LLVM programs are composed of "Module"s, each of which is a +translation unit of the input programs. Each module consists of +functions, global variables, and symbol table entries. Modules may be +combined together with the LLVM linker, which merges function (and +global variable) definitions, resolves forward declarations, and merges +symbol table entries. Here is an example of the "hello world" module:
+ +; Declare the string constant as a global constant... +%.LC0 = internal constant [13 x sbyte] c"hello world\0A\00" ; [13 x sbyte]* + +; External declaration of the puts function +declare int %puts(sbyte*) ; int(sbyte*)* + +; Definition of main function +int %main() { ; int()* + ; Convert [13x sbyte]* to sbyte *... + %cast210 = getelementptr [13 x sbyte]* %.LC0, long 0, long 0 ; sbyte* + + ; Call puts function to write out the string to stdout... + call int %puts(sbyte* %cast210) ; int + ret int 0+ +
}
This example is made up of a global variable +named ".LC0", an external declaration of the "puts" +function, and a function definition +for "main".
+ + In general, a module is made up of a list of global +values, where both functions and global variables are global values. +Global values are represented by a pointer to a memory location (in +this case, a pointer to an array of char, and a pointer to a function), +and have one of the following linkage types: + ++ +
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Global variables define regions of memory allocated at compilation +time instead of run-time. Global variables may optionally be +initialized. A variable may be defined as a global "constant", which +indicates that the contents of the variable will never be modified +(enabling better optimization, allowing the global data to be placed in the +read-only section of an executable, etc).
+ +As SSA values, global variables define pointer values that are in +scope (i.e. they dominate) all basic blocks in the program. Global +variables always define a pointer to their "content" type because they +describe a region of memory, and all memory objects in LLVM are +accessed through pointers.
+ +LLVM function definitions are composed of a (possibly empty) argument list, +an opening curly brace, a list of basic blocks, and a closing curly brace. LLVM +function declarations are defined with the "declare" keyword, a +function name, and a function signature.
+ +A function definition contains a list of basic blocks, forming the CFG for +the function. Each basic block may optionally start with a label (giving the +basic block a symbol table entry), contains a list of instructions, and ends +with a terminator instruction (such as a branch or +function return).
+ +The first basic block in program is special in two ways: it is immediately +executed on entrance to the function, and it is not allowed to have predecessor +basic blocks (i.e. there can not be any branches to the entry block of a +function). Because the block can have no predecessors, it also cannot have any +PHI nodes.
+ +LLVM functions are identified by their name and type signature. Hence, two +functions with the same name but different parameter lists or return values are +considered different functions, and LLVM will resolves references to each +appropriately.
+ +The LLVM type system is one of the most important features of the intermediate representation. Being typed enables a number of optimizations to be performed on the IR directly, without having to do @@ -290,9 +454,9 @@ extra analyses on the side before the transformation. A strong type system makes it easier to read the generated code and enables novel analyses and transformations that are not feasible to perform on normal three address code representations.
-LLVM programs are composed of "Module"s, each of which is a -translation unit of the input programs. Each module consists of -functions, global variables, and symbol table entries. Modules may be -combined together with the LLVM linker, which merges function (and -global variable) definitions, resolves forward declarations, and merges -symbol table entries. Here is an example of the "hello world" module:
-; Declare the string constant as a global constant... -%.LC0 = internal constant [13 x sbyte] c"hello world\0A\00" ; [13 x sbyte]* - -; External declaration of the puts function -declare int %puts(sbyte*) ; int(sbyte*)* - -; Definition of main function -int %main() { ; int()* - ; Convert [13x sbyte]* to sbyte *... - %cast210 = getelementptr [13 x sbyte]* %.LC0, long 0, long 0 ; sbyte* - - ; Call puts function to write out the string to stdout... - call int %puts(sbyte* %cast210) ; int - ret int 0-
}
This example is made up of a global variable -named ".LC0", an external declaration of the "puts" -function, and a function definition -for "main".
- In general, a module is made up of a list of global -values, where both functions and global variables are global values. -Global values are represented by a pointer to a memory location (in -this case, a pointer to an array of char, and a pointer to a function), -and have one of the following linkage types: --
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Global variables define regions of memory allocated at compilation -time instead of run-time. Global variables may optionally be -initialized. A variable may be defined as a global "constant", which -indicates that the contents of the variable will never be modified -(opening options for optimization).
- -As SSA values, global variables define pointer values that are in -scope (i.e. they dominate) for all basic blocks in the program. Global -variables always define a pointer to their "content" type because they -describe a region of memory, and all memory objects in LLVM are -accessed through pointers.
- -LLVM function definitions are composed of a (possibly empty) argument list, -an opening curly brace, a list of basic blocks, and a closing curly brace. LLVM -function declarations are defined with the "declare" keyword, a -function name, and a function signature.
- -A function definition contains a list of basic blocks, forming the CFG for -the function. Each basic block may optionally start with a label (giving the -basic block a symbol table entry), contains a list of instructions, and ends -with a terminator instruction (such as a branch or -function return).
- -The first basic block in program is special in two ways: it is immediately -executed on entrance to the function, and it is not allowed to have predecessor -basic blocks (i.e. there can not be any branches to the entry block of a -function). Because the block can have no predecessors, it also cannot have any -PHI nodes.
- -LLVM functions are identified by their name and type signature. Hence, two -functions with the same name but different parameter lists or return values are -considered different functions, and LLVM will resolves references to each -appropriately.
- -