reference, declarationdefinition
definition → references, declarations, derived classes, virtual overrides
reference to multiple definitions → definitions
unreferenced
    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   15
   16
   17
   18
   19
   20
   21
   22
   23
   24
   25
   26
   27
   28
   29
   30
   31
   32
   33
   34
   35
   36
   37
   38
   39
   40
   41
   42
   43
   44
   45
   46
   47
   48
   49
   50
   51
   52
   53
   54
   55
   56
   57
   58
   59
   60
   61
   62
   63
   64
   65
   66
   67
   68
   69
   70
   71
   72
   73
   74
   75
   76
   77
   78
   79
   80
   81
   82
   83
   84
   85
   86
   87
   88
   89
   90
   91
   92
   93
   94
   95
   96
   97
   98
   99
  100
  101
  102
  103
  104
  105
  106
  107
  108
  109
  110
  111
  112
  113
  114
  115
  116
  117
  118
  119
  120
  121
  122
  123
  124
  125
  126
  127
  128
  129
  130
  131
  132
  133
  134
  135
  136
  137
  138
  139
  140
  141
  142
  143
  144
  145
  146
  147
  148
  149
  150
  151
  152
  153
  154
  155
  156
  157
  158
  159
  160
  161
  162
  163
  164
  165
  166
  167
  168
  169
  170
  171
  172
  173
  174
  175
  176
  177
  178
  179
  180
  181
  182
  183
  184
  185
  186
  187
  188
  189
  190
  191
  192
  193
  194
  195
  196
  197
  198
  199
  200
  201
  202
  203
  204
  205
  206
  207
  208
  209
  210
  211
  212
  213
  214
  215
  216
  217
  218
  219
  220
  221
  222
  223
  224
  225
  226
  227
  228
  229
  230
  231
  232
  233
  234
  235
  236
  237
  238
  239
  240
  241
  242
  243
  244
  245
  246
  247
  248
  249
  250
  251
  252
  253
  254
  255
  256
  257
  258
  259
  260
  261
  262
  263
  264
  265
  266
  267
  268
  269
  270
  271
  272
  273
  274
  275
  276
  277
  278
  279
  280
  281
  282
  283
  284
  285
  286
  287
  288
  289
  290
  291
  292
  293
  294
  295
  296
  297
  298
  299
  300
  301
  302
  303
  304
  305
  306
  307
  308
  309
  310
  311
  312
  313
  314
  315
  316
  317
  318
  319
  320
  321
  322
  323
  324
  325
  326
  327
  328
  329
  330
  331
  332
  333
  334
  335
  336
  337
  338
  339
  340
  341
  342
  343
  344
  345
  346
  347
  348
  349
  350
  351
  352
  353
  354
  355
  356
  357
  358
  359
  360
  361
  362
  363
  364
  365
  366
  367
  368
  369
  370
  371
  372
  373
  374
  375
  376
  377
  378
  379
  380
  381
  382
  383
  384
  385
  386
  387
  388
  389
  390
  391
  392
  393
  394
  395
  396
  397
  398
  399
  400
  401
  402
  403
  404
  405
  406
  407
  408
  409
  410
  411
  412
  413
  414
  415
  416
  417
  418
  419
  420
  421
  422
  423
  424
  425
  426
  427
  428
  429
  430
  431
  432
  433
  434
  435
  436
  437
  438
  439
  440
  441
  442
  443
  444
  445
  446
  447
  448
  449
  450
  451
  452
  453
  454
  455
  456
  457
  458
  459
  460
  461
  462
  463
  464
  465
  466
  467
  468
  469
  470
  471
  472
  473
  474
  475
  476
  477
  478
  479
  480
  481
  482
  483
  484
  485
  486
  487
  488
  489
  490
  491
  492
  493
  494
  495
  496
  497
  498
  499
  500
  501
  502
  503
  504
  505
  506
  507
  508
  509
  510
  511
  512
  513
  514
  515
  516
  517
  518
  519
  520
  521
  522
  523
  524
  525
  526
  527
  528
  529
  530
  531
  532
  533
  534
  535
  536
  537
  538
  539
  540
  541
  542
  543
  544
  545
  546
  547
  548
  549
  550
  551
  552
  553
  554
  555
  556
  557
  558
  559
  560
  561
  562
  563
  564
  565
  566
  567
  568
  569
  570
  571
  572
  573
  574
  575
  576
  577
  578
  579
  580
  581
  582
  583
  584
  585
  586
  587
  588
  589
  590
  591
  592
  593
  594
  595
  596
  597
  598
  599
  600
  601
  602
  603
  604
  605
  606
  607
  608
  609
  610
  611
  612
  613
  614
  615
  616
  617
  618
  619
  620
  621
  622
  623
  624
  625
  626
  627
  628
  629
  630
  631
  632
  633
  634
  635
  636
  637
  638
  639
  640
  641
  642
  643
  644
  645
  646
  647
  648
  649
  650
  651
  652
  653
  654
  655
  656
  657
  658
  659
  660
  661
  662
  663
  664
  665
  666
  667
  668
  669
  670
  671
  672
  673
  674
  675
  676
  677
  678
  679
  680
  681
  682
  683
  684
  685
  686
  687
  688
  689
  690
  691
  692
  693
  694
  695
  696
  697
  698
  699
  700
  701
  702
  703
  704
  705
  706
  707
  708
  709
  710
  711
  712
  713
  714
  715
  716
  717
  718
  719
  720
  721
  722
  723
  724
  725
  726
  727
  728
  729
  730
  731
  732
  733
  734
  735
  736
  737
  738
  739
  740
  741
  742
  743
  744
  745
  746
  747
  748
  749
  750
  751
  752
  753
  754
  755
  756
  757
  758
  759
  760
  761
  762
  763
  764
  765
  766
  767
  768
  769
  770
  771
  772
  773
  774
  775
  776
  777
  778
  779
  780
  781
  782
  783
  784
  785
  786
  787
  788
  789
  790
  791
  792
  793
  794
  795
  796
  797
  798
  799
  800
  801
  802
=================================
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 user itself could
write the same thing twice :-)

