================================= | |

MergeFunctions pass, how it works | |

================================= | |

.. contents:: | |

:local: | |

Introduction | |

============ | |

Sometimes code contains equal functions, or functions that does exactly the same | |

thing even though they are non-equal on the IR level (e.g.: multiplication on 2 | |

and 'shl 1'). It could happen due to several reasons: mainly, the usage of | |

templates and automatic code generators. Though, sometimes the user itself could | |

write the same thing twice :-) | |

The main purpose of this pass is to recognize such functions and merge them. | |

This document is the extension to pass comments and describes the pass logic. It | |

describes the algorithm that is used in order to compare functions and | |

explains how we could combine equal functions correctly to keep the module | |

valid. | |

Material is brought in a top-down form, so the reader could start to learn pass | |

from high level ideas and end with low-level algorithm details, thus preparing | |

him or her for reading the sources. | |

The main goal is to describe the algorithm and logic here and the concept. If | |

you *don't want* to read the source code, but want to understand pass | |

algorithms, this document is good for you. The author tries not to repeat the | |

source-code and covers only common cases to avoid the cases of needing to | |

update this document after any minor code changes. | |

What should I know to be able to follow along with this document? | |

----------------------------------------------------------------- | |

The reader should be familiar with common compile-engineering principles and | |

LLVM code fundamentals. In this article, we assume the reader is familiar with | |

`Single Static Assignment | |

<http://en.wikipedia.org/wiki/Static_single_assignment_form>`_ | |

concept and has an understanding of | |

`IR structure <http://llvm.org/docs/LangRef.html#high-level-structure>`_. | |

We will use terms such as | |

"`module <http://llvm.org/docs/LangRef.html#high-level-structure>`_", | |

"`function <http://llvm.org/docs/ProgrammersManual.html#the-function-class>`_", | |

"`basic block <http://en.wikipedia.org/wiki/Basic_block>`_", | |

"`user <http://llvm.org/docs/ProgrammersManual.html#the-user-class>`_", | |

"`value <http://llvm.org/docs/ProgrammersManual.html#the-value-class>`_", | |

"`instruction | |

<http://llvm.org/docs/ProgrammersManual.html#the-instruction-class>`_". | |

As a good starting point, the Kaleidoscope tutorial can be used: | |

:doc:`tutorial/index` | |

It's especially important to understand chapter 3 of tutorial: | |

:doc:`tutorial/LangImpl03` | |

The reader should also know how passes work in LLVM. They could use this | |

article as a reference and start point here: | |

:doc:`WritingAnLLVMPass` | |

What else? Well perhaps the reader should also have some experience in LLVM pass | |

debugging and bug-fixing. | |

Narrative structure | |

------------------- | |

The article consists of three parts. The first part explains pass functionality | |

on the top-level. The second part describes the comparison procedure itself. | |

The third part describes the merging process. | |

In every part, the author tries to put the contents in the top-down form. | |

The top-level methods will first be described followed by the terminal ones at | |

the end, in the tail of each part. If the reader sees the reference to the | |

method that wasn't described yet, they will find its description a bit below. | |

Basics | |

====== | |

How to do it? | |

------------- | |

Do we need to merge functions? The obvious answer is: Yes, that is quite a | |

possible case. We usually *do* have duplicates and it would be good to get rid | |

of them. But how do we detect duplicates? This is the idea: we split functions | |

into smaller bricks or parts and compare the "bricks" amount. If equal, | |

we compare the "bricks" themselves, and then do our conclusions about functions | |

themselves. | |

What could the difference be? For example, on a machine with 64-bit pointers | |

(let's assume we have only one address space), one function stores a 64-bit | |

integer, while another one stores a pointer. If the target is the machine | |

mentioned above, and if functions are identical, except the parameter type (we | |

could consider it as a part of function type), then we can treat a ``uint64_t`` | |

and a ``void*`` as equal. | |

This is just an example; more possible details are described a bit below. | |

As another example, the reader may imagine two more functions. The first | |

function performs a multiplication on 2, while the second one performs an | |

arithmetic right shift on 1. | |

Possible solutions | |

^^^^^^^^^^^^^^^^^^ | |

Let's briefly consider possible options about how and what we have to implement | |

in order to create full-featured functions merging, and also what it would | |

mean for us. | |

Equal function detection obviously supposes that a "detector" method to be | |

implemented and latter should answer the question "whether functions are equal". | |

