1 //===-- NVPTXTargetTransformInfo.cpp - NVPTX specific TTI -----------------===//
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 #include "NVPTXTargetTransformInfo.h"
10 #include "NVPTXUtilities.h"
11 #include "llvm/Analysis/LoopInfo.h"
12 #include "llvm/Analysis/TargetTransformInfo.h"
13 #include "llvm/Analysis/ValueTracking.h"
14 #include "llvm/CodeGen/BasicTTIImpl.h"
15 #include "llvm/CodeGen/CostTable.h"
16 #include "llvm/CodeGen/TargetLowering.h"
17 #include "llvm/IR/IntrinsicsNVPTX.h"
18 #include "llvm/Support/Debug.h"
19 using namespace llvm;
20
21 #define DEBUG_TYPE "NVPTXtti"
22
23 // Whether the given intrinsic reads threadIdx.x/y/z.
readsThreadIndex(const IntrinsicInst * II)24 static bool readsThreadIndex(const IntrinsicInst *II) {
25 switch (II->getIntrinsicID()) {
26 default: return false;
27 case Intrinsic::nvvm_read_ptx_sreg_tid_x:
28 case Intrinsic::nvvm_read_ptx_sreg_tid_y:
29 case Intrinsic::nvvm_read_ptx_sreg_tid_z:
30 return true;
31 }
32 }
33
readsLaneId(const IntrinsicInst * II)34 static bool readsLaneId(const IntrinsicInst *II) {
35 return II->getIntrinsicID() == Intrinsic::nvvm_read_ptx_sreg_laneid;
36 }
37
38 // Whether the given intrinsic is an atomic instruction in PTX.
isNVVMAtomic(const IntrinsicInst * II)39 static bool isNVVMAtomic(const IntrinsicInst *II) {
40 switch (II->getIntrinsicID()) {
41 default: return false;
42 case Intrinsic::nvvm_atomic_load_inc_32:
43 case Intrinsic::nvvm_atomic_load_dec_32:
44
45 case Intrinsic::nvvm_atomic_add_gen_f_cta:
46 case Intrinsic::nvvm_atomic_add_gen_f_sys:
47 case Intrinsic::nvvm_atomic_add_gen_i_cta:
48 case Intrinsic::nvvm_atomic_add_gen_i_sys:
49 case Intrinsic::nvvm_atomic_and_gen_i_cta:
50 case Intrinsic::nvvm_atomic_and_gen_i_sys:
51 case Intrinsic::nvvm_atomic_cas_gen_i_cta:
52 case Intrinsic::nvvm_atomic_cas_gen_i_sys:
53 case Intrinsic::nvvm_atomic_dec_gen_i_cta:
54 case Intrinsic::nvvm_atomic_dec_gen_i_sys:
55 case Intrinsic::nvvm_atomic_inc_gen_i_cta:
56 case Intrinsic::nvvm_atomic_inc_gen_i_sys:
57 case Intrinsic::nvvm_atomic_max_gen_i_cta:
58 case Intrinsic::nvvm_atomic_max_gen_i_sys:
59 case Intrinsic::nvvm_atomic_min_gen_i_cta:
60 case Intrinsic::nvvm_atomic_min_gen_i_sys:
61 case Intrinsic::nvvm_atomic_or_gen_i_cta:
62 case Intrinsic::nvvm_atomic_or_gen_i_sys:
63 case Intrinsic::nvvm_atomic_exch_gen_i_cta:
64 case Intrinsic::nvvm_atomic_exch_gen_i_sys:
65 case Intrinsic::nvvm_atomic_xor_gen_i_cta:
66 case Intrinsic::nvvm_atomic_xor_gen_i_sys:
67 return true;
68 }
69 }
70
isSourceOfDivergence(const Value * V)71 bool NVPTXTTIImpl::isSourceOfDivergence(const Value *V) {
72 // Without inter-procedural analysis, we conservatively assume that arguments
73 // to __device__ functions are divergent.
