[PATCH] D36061: [MSP430] Implement multiplication by a constant

[Vadzim Dambrouski via Phabricator via llvm-commits]

pftbest added a comment.

> Do you need to worry about codesize here? Lowering something like "a * 0x3333" to an inline sequence like this is going to generate a lot of code.

The `a * 0x3333` generates the following sequence (the register allocation is not very good in this case):

  ; BB#0:
  	mov.b	r12, r13
  	rla.b	r13
  	add.b	r13, r12
  	rla.b	r13
  	rla.b	r13
  	rla.b	r13
  	mov.b	r13, r14
  	sub.b	r12, r14
  	rla.b	r13
  	rla.b	r13
  	sub.b	r14, r13
  	mov.w	r13, r12
  	ret

It is 24 bytes in size and takes only 12 cycles, which is both smaller and faster than a library call.
If we have the hardware multiplier, then a library function is 20 bytes in size (+4 bytes per call), and takes 20 cycles to execute (including interrupts disabling).
But if we don't have a hardware multiplier, the library function is 70 bytes in size and i don't know how much slower.

We can provide an option like Lanai did ("lanai-constant-mul-threshold"), to limit the number of operations, but I think it would only be useful if we have a hardware multiplier.


https://reviews.llvm.org/D36061