The cast<> operator is a "checked cast" operation. It - converts a pointer or reference from a base class to a derived cast, causing + converts a pointer or reference from a base class to a derived class, causing an assertion failure if it is not really an instance of the right type. This should be used in cases where you have some information that makes you believe that something is of the right type. An example of the isa<> @@ -408,6 +437,106 @@ are lots of examples in the LLVM source base.
Although LLVM generally does not do much string manipulation, we do have +several important APIs which take strings. Two important examples are the +Value class -- which has names for instructions, functions, etc. -- and the +StringMap class which is used extensively in LLVM and Clang.
+ +These are generic classes, and they need to be able to accept strings which +may have embedded null characters. Therefore, they cannot simply take +a const char *, and taking a const std::string& requires +clients to perform a heap allocation which is usually unnecessary. Instead, +many LLVM APIs use a StringRef or a const Twine& for +passing strings efficiently.
+ +The StringRef data type represents a reference to a constant string +(a character array and a length) and supports the common operations available +on std:string, but does not require heap allocation.
+ +It can be implicitly constructed using a C style null-terminated string, +an std::string, or explicitly with a character pointer and length. +For example, the StringRef find function is declared as:
+ ++ iterator find(StringRef Key); ++ +
and clients can call it using any one of:
+ +
+ Map.find("foo"); // Lookup "foo"
+ Map.find(std::string("bar")); // Lookup "bar"
+ Map.find(StringRef("\0baz", 4)); // Lookup "\0baz"
+
+
+Similarly, APIs which need to return a string may return a StringRef +instance, which can be used directly or converted to an std::string +using the str member function. See +"llvm/ADT/StringRef.h" +for more information.
+ +You should rarely use the StringRef class directly, because it contains +pointers to external memory it is not generally safe to store an instance of the +class (unless you know that the external storage will not be freed). StringRef is +small and pervasive enough in LLVM that it should always be passed by value.
+ +The Twine class is an efficient way for APIs to accept concatenated +strings. For example, a common LLVM paradigm is to name one instruction based on +the name of another instruction with a suffix, for example:
+ ++ New = CmpInst::Create(..., SO->getName() + ".cmp"); ++
The Twine class is effectively a +lightweight rope +which points to temporary (stack allocated) objects. Twines can be implicitly +constructed as the result of the plus operator applied to strings (i.e., a C +strings, an std::string, or a StringRef). The twine delays the +actual concatenation of strings until it is actually required, at which point +it can be efficiently rendered directly into a character array. This avoids +unnecessary heap allocation involved in constructing the temporary results of +string concatenation. See +"llvm/ADT/Twine.h" +for more information.
+ +As with a StringRef, Twine objects point to external memory +and should almost never be stored or mentioned directly. They are intended +solely for use when defining a function which should be able to efficiently +accept concatenated strings.
+ +-DOUT << "I am here!\n"; +DEBUG(errs() << "I am here!\n");
-DOUT << "No debug type\n"; #undef DEBUG_TYPE +DEBUG(errs() << "No debug type\n"); #define DEBUG_TYPE "foo" -DOUT << "'foo' debug type\n"; +DEBUG(errs() << "'foo' debug type\n"); #undef DEBUG_TYPE #define DEBUG_TYPE "bar" -DOUT << "'bar' debug type\n"; +DEBUG(errs() << "'bar' debug type\n")); #undef DEBUG_TYPE #define DEBUG_TYPE "" -DOUT << "No debug type (2)\n"; +DEBUG(errs() << "No debug type (2)\n");
The DEBUG_WITH_TYPE macro is also available for situations where you +would like to set DEBUG_TYPE, but only for one specific DEBUG +statement. It takes an additional first parameter, which is the type to use. For +example, the preceding example could be written as:
+ + +
+DEBUG_WITH_TYPE("", errs() << "No debug type\n");
+DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n");
+DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n"));
+DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n");
+
+ilist has the same drawbacks as std::list, and additionally requires an -ilist_traits implementation for the element type, but it provides some novel -characteristics. In particular, it can efficiently store polymorphic objects, -the traits class is informed when an element is inserted or removed from the -list, and ilists are guaranteed to support a constant-time splice operation. -
+ilist has the same drawbacks as std::list, and additionally +requires an ilist_traits implementation for the element type, but it +provides some novel characteristics. In particular, it can efficiently store +polymorphic objects, the traits class is informed when an element is inserted or +removed from the list, and ilists are guaranteed to support a +constant-time splice operation.
