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diff --git a/gnu/llvm/docs/MergeFunctions.rst b/gnu/llvm/docs/MergeFunctions.rst deleted file mode 100644 index 7c51adac681..00000000000 --- a/gnu/llvm/docs/MergeFunctions.rst +++ /dev/null @@ -1,785 +0,0 @@ -================================= -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*. - - |