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

Why would I want to read this document?
---------------------------------------
Document is the extension to pass comments and describes the pass logic. It
describes algorithm that is used in order to compare functions, it also
explains how we could combine equal functions correctly, keeping module valid.

Material is brought in top-down form, so reader could start learn pass from
ideas and end up with low-level algorithm details, thus preparing him for
reading the sources.

So main goal is do describe algorithm and logic here; the concept. This document
is good for you, if you *don't want* to read the source code, but want to
understand pass algorithms. Author tried not to repeat the source-code and
cover only common cases, and thus avoid cases when after minor code changes we
need to update this document.


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

Reader should be familiar with common compile-engineering principles and LLVM
code fundamentals. In this article we suppose reader is familiar with
`Single Static Assingment <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
concepts. Understanding of
`IR structure <http://llvm.org/docs/LangRef.html#high-level-structure>`_ is
also important.

We will use such terms 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 start point, Kaleidoscope tutorial could be used:

:doc:`tutorial/index`

Especially it's important to understand chapter 3 of tutorial:

:doc:`tutorial/LangImpl03`

Reader also should know how passes work in LLVM, they could use next article as
a reference and start point here:

:doc:`WritingAnLLVMPass`

What else? Well perhaps reader also should have some experience in LLVM pass
debugging and bug-fixing.

What I gain by reading this document?
-------------------------------------
Main purpose is to provide reader with comfortable form of algorithms
description, namely the human reading text. Since it could be hard to
understand algorithm straight from the source code: pass uses some principles
that have to be explained first.

Author wishes to everybody to avoid case, when you read code from top to bottom
again and again, and yet you don't understand why we implemented it that way.

We hope that after this article reader could easily debug and improve
MergeFunctions pass and thus help LLVM project.

Narrative structure
-------------------
Article consists of three parts. First part explains pass functionality on the
top-level. Second part describes the comparison procedure itself. The third
part describes the merging process.

In every part author also tried to put the contents into the top-down form.
First, the top-level methods will be described, while the terminal ones will be
at the end, in the tail of each part. If reader will see 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? Obvious thing is: yes that's a quite possible
case, since usually we *do* have duplicates. And it would be good to get rid of
them. But how to detect such a duplicates? The idea is next: we split functions
onto small bricks (parts), then we compare "bricks" amount, and if it equal,
compare "bricks" themselves, and then do our conclusions about functions
themselves.

What the difference it could be? For example, on machine with 64-bit pointers
(let's assume we have only one address space),  one function stores 64-bit
integer, while another one stores a pointer. So if the target is a 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 ``uint64_t``
and``void*`` as equal.

It was just an example; possible details are described a bit below.

As another example reader may imagine two more functions. First function
performs multiplication on 2, while the second one performs 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
meant for us.

Equal functions detection, obviously supposes "detector" method to be
implemented, latter should answer the question "whether functions are equal".
This "detector" method consists of tiny "sub-detectors", each of them 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 a routines in our hands, we can process whole module, and merge all
equal functions.

In this case, we have to compare every function with every another function. As
reader could 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 be done? Just convert each function to number, and gather all of
them in special hash-table. Functions with equal hash 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 hash.
Lookup-up time would be small, but such approach adds some delay due to hashing
routine.

Logarithmical search
""""""""""""""""""""
We could introduce total ordering among the functions set, once we had it 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 approaches (random-access and logarithmical) has been implemented and
tested. And both of them gave a very good improvement. And what was most
surprising, logarithmical search was faster; sometimes up to 15%. Hashing needs
some extra CPU time, and it is the main reason why it works slower; in most of
cases total "hashing" time was 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 flags comparison. In
*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
whole function body every time (note, it could be slower). Like in
"total-ordering", we will track every numbers and flags, but instead of
comparison, we should get 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 most important fields in 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 as 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 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* equal function in *FnTree* (call it *FExists*):
   merge function *FCur* with *FExists*. Otherwise add function from *worklist*
   to *FnTree*.

3. Once *worklist* scanning and merging operations is complete, check *Deferred*
list. If it is not empty: refill *worklist* contents with *Deferred* list and
do step 2 again, if *Deferred* 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 shortest time, and merge them into the single function.

Defining total ordering among the functions set allows to organize functions
into the binary tree. The lookup procedure complexity would be estimated as
O(log(N)) in this case. But how to 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
it could be? Let's declare it as "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 it was mentioned before, comparison routine consists of
"sub-comparison-routines", each of them also consists
"sub-comparison-routines", and so on, finally it ends up with a primitives
comparison.

Below, we will use the next operations:

#. ``cmpNumbers(number1, number2)`` is method that returns -1 if left is less
   than right; 0, if left and right are equal; and 1 otherwise.

#. ``cmpFlags(flag1, flag2)`` is 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 article is based on *MergeFunctions.cpp* source code
(*<llvm_dir>/lib/Transforms/IPO/MergeFunctions.cpp*). We would like to ask
reader to keep this file open nearby, 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 objects comparison (function, basic-block, etc) is mostly based on its
sub-objects comparison results. So 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*".

#. Then compare chains "*T1Items*" and "*T2Items*" in
   most-significant-item-first order. Result of items comparison would be the
   result of *T1* and *T2* comparison itself.

FunctionComparator::compare(void)
---------------------------------
Brief look at the source code tells us, that comparison starts in
“``int FunctionComparator::compare(void)``” method.

1. First parts to be compared are function's attributes and some properties that
outsides “attributes” term, but still could make function different without
changing its body. This part of comparison is usually done within simple
*cmpNumbers* or *cmpFlags* operations (e.g.
``cmpFlags(F1->hasGC(), F2->hasGC())``). Below is 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 types 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 a 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
unexpectable case.

cmpValues(const Value*, const Value*)
-------------------------------------
Method that compares local values.

This method gives us an answer on a very curious quesion: whether we could treat
local values as equal, and which value is greater otherwise. It's better to
start from example:

Consider situation when we're looking at the same place in left function "*FL*"
and in right function "*FR*". And 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.

Main purpose of this method is to determine relation between such values.

What 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 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*.

Instruction with opcode "*instr1*" from *f* is *greater* than instruction with
opcode "*instr1*" from *g*; here we have equal types and opcodes, but "*pf1* is
greater than "*pg0*".

And instructions with opcode "*instr2*" are equal, because their opcodes and
types are equal, and the same constant is used as a value.

What we assiciate 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 global value. It is constant, since its the only
  supposed global here. Method also compares:
* Constants that are of the same type.
* If right constant could 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 it was
mentioned above, to understand what is *less*, we can use order in which we
meet values. If both of values has the same order in function (met at the same
time), then treat values as *associated*. Otherwise – it depends on who was
first.

Every time we run 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 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, 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`` 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 ;-)