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//===- InstCombineAtomicRMW.cpp -------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the visit functions for atomic rmw instructions.
//
//===----------------------------------------------------------------------===//
#include "InstCombineInternal.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Transforms/InstCombine/InstCombiner.h"
using namespace llvm;
namespace {
/// Return true if and only if the given instruction does not modify the memory
/// location referenced. Note that an idemptent atomicrmw may still have
/// ordering effects on nearby instructions, or be volatile.
/// TODO: Common w/ the version in AtomicExpandPass, and change the term used.
/// Idemptotent is confusing in this context.
bool isIdempotentRMW(AtomicRMWInst& RMWI) {
if (auto CF = dyn_cast<ConstantFP>(RMWI.getValOperand()))
switch(RMWI.getOperation()) {
case AtomicRMWInst::FAdd: // -0.0
return CF->isZero() && CF->isNegative();
case AtomicRMWInst::FSub: // +0.0
return CF->isZero() && !CF->isNegative();
default:
return false;
};
auto C = dyn_cast<ConstantInt>(RMWI.getValOperand());
if(!C)
return false;
switch(RMWI.getOperation()) {
case AtomicRMWInst::Add:
case AtomicRMWInst::Sub:
case AtomicRMWInst::Or:
case AtomicRMWInst::Xor:
return C->isZero();
case AtomicRMWInst::And:
return C->isMinusOne();
case AtomicRMWInst::Min:
return C->isMaxValue(true);
case AtomicRMWInst::Max:
return C->isMinValue(true);
case AtomicRMWInst::UMin:
return C->isMaxValue(false);
case AtomicRMWInst::UMax:
return C->isMinValue(false);
default:
return false;
}
}
/// Return true if the given instruction always produces a value in memory
/// equivalent to its value operand.
bool isSaturating(AtomicRMWInst& RMWI) {
if (auto CF = dyn_cast<ConstantFP>(RMWI.getValOperand()))
switch(RMWI.getOperation()) {
case AtomicRMWInst::FAdd:
case AtomicRMWInst::FSub:
return CF->isNaN();
default:
return false;
};
auto C = dyn_cast<ConstantInt>(RMWI.getValOperand());
if(!C)
return false;
switch(RMWI.getOperation()) {
default:
return false;
case AtomicRMWInst::Xchg:
return true;
case AtomicRMWInst::Or:
return C->isAllOnesValue();
case AtomicRMWInst::And:
return C->isZero();
case AtomicRMWInst::Min:
return C->isMinValue(true);
case AtomicRMWInst::Max:
return C->isMaxValue(true);
case AtomicRMWInst::UMin:
return C->isMinValue(false);
case AtomicRMWInst::UMax:
return C->isMaxValue(false);
};
}
} // namespace
Instruction *InstCombinerImpl::visitAtomicRMWInst(AtomicRMWInst &RMWI) {
// Volatile RMWs perform a load and a store, we cannot replace this by just a
// load or just a store. We chose not to canonicalize out of general paranoia
// about user expectations around volatile.
if (RMWI.isVolatile())
return nullptr;
// Any atomicrmw op which produces a known result in memory can be
// replaced w/an atomicrmw xchg.
if (isSaturating(RMWI) &&
RMWI.getOperation() != AtomicRMWInst::Xchg) {
RMWI.setOperation(AtomicRMWInst::Xchg);
return &RMWI;
}
AtomicOrdering Ordering = RMWI.getOrdering();
assert(Ordering != AtomicOrdering::NotAtomic &&
Ordering != AtomicOrdering::Unordered &&
"AtomicRMWs don't make sense with Unordered or NotAtomic");
// Any atomicrmw xchg with no uses can be converted to a atomic store if the
// ordering is compatible.
if (RMWI.getOperation() == AtomicRMWInst::Xchg &&
RMWI.use_empty()) {
if (Ordering != AtomicOrdering::Release &&
Ordering != AtomicOrdering::Monotonic)
return nullptr;
auto *SI = new StoreInst(RMWI.getValOperand(),
RMWI.getPointerOperand(), &RMWI);
SI->setAtomic(Ordering, RMWI.getSyncScopeID());
SI->setAlignment(DL.getABITypeAlign(RMWI.getType()));
return eraseInstFromFunction(RMWI);
}
if (!isIdempotentRMW(RMWI))
return nullptr;
// We chose to canonicalize all idempotent operations to an single
// operation code and constant. This makes it easier for the rest of the
// optimizer to match easily. The choices of or w/0 and fadd w/-0.0 are
// arbitrary.
if (RMWI.getType()->isIntegerTy() &&
RMWI.getOperation() != AtomicRMWInst::Or) {
RMWI.setOperation(AtomicRMWInst::Or);
return replaceOperand(RMWI, 1, ConstantInt::get(RMWI.getType(), 0));
} else if (RMWI.getType()->isFloatingPointTy() &&
RMWI.getOperation() != AtomicRMWInst::FAdd) {
RMWI.setOperation(AtomicRMWInst::FAdd);
return replaceOperand(RMWI, 1, ConstantFP::getNegativeZero(RMWI.getType()));
}
// Check if the required ordering is compatible with an atomic load.
if (Ordering != AtomicOrdering::Acquire &&
Ordering != AtomicOrdering::Monotonic)
return nullptr;
LoadInst *Load = new LoadInst(RMWI.getType(), RMWI.getPointerOperand(), "",
false, DL.getABITypeAlign(RMWI.getType()),
Ordering, RMWI.getSyncScopeID());
return Load;
}
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