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Branch table
In computer programming, a branch table or jump table is a method of transferring program control (branching) to another part of a program (or a different program that may have been dynamically loaded) using a table of branch or jump instructions. It is a form of multiway branch. The branch table construction is commonly used when programming in assembly language but may also be generated by compilers, especially when implementing optimized switch statements whose values are densely packed together.
A branch table consists of a serial list of unconditional branch instructions that is branched into using an offset created by multiplying a sequential index by the instruction length (the number of bytes in memory occupied by each branch instruction). It relies on the fact that machine code instructions for branching have a fixed length and can be executed extremely efficiently by most hardware, and is most useful when dealing with raw data values that may be easily converted to sequential index values. Given such data, a branch table can be extremely efficient. It usually consists of the following 3 steps:
The following pseudocode illustrates the concept
Another method of implementing a branch table is with an array of pointers from which the required function's address is retrieved. Originally known as transfer vector, this method is also more recently known under such different names as "dispatch table" or "virtual method table" but essentially performing exactly the same purpose. This pointer function method can result in saving one machine instruction, and avoids the indirect jump (to one of the branch instructions).
The resulting list of pointers to functions is almost identical to direct threaded code, and is conceptually similar to a control table.
The actual method used to implement a branch table is usually based on:
Programmers frequently leave the decision of whether or not to create a branch table to the compiler, believing that it is perfectly capable of making the correct choice from the known search keys. This may be true for optimizing compilers for relatively simple cases where the range of search keys is limited. However, compilers are not as intelligent as humans and cannot have a deep knowledge of 'context', believing that a range of possible search key integer values such as 1, 2, 4, 6, 7, 20, 23, 40, 42, 50 & 1000 would generate a branch table with an excessively large number of empty entries (900+) for very little advantage. A good optimizing compiler may then presort the values and generate code for a binary chop search, as a 'second best' option. In fact, the application may be highly "time critical" and memory requirement may not really be an issue at all.
However, a little 'common sense' can transform this particular case, and many other similar cases, to a simple two-step process with very large potential savings, while still eventually leaving the ultimate choice to the compiler, but 'assisting its decision' considerably:
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Branch table AI simulator
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Branch table
In computer programming, a branch table or jump table is a method of transferring program control (branching) to another part of a program (or a different program that may have been dynamically loaded) using a table of branch or jump instructions. It is a form of multiway branch. The branch table construction is commonly used when programming in assembly language but may also be generated by compilers, especially when implementing optimized switch statements whose values are densely packed together.
A branch table consists of a serial list of unconditional branch instructions that is branched into using an offset created by multiplying a sequential index by the instruction length (the number of bytes in memory occupied by each branch instruction). It relies on the fact that machine code instructions for branching have a fixed length and can be executed extremely efficiently by most hardware, and is most useful when dealing with raw data values that may be easily converted to sequential index values. Given such data, a branch table can be extremely efficient. It usually consists of the following 3 steps:
The following pseudocode illustrates the concept
Another method of implementing a branch table is with an array of pointers from which the required function's address is retrieved. Originally known as transfer vector, this method is also more recently known under such different names as "dispatch table" or "virtual method table" but essentially performing exactly the same purpose. This pointer function method can result in saving one machine instruction, and avoids the indirect jump (to one of the branch instructions).
The resulting list of pointers to functions is almost identical to direct threaded code, and is conceptually similar to a control table.
The actual method used to implement a branch table is usually based on:
Programmers frequently leave the decision of whether or not to create a branch table to the compiler, believing that it is perfectly capable of making the correct choice from the known search keys. This may be true for optimizing compilers for relatively simple cases where the range of search keys is limited. However, compilers are not as intelligent as humans and cannot have a deep knowledge of 'context', believing that a range of possible search key integer values such as 1, 2, 4, 6, 7, 20, 23, 40, 42, 50 & 1000 would generate a branch table with an excessively large number of empty entries (900+) for very little advantage. A good optimizing compiler may then presort the values and generate code for a binary chop search, as a 'second best' option. In fact, the application may be highly "time critical" and memory requirement may not really be an issue at all.
However, a little 'common sense' can transform this particular case, and many other similar cases, to a simple two-step process with very large potential savings, while still eventually leaving the ultimate choice to the compiler, but 'assisting its decision' considerably: