4.9 Expressions

Nearly all values and operands in assembly language are expressions, which may be any of the following:

• a literal constant
• a register
• a memory reference
• a symbol
• a built-in function invocation
• a mathematical or logical operation on one or more expressions

This section defines several types of expressions that are referred to throughout this document. Some instruction operands accept limited types of expressions. For example, the .if directive requires its operand be an absolute constant expression with an integer value. Absolute in the context of assembly code means that the value of the expression must be known at assembly time.

A constant expression is any expression that does not in any way refer to a register or memory reference. An immediate operand will usually not accept a register or memory reference. It must be given a constant expression. Constant expressions may be any of the following:

• a literal constant
• an address constant expression
• a symbol whose value is a constant expression
• a built-in function invocation on a constant expression
• a mathematical or logical operation on one or more constant expressions

An address constant expression is a special case of a constant expression. Some immediate operands that require an address value can accept a symbol plus an addend; for example, some branch instructions. The symbol must have a value that is an address, and it may be an external symbol. The addend must be an absolute constant expression with an integer value. For example, a valid address constant expression is "array+4".

A constant expression may be absolute or relocatable. Absolute means known at assembly time. Relocatable means constant, but not known until link time. External symbols are relocatable, even if they refer to a symbol defined in the same module.

An absolute constant expression may not refer to any external symbols anywhere in the expression. In other words, an absolute constant expression may be any of the following:

• a literal constant
• an absolute address constant expression
• a symbol whose value is an absolute constant expression
• a built-in function invocation whose arguments are all absolute constant expressions
• a mathematical or logical operation on one or more absolute constant expressions

A relocatable constant expression refers to at least one external symbol. For ELF, such expressions may contain at most one external symbol. A relocatable constant expression may be any of the following:

• an external symbol
• a relocatable address constant expression
• a symbol whose value is a relocatable constant expression
• a built-in function invocation with any arguments that are relocatable constant expressions
• a mathematical or logical operation on one or more expressions, at least one of which is a relocatable constant expression

In some cases, the value of a relocatable address expression may be known at assembly time. For example, a relative displacement branch may branch to a label defined in the same section.