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Pass looks for equivalent functions that are mergable and folds them. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223931 91177308-0d34-0410-b5e6-96231b3b80d8
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ReStructuredText
=================================
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MergeFunctions pass, how it works
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=================================
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.. contents::
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:local:
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Introduction
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============
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Sometimes code contains equal functions, or functions that does exactly the same
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thing even though they are non-equal on the IR level (e.g.: multiplication on 2
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and 'shl 1'). It could happen due to several reasons: mainly, the usage of
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templates and automatic code generators. Though, sometimes user itself could
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write the same thing twice :-)
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The main purpose of this pass is to recognize such functions and merge them.
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Why would I want to read this document?
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---------------------------------------
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Document is the extension to pass comments and describes the pass logic. It
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describes algorithm that is used in order to compare functions, it also
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explains how we could combine equal functions correctly, keeping module valid.
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Material is brought in top-down form, so reader could start learn pass from
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ideas and end up with low-level algorithm details, thus preparing him for
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reading the sources.
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So main goal is do describe algorithm and logic here; the concept. This document
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is good for you, if you *don't want* to read the source code, but want to
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understand pass algorithms. Author tried not to repeat the source-code and
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cover only common cases, and thus avoid cases when after minor code changes we
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need to update this document.
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What should I know to be able to follow along with this document?
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-----------------------------------------------------------------
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Reader should be familiar with common compile-engineering principles and LLVM
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code fundamentals. In this article we suppose reader is familiar with
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`Single Static Assingment <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
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concepts. Understanding of
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`IR structure <http://llvm.org/docs/LangRef.html#high-level-structure>`_ is
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also important.
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We will use such terms as
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"`module <http://llvm.org/docs/LangRef.html#high-level-structure>`_",
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"`function <http://llvm.org/docs/ProgrammersManual.html#the-function-class>`_",
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"`basic block <http://en.wikipedia.org/wiki/Basic_block>`_",
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"`user <http://llvm.org/docs/ProgrammersManual.html#the-user-class>`_",
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"`value <http://llvm.org/docs/ProgrammersManual.html#the-value-class>`_",
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"`instruction <http://llvm.org/docs/ProgrammersManual.html#the-instruction-class>`_".
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As a good start point, Kaleidoscope tutorial could be used:
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:doc:`tutorial/index`
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Especially it's important to understand chapter 3 of tutorial:
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:doc:`tutorial/LangImpl3`
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Reader also should know how passes work in LLVM, he could use next article as a
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reference and start point here:
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:doc:`WritingAnLLVMPass`
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What else? Well perhaps reader also should have some experience in LLVM pass
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debugging and bug-fixing.
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What I gain by reading this document?
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-------------------------------------
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Main purpose is to provide reader with comfortable form of algorithms
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description, namely the human reading text. Since it could be hard to
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understand algorithm straight from the source code: pass uses some principles
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that have to be explained first.
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Author wishes to everybody to avoid case, when you read code from top to bottom
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again and again, and yet you don't understand why we implemented it that way.
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We hope that after this article reader could easily debug and improve
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MergeFunctions pass and thus help LLVM project.
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Narrative structure
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-------------------
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Article consists of three parts. First part explains pass functionality on the
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top-level. Second part describes the comparison procedure itself. The third
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part describes the merging process.
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In every part author also tried to put the contents into the top-down form.
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First, the top-level methods will be described, while the terminal ones will be
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at the end, in the tail of each part. If reader will see the reference to the
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method that wasn't described yet, he will find its description a bit below.
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Basics
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======
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How to do it?
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-------------
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Do we need to merge functions? Obvious thing is: yes that's a quite possible
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case, since usually we *do* have duplicates. And it would be good to get rid of
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them. But how to detect such a duplicates? The idea is next: we split functions
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onto small bricks (parts), then we compare "bricks" amount, and if it equal,
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compare "bricks" themselves, and then do our conclusions about functions
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themselves.
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What the difference it could be? For example, on machine with 64-bit pointers
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(let's assume we have only one address space), one function stores 64-bit
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integer, while another one stores a pointer. So if the target is a machine
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mentioned above, and if functions are identical, except the parameter type (we
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could consider it as a part of function type), then we can treat ``uint64_t``
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and``void*`` as equal.
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It was just an example; possible details are described a bit below.
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As another example reader may imagine two more functions. First function
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performs multiplication on 2, while the second one performs arithmetic right
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shift on 1.
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Possible solutions
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^^^^^^^^^^^^^^^^^^
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Let's briefly consider possible options about how and what we have to implement
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in order to create full-featured functions merging, and also what it would
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meant for us.
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Equal functions detection, obviously supposes "detector" method to be
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implemented, latter should answer the question "whether functions are equal".
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This "detector" method consists of tiny "sub-detectors", each of them answers
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exactly the same question, but for function parts.
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As the second step, we should merge equal functions. So it should be a "merger"
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method. "Merger" accepts two functions *F1* and *F2*, and produces *F1F2*
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function, the result of merging.
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Having such a routines in our hands, we can process whole module, and merge all
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equal functions.
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In this case, we have to compare every function with every another function. As
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reader could notice, this way seems to be quite expensive. Of course we could
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introduce hashing and other helpers, but it is still just an optimization, and
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thus the level of O(N*N) complexity.
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Can we reach another level? Could we introduce logarithmical search, or random
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access lookup? The answer is: "yes".
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Random-access
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"""""""""""""
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How it could be done? Just convert each function to number, and gather all of
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them in special hash-table. Functions with equal hash are equal. Good hashing
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means, that every function part must be taken into account. That means we have
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to convert every function part into some number, and then add it into hash.
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Lookup-up time would be small, but such approach adds some delay due to hashing
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routine.
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Logarithmical search
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""""""""""""""""""""
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We could introduce total ordering among the functions set, once we had it we
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could then implement a logarithmical search. Lookup time still depends on N,
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but adds a little of delay (*log(N)*).
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Present state
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"""""""""""""
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Both of approaches (random-access and logarithmical) has been implemented and
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tested. And both of them gave a very good improvement. And what was most
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surprising, logarithmical search was faster; sometimes up to 15%. Hashing needs
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some extra CPU time, and it is the main reason why it works slower; in most of
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cases total "hashing" time was greater than total "logarithmical-search" time.
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So, preference has been granted to the "logarithmical search".
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Though in the case of need, *logarithmical-search* (read "total-ordering") could
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be used as a milestone on our way to the *random-access* implementation.
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Every comparison is based either on the numbers or on flags comparison. In
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*random-access* approach we could use the same comparison algorithm. During
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comparison we exit once we find the difference, but here we might have to scan
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whole function body every time (note, it could be slower). Like in
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"total-ordering", we will track every numbers and flags, but instead of
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comparison, we should get numbers sequence and then create the hash number. So,
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once again, *total-ordering* could be considered as a milestone for even faster
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(in theory) random-access approach.
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MergeFunctions, main fields and runOnModule
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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There are two most important fields in class:
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``FnTree`` – the set of all unique functions. It keeps items that couldn't be
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merged with each other. It is defined as:
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``std::set<FunctionNode> FnTree;``
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Here ``FunctionNode`` is a wrapper for ``llvm::Function`` class, with
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implemented “<” operator among the functions set (below we explain how it works
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exactly; this is a key point in fast functions comparison).
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``Deferred`` – merging process can affect bodies of functions that are in
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``FnTree`` already. Obviously such functions should be rechecked again. In this
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case we remove them from ``FnTree``, and mark them as to be rescanned, namely
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put them into ``Deferred`` list.
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runOnModule
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"""""""""""
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The algorithm is pretty simple:
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1. Put all module's functions into the *worklist*.
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2. Scan *worklist*'s functions twice: first enumerate only strong functions and
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then only weak ones:
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2.1. Loop body: take function from *worklist* (call it *FCur*) and try to
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insert it into *FnTree*: check whether *FCur* is equal to one of functions
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in *FnTree*. If there *is* equal function in *FnTree* (call it *FExists*):
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merge function *FCur* with *FExists*. Otherwise add function from *worklist*
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to *FnTree*.
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3. Once *worklist* scanning and merging operations is complete, check *Deferred*
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list. If it is not empty: refill *worklist* contents with *Deferred* list and
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do step 2 again, if *Deferred* is empty, then exit from method.
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Comparison and logarithmical search
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"""""""""""""""""""""""""""""""""""
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Let's recall our task: for every function *F* from module *M*, we have to find
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equal functions *F`* in shortest time, and merge them into the single function.
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Defining total ordering among the functions set allows to organize functions
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into the binary tree. The lookup procedure complexity would be estimated as
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O(log(N)) in this case. But how to define *total-ordering*?
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We have to introduce a single rule applicable to every pair of functions, and
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following this rule then evaluate which of them is greater. What kind of rule
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it could be? Let's declare it as "compare" method, that returns one of 3
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possible values:
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-1, left is *less* than right,
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0, left and right are *equal*,
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1, left is *greater* than right.
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Of course it means, that we have to maintain
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*strict and non-strict order relation properties*:
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* reflexivity (``a <= a``, ``a == a``, ``a >= a``),
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* antisymmetry (if ``a <= b`` and ``b <= a`` then ``a == b``),
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* transitivity (``a <= b`` and ``b <= c``, then ``a <= c``)
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* asymmetry (if ``a < b``, then ``a > b`` or ``a == b``).
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As it was mentioned before, comparison routine consists of
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"sub-comparison-routines", each of them also consists
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"sub-comparison-routines", and so on, finally it ends up with a primitives
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comparison.
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Below, we will use the next operations:
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#. ``cmpNumbers(number1, number2)`` is method that returns -1 if left is less
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than right; 0, if left and right are equal; and 1 otherwise.
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#. ``cmpFlags(flag1, flag2)`` is hypothetical method that compares two flags.
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The logic is the same as in ``cmpNumbers``, where ``true`` is 1, and
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``false`` is 0.
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The rest of article is based on *MergeFunctions.cpp* source code
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(*<llvm_dir>/lib/Transforms/IPO/MergeFunctions.cpp*). We would like to ask
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reader to keep this file open nearby, so we could use it as a reference for
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further explanations.
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Now we're ready to proceed to the next chapter and see how it works.
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Functions comparison
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====================
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At first, let's define how exactly we compare complex objects.
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Complex objects comparison (function, basic-block, etc) is mostly based on its
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sub-objects comparison results. So it is similar to the next "tree" objects
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comparison:
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#. For two trees *T1* and *T2* we perform *depth-first-traversal* and have
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two sequences as a product: "*T1Items*" and "*T2Items*".
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#. Then compare chains "*T1Items*" and "*T2Items*" in
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most-significant-item-first order. Result of items comparison would be the
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result of *T1* and *T2* comparison itself.
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FunctionComparator::compare(void)
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---------------------------------
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Brief look at the source code tells us, that comparison starts in
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“``int FunctionComparator::compare(void)``” method.
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1. First parts to be compared are function's attributes and some properties that
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outsides “attributes” term, but still could make function different without
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changing its body. This part of comparison is usually done within simple
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*cmpNumbers* or *cmpFlags* operations (e.g.
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``cmpFlags(F1->hasGC(), F2->hasGC())``). Below is full list of function's
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properties to be compared on this stage:
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* *Attributes* (those are returned by ``Function::getAttributes()``
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method).
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* *GC*, for equivalence, *RHS* and *LHS* should be both either without
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*GC* or with the same one.
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* *Section*, just like a *GC*: *RHS* and *LHS* should be defined in the
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same section.
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* *Variable arguments*. *LHS* and *RHS* should be both either with or
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without *var-args*.
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* *Calling convention* should be the same.
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2. Function type. Checked by ``FunctionComparator::cmpType(Type*, Type*)``
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method. It checks return type and parameters type; the method itself will be
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described later.
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3. Associate function formal parameters with each other. Then comparing function
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bodies, if we see the usage of *LHS*'s *i*-th argument in *LHS*'s body, then,
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we want to see usage of *RHS*'s *i*-th argument at the same place in *RHS*'s
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body, otherwise functions are different. On this stage we grant the preference
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to those we met later in function body (value we met first would be *less*).
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This is done by “``FunctionComparator::cmpValues(const Value*, const Value*)``”
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method (will be described a bit later).
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4. Function body comparison. As it written in method comments:
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“We do a CFG-ordered walk since the actual ordering of the blocks in the linked
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list is immaterial. Our walk starts at the entry block for both functions, then
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takes each block from each terminator in order. As an artifact, this also means
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that unreachable blocks are ignored.”
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So, using this walk we get BBs from *left* and *right* in the same order, and
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compare them by “``FunctionComparator::compare(const BasicBlock*, const
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BasicBlock*)``” method.
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We also associate BBs with each other, like we did it with function formal
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arguments (see ``cmpValues`` method below).
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FunctionComparator::cmpType
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---------------------------
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Consider how types comparison works.
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1. Coerce pointer to integer. If left type is a pointer, try to coerce it to the
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integer type. It could be done if its address space is 0, or if address spaces
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are ignored at all. Do the same thing for the right type.
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2. If left and right types are equal, return 0. Otherwise we need to give
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preference to one of them. So proceed to the next step.
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3. If types are of different kind (different type IDs). Return result of type
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IDs comparison, treating them as a numbers (use ``cmpNumbers`` operation).
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4. If types are vectors or integers, return result of their pointers comparison,
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comparing them as numbers.
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5. Check whether type ID belongs to the next group (call it equivalent-group):
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* Void
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* Float
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* Double
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* X86_FP80
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* FP128
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* PPC_FP128
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* Label
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* Metadata.
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If ID belongs to group above, return 0. Since it's enough to see that
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types has the same ``TypeID``. No additional information is required.
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6. Left and right are pointers. Return result of address space comparison
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(numbers comparison).
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7. Complex types (structures, arrays, etc.). Follow complex objects comparison
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technique (see the very first paragraph of this chapter). Both *left* and
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*right* are to be expanded and their element types will be checked the same
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way. If we get -1 or 1 on some stage, return it. Otherwise return 0.
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8. Steps 1-6 describe all the possible cases, if we passed steps 1-6 and didn't
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get any conclusions, then invoke ``llvm_unreachable``, since it's quite
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unexpectable case.
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cmpValues(const Value*, const Value*)
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-------------------------------------
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Method that compares local values.
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This method gives us an answer on a very curious quesion: whether we could treat
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local values as equal, and which value is greater otherwise. It's better to
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start from example:
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Consider situation when we're looking at the same place in left function "*FL*"
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and in right function "*FR*". And every part of *left* place is equal to the
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corresponding part of *right* place, and (!) both parts use *Value* instances,
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for example:
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.. code-block:: llvm
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instr0 i32 %LV ; left side, function FL
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instr0 i32 %RV ; right side, function FR
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So, now our conclusion depends on *Value* instances comparison.
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Main purpose of this method is to determine relation between such values.
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What we expect from equal functions? At the same place, in functions "*FL*" and
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"*FR*" we expect to see *equal* values, or values *defined* at the same place
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in "*FL*" and "*FR*".
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Consider small example here:
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.. code-block:: llvm
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define void %f(i32 %pf0, i32 %pf1) {
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instr0 i32 %pf0 instr1 i32 %pf1 instr2 i32 123
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}
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.. code-block:: llvm
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define void %g(i32 %pg0, i32 %pg1) {
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instr0 i32 %pg0 instr1 i32 %pg0 instr2 i32 123
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}
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In this example, *pf0* is associated with *pg0*, *pf1* is associated with *pg1*,
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and we also declare that *pf0* < *pf1*, and thus *pg0* < *pf1*.
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Instructions with opcode "*instr0*" would be *equal*, since their types and
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opcodes are equal, and values are *associated*.
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Instruction with opcode "*instr1*" from *f* is *greater* than instruction with
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opcode "*instr1*" from *g*; here we have equal types and opcodes, but "*pf1* is
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greater than "*pg0*".
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And instructions with opcode "*instr2*" are equal, because their opcodes and
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types are equal, and the same constant is used as a value.
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What we assiciate in cmpValues?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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* Function arguments. *i*-th argument from left function associated with
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*i*-th argument from right function.
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* BasicBlock instances. In basic-block enumeration loop we associate *i*-th
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BasicBlock from the left function with *i*-th BasicBlock from the right
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function.
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* Instructions.
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* Instruction operands. Note, we can meet *Value* here we have never seen
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before. In this case it is not a function argument, nor *BasicBlock*, nor
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*Instruction*. It is global value. It is constant, since its the only
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supposed global here. Method also compares:
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* Constants that are of the same type.
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* If right constant could be losslessly bit-casted to the left one, then we
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also compare them.
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How to implement cmpValues?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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*Association* is a case of equality for us. We just treat such values as equal.
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But, in general, we need to implement antisymmetric relation. As it was
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mentioned above, to understand what is *less*, we can use order in which we
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meet values. If both of values has the same order in function (met at the same
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time), then treat values as *associated*. Otherwise – it depends on who was
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first.
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Every time we run top-level compare method, we initialize two identical maps
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(one for the left side, another one for the right side):
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``map<Value, int> sn_mapL, sn_mapR;``
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The key of the map is the *Value* itself, the *value* – is its order (call it
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*serial number*).
|
||
|
||
To add value *V* we need to perform the next procedure:
|
||
|
||
``sn_map.insert(std::make_pair(V, sn_map.size()));``
|
||
|
||
For the first *Value*, map will return *0*, for second *Value* map will return
|
||
*1*, and so on.
|
||
|
||
Then we can check whether left and right values met at the same time with simple
|
||
comparison:
|
||
|
||
``cmpNumbers(sn_mapL[Left], sn_mapR[Right]);``
|
||
|
||
Of course, we can combine insertion and comparison:
|
||
|
||
.. code-block:: c++
|
||
|
||
std::pair<iterator, bool>
|
||
LeftRes = sn_mapL.insert(std::make_pair(Left, sn_mapL.size())), RightRes
|
||
= sn_mapR.insert(std::make_pair(Right, sn_mapR.size()));
|
||
return cmpNumbers(LeftRes.first->second, RightRes.first->second);
|
||
|
||
Let's look, how whole method could be implemented.
|
||
|
||
1. we have to start from the bad news. Consider function self and
|
||
cross-referencing cases:
|
||
|
||
.. code-block:: c++
|
||
|
||
// self-reference unsigned fact0(unsigned n) { return n > 1 ? n
|
||
* fact0(n-1) : 1; } unsigned fact1(unsigned n) { return n > 1 ? n *
|
||
fact1(n-1) : 1; }
|
||
|
||
// cross-reference unsigned ping(unsigned n) { return n!= 0 ? pong(n-1) : 0;
|
||
} unsigned pong(unsigned n) { return n!= 0 ? ping(n-1) : 0; }
|
||
|
||
..
|
||
|
||
This comparison has been implemented in initial *MergeFunctions* pass
|
||
version. But, unfortunately, it is not transitive. And this is the only case
|
||
we can't convert to less-equal-greater comparison. It is a seldom case, 4-5
|
||
functions of 10000 (checked on test-suite), and, we hope, reader would
|
||
forgive us for such a sacrifice in order to get the O(log(N)) pass time.
|
||
|
||
2. If left/right *Value* is a constant, we have to compare them. Return 0 if it
|
||
is the same constant, or use ``cmpConstants`` method otherwise.
|
||
|
||
3. If left/right is *InlineAsm* instance. Return result of *Value* pointers
|
||
comparison.
|
||
|
||
4. Explicit association of *L* (left value) and *R* (right value). We need to
|
||
find out whether values met at the same time, and thus are *associated*. Or we
|
||
need to put the rule: when we treat *L* < *R*. Now it is easy: just return
|
||
result of numbers comparison:
|
||
|
||
.. code-block:: c++
|
||
|
||
std::pair<iterator, bool>
|
||
LeftRes = sn_mapL.insert(std::make_pair(Left, sn_mapL.size())),
|
||
RightRes = sn_mapR.insert(std::make_pair(Right, sn_mapR.size()));
|
||
if (LeftRes.first->second == RightRes.first->second) return 0;
|
||
if (LeftRes.first->second < RightRes.first->second) return -1;
|
||
return 1;
|
||
|
||
Now when *cmpValues* returns 0, we can proceed comparison procedure. Otherwise,
|
||
if we get (-1 or 1), we need to pass this result to the top level, and finish
|
||
comparison procedure.
