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73e1a13fdc
Summary: Implements PR889 Removing the virtual table pointer from Value saves 1% of RSS when doing LTO of llc on Linux. The impact on time was positive, but too noisy to conclusively say that performance improved. Here is a link to the spreadsheet with the original data: https://docs.google.com/spreadsheets/d/1F4FHir0qYnV0MEp2sYYp_BuvnJgWlWPhWOwZ6LbW7W4/edit?usp=sharing This change makes it invalid to directly delete a Value, User, or Instruction pointer. Instead, such code can be rewritten to a null check and a call Value::deleteValue(). Value objects tend to have their lifetimes managed through iplist, so for the most part, this isn't a big deal. However, there are some places where LLVM deletes values, and those places had to be migrated to deleteValue. I have also created llvm::unique_value, which has a custom deleter, so it can be used in place of std::unique_ptr<Value>. I had to add the "DerivedUser" Deleter escape hatch for MemorySSA, which derives from User outside of lib/IR. Code in IR cannot include MemorySSA headers or call the MemoryAccess object destructors without introducing a circular dependency, so we need some level of indirection. Unfortunately, no class derived from User may have any virtual methods, because adding a virtual method would break User::getHungOffOperands(), which assumes that it can find the use list immediately prior to the User object. I've added a static_assert to the appropriate OperandTraits templates to help people avoid this trap. Reviewers: chandlerc, mehdi_amini, pete, dberlin, george.burgess.iv Reviewed By: chandlerc Subscribers: krytarowski, eraman, george.burgess.iv, mzolotukhin, Prazek, nlewycky, hans, inglorion, pcc, tejohnson, dberlin, llvm-commits Differential Revision: https://reviews.llvm.org/D31261 llvm-svn: 303362
42 lines
1.3 KiB
C++
42 lines
1.3 KiB
C++
//===-- DerivedUser.h - Base for non-IR Users -------------------*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_IR_DERIVEDUSER_H
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#define LLVM_IR_DERIVEDUSER_H
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#include "llvm/IR/User.h"
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namespace llvm {
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/// Extension point for the Value hierarchy. All classes outside of lib/IR
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/// that wish to inherit from User should instead inherit from DerivedUser
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/// instead. Inheriting from this class is discouraged.
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///
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/// Generally speaking, Value is the base of a closed class hierarchy
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/// that can't be extended by code outside of lib/IR. This class creates a
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/// loophole that allows classes outside of lib/IR to extend User to leverage
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/// its use/def list machinery.
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class DerivedUser : public User {
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protected:
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typedef void (*DeleteValueTy)(DerivedUser *);
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private:
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friend Value;
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DeleteValueTy DeleteValue;
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public:
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DerivedUser(Type *Ty, unsigned VK, Use *U, unsigned NumOps,
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DeleteValueTy DeleteValue)
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: User(Ty, VK, U, NumOps), DeleteValue(DeleteValue) {}
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};
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} // namespace llvm
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#endif // LLVM_IR_DERIVEDUSER_H
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