+ +These properties are exactly what we want for things like +Instructions and basic blocks, which is why these are implemented with +ilists.
+ +Related classes of interest are explained in the following subsections: +-
+
- ilist_traits +
- iplist +
- llvm/ADT/ilist_node.h +
- Sentinels +
ilist_traits<T> is ilist<T>'s customization +mechanism. iplist<T> (and consequently ilist<T>) +publicly derive from this traits class.
+iplist<T> is ilist<T>'s base and as such +supports a slightly narrower interface. Notably, inserters from +T& are absent.
+ +ilist_traits<T> is a public base of this class and can be +used for a wide variety of customizations.
+ilist_node<T> implements a the forward and backward links +that are expected by the ilist<T> (and analogous containers) +in the default manner.
+ +ilist_node<T>s are meant to be embedded in the node type +T, usually T publicly derives from +ilist_node<T>.
+ilists have another specialty that must be considered. To be a good +citizen in the C++ ecosystem, it needs to support the standard container +operations, such as begin and end iterators, etc. Also, the +operator-- must work correctly on the end iterator in the +case of non-empty ilists.
-These properties are exactly what we want for things like Instructions and -basic blocks, which is why these are implemented with ilists.
+The only sensible solution to this problem is to allocate a so-called +sentinel along with the intrusive list, which serves as the end +iterator, providing the back-link to the last element. However conforming to the +C++ convention it is illegal to operator++ beyond the sentinel and it +also must not be dereferenced.
+ +These constraints allow for some implementation freedom to the ilist +how to allocate and store the sentinel. The corresponding policy is dictated +by ilist_traits<T>. By default a T gets heap-allocated +whenever the need for a sentinel arises.
+ +While the default policy is sufficient in most cases, it may break down when +T does not provide a default constructor. Also, in the case of many +instances of ilists, the memory overhead of the associated sentinels +is wasted. To alleviate the situation with numerous and voluminous +T-sentinels, sometimes a trick is employed, leading to ghostly +sentinels.
+ +Ghostly sentinels are obtained by specially-crafted ilist_traits<T> +which superpose the sentinel with the ilist instance in memory. Pointer +arithmetic is used to obtain the sentinel, which is relative to the +ilist's this pointer. The ilist is augmented by an +extra pointer, which serves as the back-link of the sentinel. This is the only +field in the ghostly sentinel which can be legally accessed.
SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is -transparently implemented with a SmallPtrSet), but also supports iterators. If +
SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is +transparently implemented with a SmallPtrSet), but also supports iterators. If more than 'N' insertions are performed, a single quadratically probed hash table is allocated and grows as needed, providing extremely efficient access (constant time insertion/deleting/queries with low constant factors) and is very stingy with malloc traffic.
-Note that, unlike std::set, the iterators of SmallPtrSet are invalidated +
Note that, unlike std::set, the iterators of SmallPtrSet are invalidated whenever an insertion occurs. Also, the values visited by the iterators are not visited in sorted order.
+DenseSet is a simple quadratically probed hash table. It excels at supporting +small values: it uses a single allocation to hold all of the pairs that +are currently inserted in the set. DenseSet is a great way to unique small +values that are not simple pointers (use SmallPtrSet for pointers). Note that DenseSet has +the same requirements for the value type that DenseMap has. +
+ +The STL provides several other options, such as std::multiset and the various -"hash_set" like containers (whether from C++ TR1 or from the SGI library).
+"hash_set" like containers (whether from C++ TR1 or from the SGI library). We +never use hash_set and unordered_set because they are generally very expensive +(each insertion requires a malloc) and very non-portable. +std::multiset is useful if you're not interested in elimination of duplicates, but has all the drawbacks of std::set. A sorted vector (where you don't delete duplicate entries) or some other approach is almost always better.
-The various hash_set implementations (exposed portably by -"llvm/ADT/hash_set") is a simple chained hashtable. This algorithm is as malloc -intensive as std::set (performing an allocation for each element inserted, -thus having really high constant factors) but (usually) provides O(1) -insertion/deletion of elements. This can be useful if your elements are large -(thus making the constant-factor cost relatively low) or if comparisons are -expensive. Element iteration does not visit elements in a useful order.
-The StringMap is very fast for several reasons: quadratic probing is very cache efficient for lookups, the hash value of strings in buckets is not -recomputed when lookup up an element, StringMap rarely has to touch the +recomputed when looking up an element, StringMap rarely has to touch the memory for unrelated objects when looking up a value (even when hash collisions happen), hash table growth does not recompute the hash values for strings already in the table, and each pair in the map is store in a single allocation @@ -1241,6 +1490,23 @@ inserted into the map) that it needs internally.
+ + + +
+ValueMap is a wrapper around a DenseMap mapping
+Value*s (or subclasses) to another type. When a Value is deleted or RAUW'ed,
+ValueMap will update itself so the new version of the key is mapped to the same
+value, just as if the key were a WeakVH. You can configure exactly how this
+happens, and what else happens on these two events, by passing
+a Config parameter to the ValueMap template.
The STL provides several other options, such as std::multimap and the various -"hash_map" like containers (whether from C++ TR1 or from the SGI library).
+"hash_map" like containers (whether from C++ TR1 or from the SGI library). We +never use hash_set and unordered_set because they are generally very expensive +(each insertion requires a malloc) and very non-portable.std::multimap is useful if you want to map a key to multiple values, but has all the drawbacks of std::map. A sorted vector or some other approach is almost always better.
-The various hash_map implementations (exposed portably by -"llvm/ADT/hash_map") are simple chained hash tables. This algorithm is as -malloc intensive as std::map (performing an allocation for each element -inserted, thus having really high constant factors) but (usually) provides O(1) -insertion/deletion of elements. This can be useful if your elements are large -(thus making the constant-factor cost relatively low) or if comparisons are -expensive. Element iteration does not visit elements in a useful order.
++TODO: const char* vs stringref vs smallstring vs std::string. Describe twine, +xref to #string_apis. +
Unlike the other containers, there are only two bit storage containers, and +choosing when to use each is relatively straightforward.
+ +One additional option is +std::vector<bool>: we discourage its use for two reasons 1) the +implementation in many common compilers (e.g. commonly available versions of +GCC) is extremely inefficient and 2) the C++ standards committee is likely to +deprecate this container and/or change it significantly somehow. In any case, +please don't use it.
The BitVector container provides a fixed size set of bits for manipulation. +
The BitVector container provides a dynamic size set of bits for manipulation. It supports individual bit setting/testing, as well as set operations. The set operations take time O(size of bitvector), but operations are performed one word at a time, instead of one bit at a time. This makes the BitVector very fast for @@ -1311,6 +1592,25 @@ the number of set bits to be high (IE a dense set).
The SmallBitVector container provides the same interface as BitVector, but +it is optimized for the case where only a small number of bits, less than +25 or so, are needed. It also transparently supports larger bit counts, but +slightly less efficiently than a plain BitVector, so SmallBitVector should +only be used when larger counts are rare. +
+ ++At this time, SmallBitVector does not support set operations (and, or, xor), +and its operator[] does not provide an assignable lvalue. +
+However, this isn't really the best way to print out the contents of a BasicBlock! Since the ostream operators are overloaded for virtually anything you'll care about, you could have just invoked the print routine on the -basic block itself: llvm::cerr << *blk << "\n";.
+basic block itself: errs() << *blk << "\n";. @@ -1455,8 +1755,8 @@ small example that shows how to dump all instructions in a function to the stand #include "llvm/Support/InstIterator.h" // F is a pointer to a Function instance -for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) - llvm::cerr << *i << "\n"; +for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) + errs() << *I << "\n"; @@ -1468,7 +1768,10 @@ F, all you would need to do is something like:std::set<Instruction*> worklist; -worklist.insert(inst_begin(F), inst_end(F)); +// or better yet, SmallPtrSet<Instruction*, 64> worklist; + +for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) + worklist.insert(&*I);
-Instruction* pinst = &*i; +Instruction *pinst = &*i;
-Instruction* pinst = i; +Instruction *pinst = i;
Unfortunately, these implicit conversions come at a cost; they prevent +these iterators from conforming to standard iterator conventions, and thus +from being usable with standard algorithms and containers. For example, they +prevent the following code, where B is a BasicBlock, +from compiling:
+ ++ llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end()); ++
Because of this, these implicit conversions may be removed some day, +and operator* changed to return a pointer instead of a reference.
+ @@ -1580,13 +1898,12 @@ class OurFunctionPass : public FunctionPass { virtual runOnFunction(Function& F) { for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) { - for (BasicBlock::iterator i = b->begin(); ie = b->end(); i != ie; ++i) { + for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) { if (CallInst* callInst = dyn_cast<CallInst>(&*i)) { // We know we've encountered a call instruction, so we // need to determine if it's a call to the - // function pointed to by m_func or not - + // function pointed to by m_func or not. if (callInst->getCalledFunction() == targetFunc) ++callCounter; } @@ -1595,7 +1912,7 @@ class OurFunctionPass : public FunctionPass { } private: - unsigned callCounter; + unsigned callCounter; }; @@ -1647,17 +1964,21 @@ of F:
-Function* F = ...;
+Function *F = ...;
for (Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(*i)) {
- llvm::cerr << "F is used in instruction:\n";
- llvm::cerr << *Inst << "\n";
+ errs() << "F is used in instruction:\n";
+ errs() << *Inst << "\n";
}
Alternately, it's common to have an instance of the Note that dereferencing a Value::use_iterator is not a very cheap +operation. Instead of performing *i above several times, consider +doing it only once in the loop body and reusing its result.
+ +Alternatively, it's common to have an instance of the User Class and need to know what Values are used by it. The list of all Values used by a User is known as a use-def chain. Instances of class @@ -1667,22 +1988,55 @@ the particular Instruction):
-Instruction* pi = ...;
+Instruction *pi = ...;
for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
- Value* v = *i;
+ Value *v = *i;
// ...
}
Declaring objects as const is an important tool of enforcing +mutation free algorithms (such as analyses, etc.). For this purpose above +iterators come in constant flavors as Value::const_use_iterator +and Value::const_op_iterator. They automatically arise when +calling use/op_begin() on const Value*s or +const User*s respectively. Upon dereferencing, they return +const Use*s. Otherwise the above patterns remain unchanged.
+ + + + +Iterating over the predecessors and successors of a block is quite easy +with the routines defined in "llvm/Support/CFG.h". Just use code like +this to iterate over all predecessors of BB:
+ +
+#include "llvm/Support/CFG.h"
+BasicBlock *BB = ...;
+
+for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
+ BasicBlock *Pred = *PI;
+ // ...
+}
+
+Similarly, to iterate over successors use +succ_iterator/succ_begin/succ_end.