|
||
|
||
cmpConstants
|
||
------------
|
||
Performs constants comparison as follows:
|
||
|
||
1. Compare constant types using ``cmpType`` method. If result is -1 or 1, goto
|
||
step 2, otherwise proceed to step 3.
|
||
|
||
2. If types are different, we still can check whether constants could be
|
||
losslessly bitcasted to each other. The further explanation is modification of
|
||
``canLosslesslyBitCastTo`` method.
|
||
|
||
2.1 Check whether constants are of the first class types
|
||
(``isFirstClassType`` check):
|
||
|
||
2.1.1. If both constants are *not* of the first class type: return result
|
||
of ``cmpType``.
|
||
|
||
2.1.2. Otherwise, if left type is not of the first class, return -1. If
|
||
right type is not of the first class, return 1.
|
||
|
||
2.1.3. If both types are of the first class type, proceed to the next step
|
||
(2.1.3.1).
|
||
|
||
2.1.3.1. If types are vectors, compare their bitwidth using the
|
||
*cmpNumbers*. If result is not 0, return it.
|
||
|
||
2.1.3.2. Different types, but not a vectors:
|
||
|
||
* if both of them are pointers, good for us, we can proceed to step 3.
|
||
* if one of types is pointer, return result of *isPointer* flags
|
||
comparison (*cmpFlags* operation).
|
||
* otherwise we have no methods to prove bitcastability, and thus return
|
||
result of types comparison (-1 or 1).
|
||
|
||
Steps below are for the case when types are equal, or case when constants are
|
||
bitcastable:
|
||
|
||
3. One of constants is a "*null*" value. Return the result of
|
||
``cmpFlags(L->isNullValue, R->isNullValue)`` comparison.
|
||
|
||
4. Compare value IDs, and return result if it is not 0:
|
||
|
||
.. code-block:: c++
|
||
|
||
if (int Res = cmpNumbers(L->getValueID(), R->getValueID()))
|
||
return Res;
|
||
|
||
5. Compare the contents of constants. The comparison depends on kind of
|
||
constants, but on this stage it is just a lexicographical comparison. Just see
|
||
how it was described in the beginning of "*Functions comparison*" paragraph.
|
||
Mathematically it is equal to the next case: we encode left constant and right
|
||
constant (with similar way *bitcode-writer* does). Then compare left code
|
||
sequence and right code sequence.
|
||
|
||
compare(const BasicBlock*, const BasicBlock*)
|
||
---------------------------------------------
|
||
Compares two *BasicBlock* instances.
|
||
|
||
It enumerates instructions from left *BB* and right *BB*.
|
||
|
||
1. It assigns serial numbers to the left and right instructions, using
|
||
``cmpValues`` method.
|
||
|
||
2. If one of left or right is *GEP* (``GetElementPtr``), then treat *GEP* as
|
||
greater than other instructions, if both instructions are *GEPs* use ``cmpGEP``
|
||
method for comparison. If result is -1 or 1, pass it to the top-level
|
||
comparison (return it).
|
||
|
||
3.1. Compare operations. Call ``cmpOperation`` method. If result is -1 or
|
||
1, return it.
|
||
|
||
3.2. Compare number of operands, if result is -1 or 1, return it.
|
||
|
||
3.3. Compare operands themselves, use ``cmpValues`` method. Return result
|
||
if it is -1 or 1.
|
||
|
||
3.4. Compare type of operands, using ``cmpType`` method. Return result if
|
||
it is -1 or 1.
|
||
|
||
3.5. Proceed to the next instruction.
|
||
|
||
4. We can finish instruction enumeration in 3 cases:
|
||
|
||
4.1. We reached the end of both left and right basic-blocks. We didn't
|
||
exit on steps 1-3, so contents is equal, return 0.
