1 //===- InstCombineAtomicRMW.cpp -------------------------------------------===//
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 file implements the visit functions for atomic rmw instructions.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "InstCombineInternal.h"
14 #include "llvm/IR/Instructions.h"
15
16 using namespace llvm;
17
18 namespace {
19 /// Return true if and only if the given instruction does not modify the memory
20 /// location referenced. Note that an idemptent atomicrmw may still have
21 /// ordering effects on nearby instructions, or be volatile.
22 /// TODO: Common w/ the version in AtomicExpandPass, and change the term used.
23 /// Idemptotent is confusing in this context.
isIdempotentRMW(AtomicRMWInst & RMWI)24 bool isIdempotentRMW(AtomicRMWInst& RMWI) {
25 if (auto CF = dyn_cast<ConstantFP>(RMWI.getValOperand()))
26 switch(RMWI.getOperation()) {
27 case AtomicRMWInst::FAdd: // -0.0
28 return CF->isZero() && CF->isNegative();
29 case AtomicRMWInst::FSub: // +0.0
30 return CF->isZero() && !CF->isNegative();
31 default:
32 return false;
33 };
34
35 auto C = dyn_cast<ConstantInt>(RMWI.getValOperand());
36 if(!C)
37 return false;
38
39 switch(RMWI.getOperation()) {
40 case AtomicRMWInst::Add:
41 case AtomicRMWInst::Sub:
42 case AtomicRMWInst::Or:
43 case AtomicRMWInst::Xor:
44 return C->isZero();
45 case AtomicRMWInst::And:
46 return C->isMinusOne();
47 case AtomicRMWInst::Min:
48 return C->isMaxValue(true);
49 case AtomicRMWInst::Max:
50 return C->isMinValue(true);
51 case AtomicRMWInst::UMin:
52 return C->isMaxValue(false);
53 case AtomicRMWInst::UMax:
54 return C->isMinValue(false);
55 default:
56 return false;
57 }
58 }
59
60 /// Return true if the given instruction always produces a value in memory
61 /// equivalent to its value operand.
isSaturating(AtomicRMWInst & RMWI)62 bool isSaturating(AtomicRMWInst& RMWI) {
63 if (auto CF = dyn_cast<ConstantFP>(RMWI.getValOperand()))
64 switch (RMWI.getOperation()) {
65 case AtomicRMWInst::FMax:
66 // maxnum(x, +inf) -> +inf
67 return !CF->isNegative() && CF->isInfinity();
68 case AtomicRMWInst::FMin:
69 // minnum(x, -inf) -> +inf
70 return CF->isNegative() && CF->isInfinity();
71 case AtomicRMWInst::FAdd:
72 case AtomicRMWInst::FSub:
73 return CF->isNaN();
74 default:
75 return false;
76 };
77
78 auto C = dyn_cast<ConstantInt>(RMWI.getValOperand());
79 if(!C)
80 return false;
81
82 switch(RMWI.getOperation()) {
83 default:
84 return false;
85 case AtomicRMWInst::Xchg:
86 return true;
87 case AtomicRMWInst::Or:
88 return C->isAllOnesValue();
89 case AtomicRMWInst::And:
90 return C->isZero();
91 case AtomicRMWInst::Min:
92 return C->isMinValue(true);
93 case AtomicRMWInst::Max:
94 return C->isMaxValue(true);
95 case AtomicRMWInst::UMin:
96 return C->isMinValue(false);
97 case AtomicRMWInst::UMax:
98 return C->isMaxValue(false);
99 };
100 }
101 } // namespace
102
visitAtomicRMWInst(AtomicRMWInst & RMWI)103 Instruction *InstCombinerImpl::visitAtomicRMWInst(AtomicRMWInst &RMWI) {
104
105 // Volatile RMWs perform a load and a store, we cannot replace this by just a
106 // load or just a store. We chose not to canonicalize out of general paranoia
107 // about user expectations around volatile.
108 if (RMWI.isVolatile())
109 return nullptr;
110
111 // Any atomicrmw op which produces a known result in memory can be
112 // replaced w/an atomicrmw xchg.
113 if (isSaturating(RMWI) &&
114 RMWI.getOperation() != AtomicRMWInst::Xchg) {
115 RMWI.setOperation(AtomicRMWInst::Xchg);
116 return &RMWI;
117 }
118
119 assert(RMWI.getOrdering() != AtomicOrdering::NotAtomic &&
120 RMWI.getOrdering() != AtomicOrdering::Unordered &&
121 "AtomicRMWs don't make sense with Unordered or NotAtomic");
122
123 if (!isIdempotentRMW(RMWI))
124 return nullptr;
125
126 // We chose to canonicalize all idempotent operations to an single
127 // operation code and constant. This makes it easier for the rest of the
128 // optimizer to match easily. The choices of or w/0 and fadd w/-0.0 are
129 // arbitrary.
130 if (RMWI.getType()->isIntegerTy() &&
131 RMWI.getOperation() != AtomicRMWInst::Or) {
132 RMWI.setOperation(AtomicRMWInst::Or);
133 return replaceOperand(RMWI, 1, ConstantInt::get(RMWI.getType(), 0));
134 } else if (RMWI.getType()->isFloatingPointTy() &&
135 RMWI.getOperation() != AtomicRMWInst::FAdd) {
136 RMWI.setOperation(AtomicRMWInst::FAdd);
137 return replaceOperand(RMWI, 1, ConstantFP::getNegativeZero(RMWI.getType()));
138 }
139
140 return nullptr;
141 }
142