1 //===- AMDGPUAliasAnalysis ------------------------------------------------===//
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 /// \file
9 /// This is the AMGPU address space based alias analysis pass.
10 //===----------------------------------------------------------------------===//
11 
12 #include "AMDGPUAliasAnalysis.h"
13 #include "AMDGPU.h"
14 #include "llvm/Analysis/ValueTracking.h"
15 #include "llvm/IR/Instructions.h"
16 
17 using namespace llvm;
18 
19 #define DEBUG_TYPE "amdgpu-aa"
20 
21 AnalysisKey AMDGPUAA::Key;
22 
23 // Register this pass...
24 char AMDGPUAAWrapperPass::ID = 0;
25 char AMDGPUExternalAAWrapper::ID = 0;
26 
27 INITIALIZE_PASS(AMDGPUAAWrapperPass, "amdgpu-aa",
28                 "AMDGPU Address space based Alias Analysis", false, true)
29 
30 INITIALIZE_PASS(AMDGPUExternalAAWrapper, "amdgpu-aa-wrapper",
31                 "AMDGPU Address space based Alias Analysis Wrapper", false, true)
32 
33 ImmutablePass *llvm::createAMDGPUAAWrapperPass() {
34   return new AMDGPUAAWrapperPass();
35 }
36 
37 ImmutablePass *llvm::createAMDGPUExternalAAWrapperPass() {
38   return new AMDGPUExternalAAWrapper();
39 }
40 
41 AMDGPUAAWrapperPass::AMDGPUAAWrapperPass() : ImmutablePass(ID) {
42   initializeAMDGPUAAWrapperPassPass(*PassRegistry::getPassRegistry());
43 }
44 
45 void AMDGPUAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
46   AU.setPreservesAll();
47 }
48 
49 static AliasResult getAliasResult(unsigned AS1, unsigned AS2) {
50   static_assert(AMDGPUAS::MAX_AMDGPU_ADDRESS <= 7, "Addr space out of range");
51 
52   if (AS1 > AMDGPUAS::MAX_AMDGPU_ADDRESS || AS2 > AMDGPUAS::MAX_AMDGPU_ADDRESS)
53     return AliasResult::MayAlias;
54 
55 #define ASMay AliasResult::MayAlias
56 #define ASNo AliasResult::NoAlias
57   // This array is indexed by address space value enum elements 0 ... to 7
58   static const AliasResult ASAliasRules[8][8] = {
59     /*                    Flat    Global Region Group  Constant Private Const32 Buf Fat Ptr */
60     /* Flat     */        {ASMay, ASMay, ASNo,  ASMay, ASMay,   ASMay,  ASMay,  ASMay},
61     /* Global   */        {ASMay, ASMay, ASNo,  ASNo,  ASMay,   ASNo,   ASMay,  ASMay},
62     /* Region   */        {ASNo,  ASNo,  ASMay, ASNo,  ASNo,    ASNo,   ASNo,   ASNo},
63     /* Group    */        {ASMay, ASNo,  ASNo,  ASMay, ASNo,    ASNo,   ASNo,   ASNo},
64     /* Constant */        {ASMay, ASMay, ASNo,  ASNo,  ASNo,    ASNo,   ASMay,  ASMay},
65     /* Private  */        {ASMay, ASNo,  ASNo,  ASNo,  ASNo,    ASMay,  ASNo,   ASNo},
66     /* Constant 32-bit */ {ASMay, ASMay, ASNo,  ASNo,  ASMay,   ASNo,   ASNo,   ASMay},
67     /* Buffer Fat Ptr  */ {ASMay, ASMay, ASNo,  ASNo,  ASMay,   ASNo,   ASMay,  ASMay}
68   };
69 #undef ASMay
70 #undef ASNo
71 
72   return ASAliasRules[AS1][AS2];
73 }
74 
75 AliasResult AMDGPUAAResult::alias(const MemoryLocation &LocA,
76                                   const MemoryLocation &LocB,
77                                   AAQueryInfo &AAQI) {
78   unsigned asA = LocA.Ptr->getType()->getPointerAddressSpace();
79   unsigned asB = LocB.Ptr->getType()->getPointerAddressSpace();
80 
81   AliasResult Result = getAliasResult(asA, asB);
82   if (Result == AliasResult::NoAlias)
83     return Result;
84 
85   // In general, FLAT (generic) pointers could be aliased to LOCAL or PRIVATE
86   // pointers. However, as LOCAL or PRIVATE pointers point to local objects, in
87   // certain cases, it's still viable to check whether a FLAT pointer won't
88   // alias to a LOCAL or PRIVATE pointer.
