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/ADT/Triple.h"
15 #include "llvm/Analysis/AliasAnalysis.h"
16 #include "llvm/Analysis/MemoryLocation.h"
17 #include "llvm/Analysis/ValueTracking.h"
18 #include "llvm/IR/Argument.h"
19 #include "llvm/IR/Attributes.h"
20 #include "llvm/IR/CallingConv.h"
21 #include "llvm/IR/Function.h"
22 #include "llvm/IR/GlobalVariable.h"
23 #include "llvm/IR/Instructions.h"
24 #include "llvm/IR/Type.h"
25 #include "llvm/IR/Value.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/Casting.h"
28 #include "llvm/Support/ErrorHandling.h"
29 #include <cassert>
30 
31 using namespace llvm;
32 
33 #define DEBUG_TYPE "amdgpu-aa"
34 
35 AnalysisKey AMDGPUAA::Key;
36 
37 // Register this pass...
38 char AMDGPUAAWrapperPass::ID = 0;
39 char AMDGPUExternalAAWrapper::ID = 0;
40 
41 INITIALIZE_PASS(AMDGPUAAWrapperPass, "amdgpu-aa",
42                 "AMDGPU Address space based Alias Analysis", false, true)
43 
44 INITIALIZE_PASS(AMDGPUExternalAAWrapper, "amdgpu-aa-wrapper",
45                 "AMDGPU Address space based Alias Analysis Wrapper", false, true)
46 
47 ImmutablePass *llvm::createAMDGPUAAWrapperPass() {
48   return new AMDGPUAAWrapperPass();
49 }
50 
51 ImmutablePass *llvm::createAMDGPUExternalAAWrapperPass() {
52   return new AMDGPUExternalAAWrapper();
53 }
54 
55 void AMDGPUAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
56   AU.setPreservesAll();
57 }
58 
59 // These arrays are indexed by address space value enum elements 0 ... to 7
60 static const AliasResult ASAliasRules[8][8] = {
61   /*                    Flat       Global    Region    Group     Constant  Private   Constant 32-bit  Buffer Fat Ptr */
62   /* Flat     */        {MayAlias, MayAlias, NoAlias,  MayAlias, MayAlias, MayAlias, MayAlias,        MayAlias},
63   /* Global   */        {MayAlias, MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , MayAlias,        MayAlias},
64   /* Region   */        {NoAlias,  NoAlias , MayAlias, NoAlias , NoAlias,  NoAlias , NoAlias,         NoAlias},
65   /* Group    */        {MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , NoAlias , NoAlias ,        NoAlias},
66   /* Constant */        {MayAlias, MayAlias, NoAlias,  NoAlias , NoAlias , NoAlias , MayAlias,        MayAlias},
67   /* Private  */        {MayAlias, NoAlias , NoAlias , NoAlias , NoAlias , MayAlias, NoAlias ,        NoAlias},
68   /* Constant 32-bit */ {MayAlias, MayAlias, NoAlias,  NoAlias , MayAlias, NoAlias , NoAlias ,        MayAlias},
69   /* Buffer Fat Ptr  */ {MayAlias, MayAlias, NoAlias , NoAlias , MayAlias, NoAlias , MayAlias,        MayAlias}
70 };
71 
72 static AliasResult getAliasResult(unsigned AS1, unsigned AS2) {
73   static_assert(AMDGPUAS::MAX_AMDGPU_ADDRESS <= 7, "Addr space out of range");
74 
75   if (AS1 > AMDGPUAS::MAX_AMDGPU_ADDRESS || AS2 > AMDGPUAS::MAX_AMDGPU_ADDRESS)
76     return MayAlias;
77 
78   return ASAliasRules[AS1][AS2];
79 }
80 
81 AliasResult AMDGPUAAResult::alias(const MemoryLocation &LocA,
82                                   const MemoryLocation &LocB,
83                                   AAQueryInfo &AAQI) {
84   unsigned asA = LocA.Ptr->getType()->getPointerAddressSpace();
85   unsigned asB = LocB.Ptr->getType()->getPointerAddressSpace();
86 
87   AliasResult Result = getAliasResult(asA, asB);
88   if (Result == NoAlias)
89     return Result;
90 
91   // In general, FLAT (generic) pointers could be aliased to LOCAL or PRIVATE
92   // pointers. However, as LOCAL or PRIVATE pointers point to local objects, in
93   // certain cases, it's still viable to check whether a FLAT pointer won't
94   // alias to a LOCAL or PRIVATE pointer.
