Constants

There are two different kinds of constants:

  • Constants that do not occupy allocated memory.
  • Constants that do occupy allocated memory.

The former are always expanded inline by the compiler as there is no LLVM IR equivalent of those. In other words, the compiler simply inserts the constant value wherever it is being used in a computation:

%1 = add i32 %0, 17     ; 17 is an inlined constant

Constants that do occupy memory are defined using the constant keyword:

@hello = internal constant [6 x i8] c"hello\00"
%struct = type { i32, i8 }
@struct_constant = internal constant %struct { i32 16, i8 4 }

Such a constant is really a global variable whose visibility can be limited with private or internal so that it is invisible outside the current module.

Constant Expressions

An example for constant expressions are sizeof-style computations. Even though the compiler ought to know the exact size of everything in use (for statically checked languages), it can at times be convenient to ask LLVM to figure out the size of a structure for you. This is done with the following little snippet of code:

%Struct = type { i8, i32, i8* }
@Struct_size = constant i32 ptrtoint (%Struct* getelementptr (%Struct* null, i32 1)) to i32

@Struct_size will now contain the size of the structure %Struct. The trick is to compute the offset of the second element

in the zero-based array starting at null and that way get the size of the structure.