+ +-AllocaInst* ai = new AllocaInst(Type::IntTy); +AllocaInst* ai = new AllocaInst(Type::Int32Ty);
-AllocaInst* pa = new AllocaInst(Type::IntTy, 0, "indexLoc"); +AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
Instruction *I = .. ; -BasicBlock *BB = I->getParent(); - -BB->getInstList().erase(I); +I->eraseFromParent();
- ReplaceInstWithValue
-
This function replaces all uses (within a basic block) of a given - instruction with a value, and then removes the original instruction. The - following example illustrates the replacement of the result of a particular +
This function replaces all uses of a given instruction with a value, + and then removes the original instruction. The following example + illustrates the replacement of the result of a particular AllocaInst that allocates memory for a single integer with a null pointer to an integer.
@@ -1895,14 +2247,16 @@ AllocaInst* instToReplace = ...; BasicBlock::iterator ii(instToReplace); ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii, - Constant::getNullValue(PointerType::get(Type::IntTy))); + Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
This function replaces a particular instruction with another - instruction. The following example illustrates the replacement of one - AllocaInst with another.
+ instruction, inserting the new instruction into the basic block at the + location where the old instruction was, and replacing any uses of the old + instruction with the new instruction. The following example illustrates + the replacement of one AllocaInst with another.@@ -1910,7 +2264,7 @@ AllocaInst* instToReplace = ...; BasicBlock::iterator ii(instToReplace); ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii, - new AllocaInst(Type::IntTy, 0, "ptrToReplacedInt")); + new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
In generating IR, you may need some complex types. If you know these types +statically, you can use TypeBuilder<...>::get(), defined +in llvm/Support/TypeBuilder.h, to retrieve them. TypeBuilder +has two forms depending on whether you're building types for cross-compilation +or native library use. TypeBuilder<T, true> requires +that T be independent of the host environment, meaning that it's built +out of types from +the llvm::types +namespace and pointers, functions, arrays, etc. built of +those. TypeBuilder<T, false> additionally allows native C types +whose size may depend on the host compiler. For example,
+ ++FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true>::get(); ++
is easier to read and write than the equivalent
+ ++std::vector<const Type*> params; +params.push_back(PointerType::getUnqual(Type::Int32Ty)); +FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false); ++
See the class +comment for more details.
+ ++This section describes the interaction of the LLVM APIs with multithreading, +both on the part of client applications, and in the JIT, in the hosted +application. +
+ ++Note that LLVM's support for multithreading is still relatively young. Up +through version 2.5, the execution of threaded hosted applications was +supported, but not threaded client access to the APIs. While this use case is +now supported, clients must adhere to the guidelines specified below to +ensure proper operation in multithreaded mode. +
+ ++Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic +intrinsics in order to support threaded operation. If you need a +multhreading-capable LLVM on a platform without a suitably modern system +compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and +using the resultant compiler to build a copy of LLVM with multithreading +support. +
++In order to properly protect its internal data structures while avoiding +excessive locking overhead in the single-threaded case, the LLVM must intialize +certain data structures necessary to provide guards around its internals. To do +so, the client program must invoke llvm_start_multithreaded() before +making any concurrent LLVM API calls. To subsequently tear down these +structures, use the llvm_stop_multithreaded() call. You can also use +the llvm_is_multithreaded() call to check the status of multithreaded +mode. +
+ ++Note that both of these calls must be made in isolation. That is to +say that no other LLVM API calls may be executing at any time during the +execution of llvm_start_multithreaded() or llvm_stop_multithreaded +. It's is the client's responsibility to enforce this isolation. +
+ ++The return value of llvm_start_multithreaded() indicates the success or +failure of the initialization. Failure typically indicates that your copy of +LLVM was built without multithreading support, typically because GCC atomic +intrinsics were not found in your system compiler. In this case, the LLVM API +will not be safe for concurrent calls. However, it will be safe for +hosting threaded applications in the JIT, though care +must be taken to ensure that side exits and the like do not accidentally +result in concurrent LLVM API calls. +
