[Veserv]

In general, having a fused instruction is beneficial for performance in that it gives you code size savings which helps with respect to the instruction cache. There are likely other microarchitectural benefits, but that is the obvious one. However, there is a limit to the number of instructions you can support efficiently, so you generally only want to add instructions that will be commonly used.

Multiply-add is a good choice because it corresponds to the relatively common operation of computing the address of a field of a struct in an array so you can operate on that field.

(e.g. &(points[5].x) is &points + (5 * sizeof(point)) + offsetof(point, x)).

[pm215]

Note also that the 'mul' instruction is described in the Arm docs the article links to as an "alias" of madd. That is, the CPU itself has no pure multiply-only insn at all, only a multiply-and-add. When you write 'mul' in assembly, the assembler turns it into a 'madd' where the register to add is XZR (the reads-as-zero register).

There are a fair number of insns in the A64 instruction set that make use of this trick to provide one flexible instruction that as a special case provides useful simpler functionality under an alias. (Register-to-register 'mov' being an alias of 'orr' is another.)

[Narishma]

RISC-V similarly has a ton of these aliased instructions.

[tom_mellior]

> relatively common operation of computing the address of a field of a struct in an array

This is only relatively common inside loops. Inside loops you will usually index with the loop counter or some other value that is derived from it linearly. Compilers will typically use induction variable arithmetic that doesn't involve multiplication.