(unsigned int Sm) : FunctionPass(ID), SmVersion(Sm) {
58     initializeNVVMReflectPass(*PassRegistry::getPassRegistry());
59   }
60 
61   bool runOnFunction(Function &) override;
62 };
63 }
64 
65 FunctionPass *llvm::createNVVMReflectPass(unsigned int SmVersion) {
66   return new NVVMReflect(SmVersion);
67 }
68 
69 static cl::opt<bool>
70 NVVMReflectEnabled("nvvm-reflect-enable", cl::init(true), cl::Hidden,
71                    cl::desc("NVVM reflection, enabled by default"));
72 
73 char NVVMReflect::ID = 0;
74 INITIALIZE_PASS(NVVMReflect, "nvvm-reflect",
75                 "Replace occurrences of __nvvm_reflect() calls with 0/1", false,
76                 false)
77 
78 static bool runNVVMReflect(Function &F, unsigned SmVersion) {
79   if (!NVVMReflectEnabled)
80     return false;
81 
82   if (F.getName() == NVVM_REFLECT_FUNCTION ||
83       F.getName() == NVVM_REFLECT_OCL_FUNCTION) {
84     assert(F.isDeclaration() && "_reflect function should not have a body");
85     assert(F.getReturnType()->isIntegerTy() &&
86            "_reflect's return type should be integer");
87     return false;
88   }
89 
90   SmallVector<Instruction *, 4> ToRemove;
91 
92   // Go through the calls in this function.  Each call to __nvvm_reflect or
93   // llvm.nvvm.reflect should be a CallInst with a ConstantArray argument.
94   // First validate that. If the c-string corresponding to the ConstantArray can
95   // be found successfully, see if it can be found in VarMap. If so, replace the
96   // uses of CallInst with the value found in VarMap. If not, replace the use
97   // with value 0.
98 
99   // The IR for __nvvm_reflect calls differs between CUDA versions.
100   //
101   // CUDA 6.5 and earlier uses this sequence:
102   //    %ptr = tail call i8* @llvm.nvvm.ptr.constant.to.gen.p0i8.p4i8
103   //        (i8 addrspace(4)* getelementptr inbounds
104   //           ([8 x i8], [8 x i8] addrspace(4)* @str, i32 0, i32 0))
105   //    %reflect = tail call i32 @__nvvm_reflect(i8* %ptr)
106   //
107   // The value returned by Sym->getOperand(0) is a Constant with a
108   // ConstantDataSequential operand which can be converted to string and used
109   // for lookup.
110   //
111   // CUDA 7.0 does it slightly differently:
112   //   %reflect = call i32 @__nvvm_reflect(i8* addrspacecast
113   //        (i8 addrspace(1)* getelementptr inbounds
114   //           ([8 x i8], [8 x i8] addrspace(1)* @str, i32 0, i32 0) to i8*))
115   //
116   // In this case, we get a Constant with a GlobalVariable operand and we need
117   // to dig deeper to find its initializer with the string we'll use for lookup.
118   for (Instruction &I : instructions(F)) {
119     CallInst *Call = dyn_cast<CallInst>(&I);
120     if (!Call)
121       continue;
122     Function *Callee = Call->getCalledFunction();
123     if (!Callee || (Callee->getName() != NVVM_REFLECT_FUNCTION &&
124                     Callee->getName() != NVVM_REFLECT_OCL_FUNCTION &&
125                     Callee->getIntrinsicID() != Intrinsic::nvvm_reflect))
126       continue;
127 
128     // FIXME: Improve error handling here and elsewhere in this pass.
129     assert(Call->getNumOperands() == 2 &&
130            "Wrong number of operands to __nvvm_reflect function");
131 
132     // In cuda 6.5 and earlier, we will have an extra constant-to-generic
133     // conversion of the string.
134     const Value *Str = Call->getArgOperand(0);
135     if (const CallInst *ConvCall = dyn_cast<CallInst>(Str)) {
136       // FIXME: Add assertions about ConvCall.
137       Str = ConvCall->getArgOperand(0);
138     }
139     // Pre opaque pointers we have a constant expression wrapping the constant
140     // string.
141     Str = Str->stripPointerCasts();
142     assert(isa<Constant>(Str) &&
143            "Format of __nvvm_reflect function not recognized");
144 
145     const Value *Operand = cast<Constant>(Str)->getOperand(0);
146     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Operand)) {
147       // For CUDA-7.0 style __nvvm_reflect calls, we need to find the operand's
148       // initializer.
149       assert(GV->hasInitializer() &&
150              "Format of _reflect function not recognized");
151       const Constant *Initializer = GV->getInitializer();
152       Operand = Initializer;
153     }
154 
155     assert(isa<ConstantDataSequential>(Operand) &&
156            "Format of _reflect function not recognized");
157     assert(cast<ConstantDataSequential>(Operand)->isCString() &&
158            "Format of _reflect function not recognized");
159 
160     StringRef ReflectArg = cast<ConstantDataSequential>(Operand)->getAsString();
161     ReflectArg = ReflectArg.substr(0, ReflectArg.size() - 1);
162     LLVM_DEBUG(dbgs() << "Arg of _reflect : " << ReflectArg << "\n");
163 
164     int ReflectVal = 0; // The default value is 0
165     if (ReflectArg == "__CUDA_FTZ") {
166       // Try to pull __CUDA_FTZ from the nvvm-reflect-ftz module flag.  Our
167       // choice here must be kept in sync with AutoUpgrade, which uses the same
168       // technique to detect whether ftz is enabled.
169       if (auto *Flag = mdconst::extract_or_null<ConstantInt>(
170               F.getParent()->getModuleFlag("nvvm-reflect-ftz")))
171         ReflectVal = Flag->getSExtValue();
172     } else if (ReflectArg == "__CUDA_ARCH") {
173       ReflectVal = SmVersion * 10;
174     }
175     Call->replaceAllUsesWith(ConstantInt::get(Call->getType(), ReflectVal));
176     ToRemove.push_back(Call);
177   }
178 
179   for (Instruction *I : ToRemove)
180     I->eraseFromParent();
181 
182   return ToRemove.size() > 0;
183 }
184 
185 bool NVVMReflect::runOnFunction(Function &F) {
186   return runNVVMReflect(F, SmVersion);
187 }
188 
189 NVVMReflectPass::NVVMReflectPass() : NVVMReflectPass(0) {}
190 
191 PreservedAnalyses NVVMReflectPass::run(Function &F,
192                                        FunctionAnalysisManager &AM) {
193   return runNVVMReflect(F, SmVersion) ? PreservedAnalyses::none()
194                                       : PreservedAnalyses::all();
195 }
196