ARM's MULA instruction

[cherry]

Hello,

As seen in https://golang.org/issue/19141, ARM's MULA instruction (multiply and add) is assembled with its third operand as destination, and fourth operand as addend. This doesn't follow the convention that the destination is the last operand. Given that other flavors of multiply-and-add instructions (MULAWB, MULAWT, etc.) all follow the convention, this is an annoying inconsistency.

Ben has a CL https://go-review.googlesource.com/c/42028/ that fixes it. However, this change may break existing assembly code.

The uses of MULA in the standard library all have its third and fourth operands same, so they will not be affected by this change. I also searched Go Corpus and found no uses will be affected. Let me know if you have/know some code that will be affected.

Any objection on this change?

Thank you.

[David Chase]

I approve, it will only hurt more later, and this counts as a bug.

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[Russ Cox]

Yes, because all the uses we can find are unaffected (they don't set the two differently), please fix it.

[cherry]

The CL is now submitted.