++When you are done using the LLVM APIs, you should call llvm_shutdown() +to deallocate memory used for internal structures. This will also invoke +llvm_stop_multithreaded() if LLVM is operating in multithreaded mode. +As such, llvm_shutdown() requires the same isolation guarantees as +llvm_stop_multithreaded(). +
+ ++Note that, if you use scope-based shutdown, you can use the +llvm_shutdown_obj class, which calls llvm_shutdown() in its +destructor. +
+ManagedStatic is a utility class in LLVM used to implement static +initialization of static resources, such as the global type tables. Before the +invocation of llvm_shutdown(), it implements a simple lazy +initialization scheme. Once llvm_start_multithreaded() returns, +however, it uses double-checked locking to implement thread-safe lazy +initialization. +
+ ++Note that, because no other threads are allowed to issue LLVM API calls before +llvm_start_multithreaded() returns, it is possible to have +ManagedStatics of llvm::sys::Mutexs. +
+ ++The llvm_acquire_global_lock() and llvm_release_global_lock +APIs provide access to the global lock used to implement the double-checked +locking for lazy initialization. These should only be used internally to LLVM, +and only if you know what you're doing! +
++LLVMContext is an opaque class in the LLVM API which clients can use +to operate multiple, isolated instances of LLVM concurrently within the same +address space. For instance, in a hypothetical compile-server, the compilation +of an individual translation unit is conceptually independent from all the +others, and it would be desirable to be able to compile incoming translation +units concurrently on independent server threads. Fortunately, +LLVMContext exists to enable just this kind of scenario! +
+ ++Conceptually, LLVMContext provides isolation. Every LLVM entity +(Modules, Values, Types, Constants, etc.) +in LLVM's in-memory IR belongs to an LLVMContext. Entities in +different contexts cannot interact with each other: Modules in +different contexts cannot be linked together, Functions cannot be added +to Modules in different contexts, etc. What this means is that is is +safe to compile on multiple threads simultaneously, as long as no two threads +operate on entities within the same context. +
+ ++In practice, very few places in the API require the explicit specification of a +LLVMContext, other than the Type creation/lookup APIs. +Because every Type carries a reference to its owning context, most +other entities can determine what context they belong to by looking at their +own Type. If you are adding new entities to LLVM IR, please try to +maintain this interface design. +
+ ++For clients that do not require the benefits of isolation, LLVM +provides a convenience API getGlobalContext(). This returns a global, +lazily initialized LLVMContext that may be used in situations where +isolation is not a concern. +
++LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple +threads can call ExecutionEngine::getPointerToFunction() or +ExecutionEngine::runFunction() concurrently, and multiple threads can +run code output by the JIT concurrently. The user must still ensure that only +one thread accesses IR in a given LLVMContext while another thread +might be modifying it. One way to do that is to always hold the JIT lock while +accessing IR outside the JIT (the JIT modifies the IR by adding +CallbackVHs). Another way is to only +call getPointerToFunction() from the LLVMContext's thread. +
+ +When the JIT is configured to compile lazily (using +ExecutionEngine::DisableLazyCompilation(false)), there is currently a +race condition in +updating call sites after a function is lazily-jitted. It's still possible to +use the lazy JIT in a threaded program if you ensure that only one thread at a +time can call any particular lazy stub and that the JIT lock guards any IR +access, but we suggest using only the eager JIT in threaded programs. +
+The +User class provides a basis for expressing the ownership of User +towards other +Values. The +Use helper class is employed to do the bookkeeping and to facilitate O(1) +addition and removal.
+ + + + ++A subclass of User can choose between incorporating its Use objects +or refer to them out-of-line by means of a pointer. A mixed variant +(some Uses inline others hung off) is impractical and breaks the invariant +that the Use objects belonging to the same User form a contiguous array. +
++We have 2 different layouts in the User (sub)classes: +
-
+
Layout a) +The Use object(s) are inside (resp. at fixed offset) of the User +object and there are a fixed number of them.
+ +Layout b) +The Use object(s) are referenced by a pointer to an +array from the User object and there may be a variable +number of them.
+
+As of v2.4 each layout still possesses a direct pointer to the +start of the array of Uses. Though not mandatory for layout a), +we stick to this redundancy for the sake of simplicity. +The User object also stores the number of Use objects it +has. (Theoretically this information can also be calculated +given the scheme presented below.)