This "detector" method consists of tiny "sub-detectors", which each answers | |

exactly the same question, but for function parts. | |

As the second step, we should merge equal functions. So it should be a "merger" | |

method. "Merger" accepts two functions *F1* and *F2*, and produces *F1F2* | |

function, the result of merging. | |

Having such routines in our hands, we can process a whole module, and merge all | |

equal functions. | |

In this case, we have to compare every function with every another function. As | |

the reader may notice, this way seems to be quite expensive. Of course we could | |

introduce hashing and other helpers, but it is still just an optimization, and | |

thus the level of O(N*N) complexity. | |

Can we reach another level? Could we introduce logarithmical search, or random | |

access lookup? The answer is: "yes". | |

Random-access | |

""""""""""""" | |

How it could this be done? Just convert each function to a number, and gather | |

all of them in a special hash-table. Functions with equal hashes are equal. | |

Good hashing means, that every function part must be taken into account. That | |

means we have to convert every function part into some number, and then add it | |

into the hash. The lookup-up time would be small, but such a approach adds some | |

delay due to the hashing routine. | |

Logarithmical search | |

"""""""""""""""""""" | |

We could introduce total ordering among the functions set, once ordered we | |

could then implement a logarithmical search. Lookup time still depends on N, | |

but adds a little of delay (*log(N)*). | |

Present state | |

""""""""""""" | |

Both of the approaches (random-access and logarithmical) have been implemented | |

and tested and both give a very good improvement. What was most | |

surprising is that logarithmical search was faster; sometimes by up to 15%. The | |

hashing method needs some extra CPU time, which is the main reason why it works | |

slower; in most cases, total "hashing" time is greater than total | |

"logarithmical-search" time. | |

So, preference has been granted to the "logarithmical search". | |

Though in the case of need, *logarithmical-search* (read "total-ordering") could | |

be used as a milestone on our way to the *random-access* implementation. | |

Every comparison is based either on the numbers or on the flags comparison. In | |

the *random-access* approach, we could use the same comparison algorithm. | |

During comparison, we exit once we find the difference, but here we might have | |

to scan the whole function body every time (note, it could be slower). Like in | |

"total-ordering", we will track every number and flag, but instead of | |

comparison, we should get the numbers sequence and then create the hash number. | |

So, once again, *total-ordering* could be considered as a milestone for even | |

faster (in theory) random-access approach. | |

MergeFunctions, main fields and runOnModule | |

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |

There are two main important fields in the class: | |

``FnTree`` – the set of all unique functions. It keeps items that couldn't be | |

merged with each other. It is defined as: | |

``std::set<FunctionNode> FnTree;`` | |

Here ``FunctionNode`` is a wrapper for ``llvm::Function`` class, with | |

implemented “<” operator among the functions set (below we explain how it works | |

exactly; this is a key point in fast functions comparison). | |

``Deferred`` – merging process can affect bodies of functions that are in | |

``FnTree`` already. Obviously, such functions should be rechecked again. In this | |

case, we remove them from ``FnTree``, and mark them to be rescanned, namely | |

put them into ``Deferred`` list. | |

runOnModule | |

""""""""""" | |

The algorithm is pretty simple: | |

1. Put all module's functions into the *worklist*. | |

2. Scan *worklist*'s functions twice: first enumerate only strong functions and | |

then only weak ones: | |

2.1. Loop body: take a function from *worklist* (call it *FCur*) and try to | |

insert it into *FnTree*: check whether *FCur* is equal to one of functions | |

in *FnTree*. If there *is* an equal function in *FnTree* | |

(call it *FExists*): merge function *FCur* with *FExists*. Otherwise add | |

the function from the *worklist* to *FnTree*. | |

3. Once the *worklist* scanning and merging operations are complete, check the | |

*Deferred* list. If it is not empty: refill the *worklist* contents with | |

*Deferred* list and redo step 2, if the *Deferred* list is empty, then exit | |

from method. | |

Comparison and logarithmical search | |

""""""""""""""""""""""""""""""""""" | |

Let's recall our task: for every function *F* from module *M*, we have to find | |

equal functions *F`* in the shortest time possible , and merge them into a | |

single function. | |

Defining total ordering among the functions set allows us to organize | |

functions into a binary tree. The lookup procedure complexity would be | |

estimated as O(log(N)) in this case. But how do we define *total-ordering*? | |

We have to introduce a single rule applicable to every pair of functions, and | |

following this rule, then evaluate which of them is greater. What kind of rule | |

could it be? Let's declare it as the "compare" method that returns one of 3 | |

possible values: | |

-1, left is *less* than right, | |

0, left and right are *equal*, | |

1, left is *greater* than right. | |

Of course it means, that we have to maintain | |

*strict and non-strict order relation properties*: | |

* reflexivity (``a <= a``, ``a == a``, ``a >= a``), | |

* antisymmetry (if ``a <= b`` and ``b <= a`` then ``a == b``), | |

* transitivity (``a <= b`` and ``b <= c``, then ``a <= c``) | |

* asymmetry (if ``a < b``, then ``a > b`` or ``a == b``). | |

As mentioned before, the comparison routine consists of | |

"sub-comparison-routines", with each of them also consisting of | |

"sub-comparison-routines", and so on. Finally, it ends up with primitive | |

comparison. | |

Below, we will use the following operations: | |

#. ``cmpNumbers(number1, number2)`` is a method that returns -1 if left is less | |

than right; 0, if left and right are equal; and 1 otherwise. | |

#. ``cmpFlags(flag1, flag2)`` is a hypothetical method that compares two flags. | |