95   MemoryLocation A = LocA;
96   MemoryLocation B = LocB;
97   // Canonicalize the location order to simplify the following alias check.
98   if (asA != AMDGPUAS::FLAT_ADDRESS) {
99     std::swap(asA, asB);
100     std::swap(A, B);
101   }
102   if (asA == AMDGPUAS::FLAT_ADDRESS &&
103       (asB == AMDGPUAS::LOCAL_ADDRESS || asB == AMDGPUAS::PRIVATE_ADDRESS)) {
104     const auto *ObjA =
105         getUnderlyingObject(A.Ptr->stripPointerCastsAndInvariantGroups());
106     if (const LoadInst *LI = dyn_cast<LoadInst>(ObjA)) {
107       // If a generic pointer is loaded from the constant address space, it
108       // could only be a GLOBAL or CONSTANT one as that address space is soley
109       // prepared on the host side, where only GLOBAL or CONSTANT variables are
110       // visible. Note that this even holds for regular functions.
111       if (LI->getPointerAddressSpace() == AMDGPUAS::CONSTANT_ADDRESS)
112         return NoAlias;
113     } else if (const Argument *Arg = dyn_cast<Argument>(ObjA)) {
114       const Function *F = Arg->getParent();
115       switch (F->getCallingConv()) {
116       case CallingConv::AMDGPU_KERNEL:
117         // In the kernel function, kernel arguments won't alias to (local)
118         // variables in shared or private address space.
119         return NoAlias;
120       default:
121         // TODO: In the regular function, if that local variable in the
122         // location B is not captured, that argument pointer won't alias to it
123         // as well.
124         break;
125       }
126     }
127   }
128 
129   // Forward the query to the next alias analysis.
130   return AAResultBase::alias(LocA, LocB, AAQI);
131 }
132 
133 bool AMDGPUAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
134                                             AAQueryInfo &AAQI, bool OrLocal) {
135   unsigned AS = Loc.Ptr->getType()->getPointerAddressSpace();
136   if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
137       AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
138     return true;
139 
140   const Value *Base = getUnderlyingObject(Loc.Ptr);
141   AS = Base->getType()->getPointerAddressSpace();
142   if (AS == AMDGPUAS::CONSTANT_ADDRESS ||
143       AS == AMDGPUAS::CONSTANT_ADDRESS_32BIT)
144     return true;
145 
146   if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
147     if (GV->isConstant())
148       return true;
149   } else if (const Argument *Arg = dyn_cast<Argument>(Base)) {
150     const Function *F = Arg->getParent();
151 
152     // Only assume constant memory for arguments on kernels.
153     switch (F->getCallingConv()) {
154     default:
155       return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
156     case CallingConv::AMDGPU_LS:
157     case CallingConv::AMDGPU_HS:
158     case CallingConv::AMDGPU_ES:
159     case CallingConv::AMDGPU_GS:
160     case CallingConv::AMDGPU_VS:
161     case CallingConv::AMDGPU_PS:
162     case CallingConv::AMDGPU_CS:
163     case CallingConv::AMDGPU_KERNEL:
164     case CallingConv::SPIR_KERNEL:
165       break;
166     }
167 
168     unsigned ArgNo = Arg->getArgNo();
169     /* On an argument, ReadOnly attribute indicates that the function does
170        not write through this pointer argument, even though it may write
171        to the memory that the pointer points to.
172        On an argument, ReadNone attribute indicates that the function does
173        not dereference that pointer argument, even though it may read or write
174        the memory that the pointer points to if accessed through other pointers.
175      */
176     if (F->hasParamAttribute(ArgNo, Attribute::NoAlias) &&
177         (F->hasParamAttribute(ArgNo, Attribute::ReadNone) ||
178          F->hasParamAttribute(ArgNo, Attribute::ReadOnly))) {
179       return true;
180     }
181   }
182   return AAResultBase::pointsToConstantMemory(Loc, AAQI, OrLocal);
183 }
184