++Special forms of allocation operators (operator new) +enforce the following memory layouts:
+ +-
+
Layout a) is modelled by prepending the User object by the Use[] array.
+ ++...---.---.---.---.-------... + | P | P | P | P | User +'''---'---'---'---'-------''' +
+ +Layout b) is modelled by pointing at the Use[] array.
++.-------... +| User +'-------''' + | + v + .---.---.---.---... + | P | P | P | P | + '---'---'---'---''' +
+
+Since the Use objects are deprived of the direct (back)pointer to +their User objects, there must be a fast and exact method to +recover it. This is accomplished by the following scheme:
+-
+
- 00 —> binary digit 0 +
- 01 —> binary digit 1 +
- 10 —> stop and calculate (s) +
- 11 —> full stop (S) +
+Given a Use*, all we have to do is to walk till we get +a stop and we either have a User immediately behind or +we have to walk to the next stop picking up digits +and calculating the offset:
++.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---------------- +| 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*) +'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---------------- + |+15 |+10 |+6 |+3 |+1 + | | | | |__> + | | | |__________> + | | |______________________> + | |______________________________________> + |__________________________________________________________> ++
+Only the significant number of bits need to be stored between the +stops, so that the worst case is 20 memory accesses when there are +1000 Use objects associated with a User.
+ + + + ++The following literate Haskell fragment demonstrates the concept:
++> import Test.QuickCheck +> +> digits :: Int -> [Char] -> [Char] +> digits 0 acc = '0' : acc +> digits 1 acc = '1' : acc +> digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc +> +> dist :: Int -> [Char] -> [Char] +> dist 0 [] = ['S'] +> dist 0 acc = acc +> dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r +> dist n acc = dist (n - 1) $ dist 1 acc +> +> takeLast n ss = reverse $ take n $ reverse ss +> +> test = takeLast 40 $ dist 20 [] +> ++
+Printing <test> gives: "1s100000s11010s10100s1111s1010s110s11s1S"
++The reverse algorithm computes the length of the string just by examining +a certain prefix:
+ +
+> pref :: [Char] -> Int
+> pref "S" = 1
+> pref ('s':'1':rest) = decode 2 1 rest
+> pref (_:rest) = 1 + pref rest
+>
+> decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
+> decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
+> decode walk acc _ = walk + acc
+>
+
++Now, as expected, printing <pref test> gives 40.
++We can quickCheck this with following property:
+ ++> testcase = dist 2000 [] +> testcaseLength = length testcase +> +> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr +> where arr = takeLast n testcase +> ++
+As expected <quickCheck identityProp> gives:
+ ++*Main> quickCheck identityProp +OK, passed 100 tests. ++
+Let's be a bit more exhaustive:
+ +
+>
+> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
+>
+
++And here is the result of <deepCheck identityProp>:
+ ++*Main> deepCheck identityProp +OK, passed 500 tests. ++ + + + +
+To maintain the invariant that the 2 LSBits of each Use** in Use +never change after being set up, setters of Use::Prev must re-tag the +new Use** on every modification. Accordingly getters must strip the +tag bits.
++For layout b) instead of the User we find a pointer (User* with LSBit set). +Following this pointer brings us to the User. A portable trick ensures +that the first bytes of User (if interpreted as a pointer) never has +the LSBit set. (Portability is relying on the fact that all known compilers place the +vptr in the first word of the instances.)
+ +-
-
- bool isInteger() const: Returns true for any integer type. +
- bool isIntegerTy() const: Returns true for any integer type. -
- bool isFloatingPoint(): Return true if this is one of the two +
- bool isFloatingPointTy(): Return true if this is one of the five floating point types.
- bool isAbstract(): Return true if the type is abstract (contains
@@ -2227,7 +3034,7 @@ the lib/VMCore directory.