74 if (const Argument *Arg = dyn_cast<Argument>(V))
75 return !isKernelFunction(*Arg->getParent());
76
77 if (const Instruction *I = dyn_cast<Instruction>(V)) {
78 // Without pointer analysis, we conservatively assume values loaded from
79 // generic or local address space are divergent.
80 if (const LoadInst *LI = dyn_cast<LoadInst>(I)) {
81 unsigned AS = LI->getPointerAddressSpace();
82 return AS == ADDRESS_SPACE_GENERIC || AS == ADDRESS_SPACE_LOCAL;
83 }
84 // Atomic instructions may cause divergence. Atomic instructions are
85 // executed sequentially across all threads in a warp. Therefore, an earlier
86 // executed thread may see different memory inputs than a later executed
87 // thread. For example, suppose *a = 0 initially.
88 //
89 // atom.global.add.s32 d, [a], 1
90 //
91 // returns 0 for the first thread that enters the critical region, and 1 for
92 // the second thread.
93 if (I->isAtomic())
94 return true;
95 if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
96 // Instructions that read threadIdx are obviously divergent.
97 if (readsThreadIndex(II) || readsLaneId(II))
98 return true;
99 // Handle the NVPTX atomic instrinsics that cannot be represented as an
100 // atomic IR instruction.
101 if (isNVVMAtomic(II))
102 return true;
103 }
104 // Conservatively consider the return value of function calls as divergent.
105 // We could analyze callees with bodies more precisely using
106 // inter-procedural analysis.
107 if (isa<CallInst>(I))
108 return true;
109 }
110
111 return false;
112 }
113
getArithmeticInstrCost(unsigned Opcode,Type * Ty,TTI::OperandValueKind Opd1Info,TTI::OperandValueKind Opd2Info,TTI::OperandValueProperties Opd1PropInfo,TTI::OperandValueProperties Opd2PropInfo,ArrayRef<const Value * > Args,const Instruction * CxtI)114 int NVPTXTTIImpl::getArithmeticInstrCost(
115 unsigned Opcode, Type *Ty, TTI::OperandValueKind Opd1Info,
116 TTI::OperandValueKind Opd2Info, TTI::OperandValueProperties Opd1PropInfo,
117 TTI::OperandValueProperties Opd2PropInfo, ArrayRef<const Value *> Args,
118 const Instruction *CxtI) {
119 // Legalize the type.
120 std::pair<int, MVT> LT = TLI->getTypeLegalizationCost(DL, Ty);
121
122 int ISD = TLI->InstructionOpcodeToISD(Opcode);
123
124 switch (ISD) {
125 default:
126 return BaseT::getArithmeticInstrCost(Opcode, Ty, Opd1Info, Opd2Info,
127 Opd1PropInfo, Opd2PropInfo);
128 case ISD::ADD:
129 case ISD::MUL:
130 case ISD::XOR:
131 case ISD::OR:
132 case ISD::AND:
133 // The machine code (SASS) simulates an i64 with two i32. Therefore, we
134 // estimate that arithmetic operations on i64 are twice as expensive as
135 // those on types that can fit into one machine register.
136 if (LT.second.SimpleTy == MVT::i64)
137 return 2 * LT.first;
138 // Delegate other cases to the basic TTI.
139 return BaseT::getArithmeticInstrCost(Opcode, Ty, Opd1Info, Opd2Info,
140 Opd1PropInfo, Opd2PropInfo);
141 }
142 }
143
getUnrollingPreferences(Loop * L,ScalarEvolution & SE,TTI::UnrollingPreferences & UP)144 void NVPTXTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
145 TTI::UnrollingPreferences &UP) {
146 BaseT::getUnrollingPreferences(L, SE, UP);
147
148 // Enable partial unrolling and runtime unrolling, but reduce the
149 // threshold. This partially unrolls small loops which are often
150 // unrolled by the PTX to SASS compiler and unrolling earlier can be
151 // beneficial.
152 UP.Partial = UP.Runtime = true;
153 UP.PartialThreshold = UP.Threshold / 4;
154 }
155