|
||
|
||
4.2. We have reached the end of the left basic-block. Return -1.
|
||
|
||
4.3. Return 1 (the end of the right basic block).
|
||
|
||
cmpGEP
|
||
------
|
||
Compares two GEPs (``getelementptr`` instructions).
|
||
|
||
It differs from regular operations comparison with the only thing: possibility
|
||
to use ``accumulateConstantOffset`` method.
|
||
|
||
So, if we get constant offset for both left and right *GEPs*, then compare it as
|
||
numbers, and return comparison result.
|
||
|
||
Otherwise treat it like a regular operation (see previous paragraph).
|
||
|
||
cmpOperation
|
||
------------
|
||
Compares instruction opcodes and some important operation properties.
|
||
|
||
1. Compare opcodes, if it differs return the result.
|
||
|
||
2. Compare number of operands. If it differs – return the result.
|
||
|
||
3. Compare operation types, use *cmpType*. All the same – if types are
|
||
different, return result.
|
||
|
||
4. Compare *subclassOptionalData*, get it with ``getRawSubclassOptionalData``
|
||
method, and compare it like a numbers.
|
||
|
||
5. Compare operand types.
|
||
|
||
6. For some particular instructions check equivalence (relation in our case) of
|
||
some significant attributes. For example we have to compare alignment for
|
||
``load`` instructions.
|
||
|
||
O(log(N))
|
||
---------
|
||
Methods described above implement order relationship. And latter, could be used
|
||
for nodes comparison in a binary tree. So we can organize functions set into
|
||
the binary tree and reduce the cost of lookup procedure from
|
||
O(N*N) to O(log(N)).
|
||
|
||
Merging process, mergeTwoFunctions
|
||
==================================
|
||
Once *MergeFunctions* detected that current function (*G*) is equal to one that
|
||
were analyzed before (function *F*) it calls ``mergeTwoFunctions(Function*,
|
||
Function*)``.
|
||
|
||
Operation affects ``FnTree`` contents with next way: *F* will stay in
|
||
``FnTree``. *G* being equal to *F* will not be added to ``FnTree``. Calls of
|
||
*G* would be replaced with something else. It changes bodies of callers. So,
|
||
functions that calls *G* would be put into ``Deferred`` set and removed from
|
||
``FnTree``, and analyzed again.
|
||
|
||
The approach is next:
|
||
|
||
1. Most wished case: when we can use alias and both of *F* and *G* are weak. We
|
||
make both of them with aliases to the third strong function *H*. Actually *H*
|
||
is *F*. See below how it's made (but it's better to look straight into the
|
||
source code). Well, this is a case when we can just replace *G* with *F*
|
||
everywhere, we use ``replaceAllUsesWith`` operation here (*RAUW*).
|
||
|
||
2. *F* could not be overridden, while *G* could. It would be good to do the
|
||
next: after merging the places where overridable function were used, still use
|
||
overridable stub. So try to make *G* alias to *F*, or create overridable tail
|
||
call wrapper around *F* and replace *G* with that call.
|
||
|
||
3. Neither *F* nor *G* could be overridden. We can't use *RAUW*. We can just
|
||
change the callers: call *F* instead of *G*. That's what
|
||
``replaceDirectCallers`` does.
|
||
|
||
Below is detailed body description.
|
||
|
||
If “F” may be overridden
|
||
------------------------
|
||
As follows from ``mayBeOverridden`` comments: “whether the definition of this
|
||
global may be replaced by something non-equivalent at link time”. If so, thats
|
||
ok: we can use alias to *F* instead of *G* or change call instructions itself.
|
||
|
||
HasGlobalAliases, removeUsers
|
||
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||
First consider the case when we have global aliases of one function name to
|
||
another. Our purpose is make both of them with aliases to the third strong
|
||
function. Though if we keep *F* alive and without major changes we can leave it
|
||
in ``FnTree``. Try to combine these two goals.
|
||
|
||
Do stub replacement of *F* itself with an alias to *F*.