89   MemoryLocation A = LocA;
90   MemoryLocation B = LocB;
91   // Canonicalize the location order to simplify the following alias check.
92   if (asA != AMDGPUAS::FLAT_ADDRESS) {
93     std::swap(asA, asB);
94     std::swap(A, B);
95   }
96   if (asA == AMDGPUAS::FLAT_ADDRESS &&
97       (asB == AMDGPUAS::LOCAL_ADDRESS || asB == AMDGPUAS::PRIVATE_ADDRESS)) {
98     const auto *ObjA =
99         getUnderlyingObject(A.Ptr->stripPointerCastsForAliasAnalysis());
100     if (const LoadInst *LI = dyn_cast<LoadInst>(ObjA)) {
101       // If a generic pointer is loaded from the constant address space, it
102       // could only be a GLOBAL or CONSTANT one as that address space is solely
103       // prepared on the host side, where only GLOBAL or CONSTANT variables are
104       // visible. Note that this even holds for regular functions.
105       if (LI->getPointerAddressSpace() == AMDGPUAS::CONSTANT_ADDRESS)
106         return AliasResult::NoAlias;
107     } else if (const Argument *Arg = dyn_cast<Argument>(ObjA)) {
108       const Function *F = Arg->getParent();
109       switch (F->getCallingConv()) {
110       case CallingConv::AMDGPU_KERNEL:
111         // In the kernel function, kernel arguments won't alias to (local)
112         // variables in shared or private address space.
113         return AliasResult::NoAlias;
114       default:
115         // TODO: In the regular function, if that local variable in the
116         // location B is not captured, that argument pointer won't alias to it
117         // as well.
118         break;
119       }
120     }
121   }
122 
123   // Forward the query to the next alias analysis.
124   return AAResultBase::alias(LocA, LocB, AAQI);
125 }
126 
127 bool AMDGPUAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
128                                             AAQueryInfo &AAQI, bool OrLocal) {
129   unsigned AS = Loc.Ptr->getType()->getPointerAddressSpace();
130   if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
131       AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
132     return true;
133 
134   const Value *Base = getUnderlyingObject(Loc.Ptr);
135   AS = Base->getType()->getPointerAddressSpace();
136   if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
137       AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
138     return true;
139 
140   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
141     if (GV->isConstant())
142       return true;
143   } else if (const Argument *Arg = dyn_cast<Argument>(Base)) {
144     const Function *F = Arg->getParent();
145 
146     // Only assume constant memory for arguments on kernels.
147     switch (F->getCallingConv()) {
148     default:
149       return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
150     case CallingConv::AMDGPU_LS:
151     case CallingConv::AMDGPU_HS:
152     case CallingConv::AMDGPU_ES:
153     case CallingConv::AMDGPU_GS:
154     case CallingConv::AMDGPU_VS:
155     case CallingConv::AMDGPU_PS:
156     case CallingConv::AMDGPU_CS:
157     case CallingConv::AMDGPU_KERNEL:
158     case CallingConv::SPIR_KERNEL:
159       break;
160     }
161 
162     unsigned ArgNo = Arg->getArgNo();
163     /* On an argument, ReadOnly attribute indicates that the function does
164        not write through this pointer argument, even though it may write
165        to the memory that the pointer points to.
166        On an argument, ReadNone attribute indicates that the function does
167        not dereference that pointer argument, even though it may read or write
168        the memory that the pointer points to if accessed through other pointers.
169      */
170     if (F->hasParamAttribute(ArgNo, Attribute::NoAlias) &&
171         (F->hasParamAttribute(ArgNo, Attribute::ReadNone) ||
172          F->hasParamAttribute(ArgNo, Attribute::ReadOnly))) {
173       return true;
174     }
175   }
176   return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
177 }
178