The logic is the same as in ``cmpNumbers``, where ``true`` is 1, and | |

``false`` is 0. | |

The rest of the article is based on *MergeFunctions.cpp* source code | |

(found in *<llvm_dir>/lib/Transforms/IPO/MergeFunctions.cpp*). We would like | |

to ask reader to keep this file open, so we could use it as a reference | |

for further explanations. | |

Now, we're ready to proceed to the next chapter and see how it works. | |

Functions comparison | |

==================== | |

At first, let's define how exactly we compare complex objects. | |

Complex object comparison (function, basic-block, etc) is mostly based on its | |

sub-object comparison results. It is similar to the next "tree" objects | |

comparison: | |

#. For two trees *T1* and *T2* we perform *depth-first-traversal* and have | |

two sequences as a product: "*T1Items*" and "*T2Items*". | |

#. We then compare chains "*T1Items*" and "*T2Items*" in | |

the most-significant-item-first order. The result of items comparison | |

would be the result of *T1* and *T2* comparison itself. | |

FunctionComparator::compare(void) | |

--------------------------------- | |

A brief look at the source code tells us that the comparison starts in the | |

“``int FunctionComparator::compare(void)``” method. | |

1. The first parts to be compared are the function's attributes and some | |

properties that is outside the “attributes” term, but still could make the | |

function different without changing its body. This part of the comparison is | |

usually done within simple *cmpNumbers* or *cmpFlags* operations (e.g. | |

``cmpFlags(F1->hasGC(), F2->hasGC())``). Below is a full list of function's | |

properties to be compared on this stage: | |

* *Attributes* (those are returned by ``Function::getAttributes()`` | |

method). | |

* *GC*, for equivalence, *RHS* and *LHS* should be both either without | |

*GC* or with the same one. | |

* *Section*, just like a *GC*: *RHS* and *LHS* should be defined in the | |

same section. | |

* *Variable arguments*. *LHS* and *RHS* should be both either with or | |

without *var-args*. | |

* *Calling convention* should be the same. | |

2. Function type. Checked by ``FunctionComparator::cmpType(Type*, Type*)`` | |

method. It checks return type and parameters type; the method itself will be | |

described later. | |

3. Associate function formal parameters with each other. Then comparing function | |

bodies, if we see the usage of *LHS*'s *i*-th argument in *LHS*'s body, then, | |

we want to see usage of *RHS*'s *i*-th argument at the same place in *RHS*'s | |

body, otherwise functions are different. On this stage we grant the preference | |

to those we met later in function body (value we met first would be *less*). | |

This is done by “``FunctionComparator::cmpValues(const Value*, const Value*)``” | |

method (will be described a bit later). | |

4. Function body comparison. As it written in method comments: | |

“We do a CFG-ordered walk since the actual ordering of the blocks in the linked | |

list is immaterial. Our walk starts at the entry block for both functions, then | |

takes each block from each terminator in order. As an artifact, this also means | |

that unreachable blocks are ignored.” | |

So, using this walk we get BBs from *left* and *right* in the same order, and | |

compare them by “``FunctionComparator::compare(const BasicBlock*, const | |

BasicBlock*)``” method. | |

We also associate BBs with each other, like we did it with function formal | |

arguments (see ``cmpValues`` method below). | |

FunctionComparator::cmpType | |

--------------------------- | |

Consider how type comparison works. | |

1. Coerce pointer to integer. If left type is a pointer, try to coerce it to the | |

integer type. It could be done if its address space is 0, or if address spaces | |

