An indirect branch (also known as a computed jump, indirect jump and register-indirect jump) is a type of program control instruction present in some machine language

instruction set In computer science, an instruction set architecture (ISA) is an abstract model that generally defines how software controls the CPU in a computer or a family of computers. A device or program that executes instructions described by that ISA, s ...

s. Rather than specifying the

address An address is a collection of information, presented in a mostly fixed format, used to give the location of a building, apartment, or other structure or a plot of land, generally using border, political boundaries and street names as references, ...

of the next instruction to execute, as in a direct branch, the

argument An argument is a series of sentences, statements, or propositions some of which are called premises and one is the conclusion. The purpose of an argument is to give reasons for one's conclusion via justification, explanation, and/or persu ...

specifies where the address is located. An example is 'jump indirect on the r1 register', which means that the next instruction to be executed is at the address in register r1. The address to be jumped to is not known until the instruction is executed. Indirect branches can also depend on the value of a memory location. An indirect branch can be useful to make a

conditional branch A branch, jump or transfer is an instruction in a computer program that can cause a computer to begin executing a different instruction sequence and thus deviate from its default behavior of executing instructions in order. ''Branch'' (or ''br ...

, especially a

multiway branch Multiway branch is the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement. A multiway branch is often the most efficient method of passing control to one of a set of program ...

. For instance, based on program input, a value could be looked up in a jump table of pointers to

code In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communicati ...

for handling the various cases implied by the data value. The

data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...

value could be added to the address of the table, with the result stored in a register. An indirect jump could then be made based on the value of that register, efficiently dispatching program control to the code appropriate to the input. In a similar manner, subroutine call instructions can be indirect, with the address of the subroutine to be called specified in memory. Function Pointers are typically implemented with indirect subroutine calls. Indirect branches were one of the attack surfaces of Spectre. To mitigate the attack GCC 8.1 introduced the following new options: -mindirect-branch=, -mfunction-return= and -mindirect-branch-register.

Example assembler syntax

Notes

References

Control flow Machine code {{Compu-prog-stub

Example assembler syntax

See also

Notes

References