|
||
|
||
1. Create stub function *H*, with the same name and attributes like function
|
||
*F*. It takes maximum alignment of *F* and *G*.
|
||
|
||
2. Replace all uses of function *F* with uses of function *H*. It is the two
|
||
steps procedure instead. First of all, we must take into account, all functions
|
||
from whom *F* is called would be changed: since we change the call argument
|
||
(from *F* to *H*). If so we must to review these caller functions again after
|
||
this procedure. We remove callers from ``FnTree``, method with name
|
||
``removeUsers(F)`` does that (don't confuse with ``replaceAllUsesWith``):
|
||
|
||
2.1. ``Inside removeUsers(Value*
|
||
V)`` we go through the all values that use value *V* (or *F* in our context).
|
||
If value is instruction, we go to function that holds this instruction and
|
||
mark it as to-be-analyzed-again (put to ``Deferred`` set), we also remove
|
||
caller from ``FnTree``.
|
||
|
||
2.2. Now we can do the replacement: call ``F->replaceAllUsesWith(H)``.
|
||
|
||
3. *H* (that now "officially" plays *F*'s role) is replaced with alias to *F*.
|
||
Do the same with *G*: replace it with alias to *F*. So finally everywhere *F*
|
||
was used, we use *H* and it is alias to *F*, and everywhere *G* was used we
|
||
also have alias to *F*.
|
||
|
||
4. Set *F* linkage to private. Make it strong :-)
|
||
|
||
No global aliases, replaceDirectCallers
|
||
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||
If global aliases are not supported. We call ``replaceDirectCallers`` then. Just
|
||
go through all calls of *G* and replace it with calls of *F*. If you look into
|
||
method you will see that it scans all uses of *G* too, and if use is callee (if
|
||
user is call instruction and *G* is used as what to be called), we replace it
|
||
with use of *F*.
|
||
|
||
If “F” could not be overridden, fix it!
|
||
"""""""""""""""""""""""""""""""""""""""
|
||
|
||
We call ``writeThunkOrAlias(Function *F, Function *G)``. Here we try to replace
|
||
*G* with alias to *F* first. Next conditions are essential:
|
||
|
||
* target should support global aliases,
|
||
* the address itself of *G* should be not significant, not named and not
|
||
referenced anywhere,
|
||
* function should come with external, local or weak linkage.
|
||
|
||
Otherwise we write thunk: some wrapper that has *G's* interface and calls *F*,
|
||
so *G* could be replaced with this wrapper.
|
||
|
||
*writeAlias*
|
||
|
||
As follows from *llvm* reference:
|
||
|
||
“Aliases act as *second name* for the aliasee value”. So we just want to create
|
||
second name for *F* and use it instead of *G*:
|
||
|
||
1. create global alias itself (*GA*),
|
||
|
||
2. adjust alignment of *F* so it must be maximum of current and *G's* alignment;
|
||
|
||
3. replace uses of *G*:
|
||
|
||
3.1. first mark all callers of *G* as to-be-analyzed-again, using
|
||
``removeUsers`` method (see chapter above),
|
||
|
||
3.2. call ``G->replaceAllUsesWith(GA)``.
|
||
|
||
4. Get rid of *G*.
|
||
|
||
*writeThunk*
|
||
|
||
As it written in method comments:
|
||
|
||
“Replace G with a simple tail call to bitcast(F). Also replace direct uses of G
|
||
with bitcast(F). Deletes G.”
|
||
|
||
In general it does the same as usual when we want to replace callee, except the
|
||
first point:
|
||
|
||
1. We generate tail call wrapper around *F*, but with interface that allows use
|
||
it instead of *G*.
|
||
|
||
2. “As-usual”: ``removeUsers`` and ``replaceAllUsesWith`` then.
|
||
|
||
3. Get rid of *G*.
|
||
|
||
That's it.
|
||
==========
|
||
We have described how to detect equal functions, and how to merge them, and in
|
||
first chapter we have described how it works all-together. Author hopes, reader
|
||
have some picture from now, and it helps him improve and debug this pass.
|
||
|
||
Reader is welcomed to send us any questions and proposals ;-)
|