are ignored at all. Do the same thing for the right type. | |

2. If left and right types are equal, return 0. Otherwise we need to give | |

preference to one of them. So proceed to the next step. | |

3. If types are of different kind (different type IDs). Return result of type | |

IDs comparison, treating them as numbers (use ``cmpNumbers`` operation). | |

4. If types are vectors or integers, return result of their pointers comparison, | |

comparing them as numbers. | |

5. Check whether type ID belongs to the next group (call it equivalent-group): | |

* Void | |

* Float | |

* Double | |

* X86_FP80 | |

* FP128 | |

* PPC_FP128 | |

* Label | |

* Metadata. | |

If ID belongs to group above, return 0. Since it's enough to see that | |

types has the same ``TypeID``. No additional information is required. | |

6. Left and right are pointers. Return result of address space comparison | |

(numbers comparison). | |

7. Complex types (structures, arrays, etc.). Follow complex objects comparison | |

technique (see the very first paragraph of this chapter). Both *left* and | |

*right* are to be expanded and their element types will be checked the same | |

way. If we get -1 or 1 on some stage, return it. Otherwise return 0. | |

8. Steps 1-6 describe all the possible cases, if we passed steps 1-6 and didn't | |

get any conclusions, then invoke ``llvm_unreachable``, since it's quite an | |

unexpectable case. | |

cmpValues(const Value*, const Value*) | |

------------------------------------- | |

Method that compares local values. | |

This method gives us an answer to a very curious question: whether we could | |

treat local values as equal, and which value is greater otherwise. It's | |

better to start from example: | |

Consider the situation when we're looking at the same place in left | |

function "*FL*" and in right function "*FR*". Every part of *left* place is | |

equal to the corresponding part of *right* place, and (!) both parts use | |

*Value* instances, for example: | |

.. code-block:: text | |

instr0 i32 %LV ; left side, function FL | |

instr0 i32 %RV ; right side, function FR | |

So, now our conclusion depends on *Value* instances comparison. | |

The main purpose of this method is to determine relation between such values. | |

What can we expect from equal functions? At the same place, in functions | |

"*FL*" and "*FR*" we expect to see *equal* values, or values *defined* at | |

the same place in "*FL*" and "*FR*". | |

Consider a small example here: | |

.. code-block:: text | |

define void %f(i32 %pf0, i32 %pf1) { | |

instr0 i32 %pf0 instr1 i32 %pf1 instr2 i32 123 | |

} | |

.. code-block:: text | |

define void %g(i32 %pg0, i32 %pg1) { | |

instr0 i32 %pg0 instr1 i32 %pg0 instr2 i32 123 | |

} | |

In this example, *pf0* is associated with *pg0*, *pf1* is associated with | |

*pg1*, and we also declare that *pf0* < *pf1*, and thus *pg0* < *pf1*. | |

Instructions with opcode "*instr0*" would be *equal*, since their types and | |

opcodes are equal, and values are *associated*. | |

Instructions with opcode "*instr1*" from *f* is *greater* than instructions | |

with opcode "*instr1*" from *g*; here we have equal types and opcodes, but | |

"*pf1* is greater than "*pg0*". | |

Instructions with opcode "*instr2*" are equal, because their opcodes and | |

types are equal, and the same constant is used as a value. | |

What we associate in cmpValues? | |

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |

* Function arguments. *i*-th argument from left function associated with | |

*i*-th argument from right function. | |

* BasicBlock instances. In basic-block enumeration loop we associate *i*-th | |

BasicBlock from the left function with *i*-th BasicBlock from the right | |

function. | |

* Instructions. | |

* Instruction operands. Note, we can meet *Value* here we have never seen | |

before. In this case it is not a function argument, nor *BasicBlock*, nor | |

*Instruction*. It is a global value. It is a constant, since it's the only | |

supposed global here. The method also compares: Constants that are of the | |

same type and if right constant can be losslessly bit-casted to the left | |

one, then we also compare them. | |

How to implement cmpValues? | |

^^^^^^^^^^^^^^^^^^^^^^^^^^^ | |

*Association* is a case of equality for us. We just treat such values as equal, | |

but, in general, we need to implement antisymmetric relation. As mentioned | |

above, to understand what is *less*, we can use order in which we | |

meet values. If both values have the same order in a function (met at the same | |

time), we then treat values as *associated*. Otherwise – it depends on who was | |

first. | |

Every time we run the top-level compare method, we initialize two identical | |

maps (one for the left side, another one for the right side): | |

``map<Value, int> sn_mapL, sn_mapR;`` | |

The key of the map is the *Value* itself, the *value* – is its order (call it | |

*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 the second *Value* map will | |

return *1*, and so on. | |

We can then check whether left and right values met at the same time with | |

a 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 with 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 in test-suite), and, we hope, the 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: we just return | |

the 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 the 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 the 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 the 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 are equal, return 0. | |

4.2. We have reached the end of the left basic-block. Return -1. | |

4.3. Return 1 (we reached 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 a 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, that's | |

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``. Just | |

go through all calls of *G* and replace it with calls of *F*. If you look into | |

the 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. The 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 | |

a 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*. | |