-
@@ -2262,7 +3069,7 @@ the lib/VMCore directory.
- VectorType
- Subclass of SequentialType for vector types. A vector type is similar to an ArrayType but is distinguished because it is - a first class type wherease ArrayType is not. Vector types are used for + a first class type whereas ArrayType is not. Vector types are used for vector operations and are usually small vectors of of an integer or floating point type.
- StructType @@ -2270,7 +3077,7 @@ the lib/VMCore directory.
- FunctionType
- Subclass of DerivedTypes for function types.
-
-
- bool isVarArg() const: Returns true if its a vararg +
- bool isVarArg() const: Returns true if it's a vararg function
- const Type * getReturnType() const: Returns the return type of the function. @@ -2488,7 +3295,7 @@ simplifies the representation and makes it easier to manipulate.
- Value::use_iterator - Typedef for iterator over the
use-list
- Value::use_const_iterator - Typedef for const_iterator over + Value::const_use_iterator - Typedef for const_iterator over the use-list
unsigned use_size() - Returns the number of users of the value.
@@ -2822,7 +3629,7 @@ Superclasses: GlobalValue, ValueThe Function class represents a single procedure in LLVM. It is -actually one of the more complex classes in the LLVM heirarchy because it must +actually one of the more complex classes in the LLVM hierarchy because it must keep track of a large amount of data. The Function class keeps track of a list of BasicBlocks, a list of formal Arguments, and a @@ -2831,7 +3638,7 @@ of a list of BasicBlocks, a list of formal
The list of BasicBlocks is the most commonly used part of Function objects. The list imposes an implicit ordering of the blocks in the function, which indicate how the code will be -layed out by the backend. Additionally, the first BasicBlock is the implicit entry node for the Function. It is not legal in LLVM to explicitly branch to this initial block. There are no implicit exit nodes, and in fact there may be multiple exit @@ -2883,7 +3690,7 @@ is its address (after linking) which is guaranteed to be constant.
will automatically be inserted into that module's list of functions.
- - bool isExternal() +
- bool isDeclaration()
Return whether or not the Function has a body defined. If the function is "external", it does not have a body, and thus must be resolved @@ -2961,7 +3768,7 @@ Superclasses: GlobalValue, User, Value
-Global variables are represented with the (suprise suprise) +
Global variables are represented with the (surprise surprise) GlobalVariable class. Like functions, GlobalVariables are also subclasses of GlobalValue, and as such are always referenced by their address (global values must live in memory, so their @@ -2989,11 +3796,12 @@ never change at runtime).
Create a new global variable of the specified type. If isConstant is true then the global variable will be marked as unchanging for the program. The Linkage parameter specifies the type of - linkage (internal, external, weak, linkonce, appending) for the variable. If - the linkage is InternalLinkage, WeakLinkage, or LinkOnceLinkage, then - the resultant global variable will have internal linkage. AppendingLinkage - concatenates together all instances (in different translation units) of the - variable into a single variable but is only applicable to arrays. See + linkage (internal, external, weak, linkonce, appending) for the variable. + If the linkage is InternalLinkage, WeakAnyLinkage, WeakODRLinkage, + LinkOnceAnyLinkage or LinkOnceODRLinkage, then the resultant + global variable will have internal linkage. AppendingLinkage concatenates + together all instances (in different translation units) of the variable + into a single variable but is only applicable to arrays. See the LLVM Language Reference for further details on linkage types. Optionally an initializer, a name, and the module to put the variable into may be specified for the global variable as @@ -3010,7 +3818,7 @@ never change at runtime).
- Constant *getInitializer()
-
Returns the intial value for a GlobalVariable. It is not legal +
Returns the initial value for a GlobalVariable. It is not legal to call this method if there is no initializer.
#include "llvm/BasicBlock.h"
@@ -3125,9 +3933,9 @@ arguments. An argument has a pointer to the parent Function.
-doxygen info: BasicBlock +doxygen info: BasicBlock Class
Superclass: Value
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