Written by Chris Lattner, Dinakar Dhurjati, Gabor Greif, - Joel Stanley and - Reid Spencer
+ Joel Stanley, + Reid Spencer and + Owen Anderson @@ -243,9 +269,9 @@ can get, so it will not be discussed in this document.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");
+
+ilists have another speciality that must be considered. To be a good +
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 @@ -1065,14 +1208,14 @@ and erasing, but does not support iteration.
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.
@@ -1291,7 +1434,7 @@ to the key string for a value.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 @@ -1347,6 +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.
+TODO: const char* vs stringref vs smallstring vs std::string. Describe twine, +xref to #string_apis. +
+ +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 @@ -1418,6 +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";. @@ -1563,7 +1756,7 @@ small example that shows how to dump all instructions in a function to the stand // 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"; + errs() << *I << "\n"; @@ -1642,11 +1835,26 @@ without actually obtaining it via iteration over some structure: void printNextInstruction(Instruction* inst) { BasicBlock::iterator it(inst); ++it; // After this line, it refers to the instruction after *inst - if (it != inst->getParent()->end()) llvm::cerr << *it << "\n"; + if (it != inst->getParent()->end()) errs() << *it << "\n"; } +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.
+ @@ -1760,13 +1968,17 @@ 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 @@ -1785,10 +1997,13 @@ for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++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.
@@ -2118,7 +2333,7 @@ FunctionType *ft = TypeBuilder<types::i<8>(types::i<32>*), true&g-std::vector@@ -2129,6 +2344,195 @@ comment for more details.params; +std::vector<const Type*> params; params.push_back(PointerType::getUnqual(Type::Int32Ty)); FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false);
+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 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 @@ -3234,7 +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 @@ -3364,7 +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 @@ -3414,7 +3818,7 @@ never change at runtime).
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"
-doxygen info: BasicBlock
+doxygen info: BasicBlock
Class
Superclass: Value