[GIT PULL] RISC-V Patches for the 4.20 Merge Window, Part 1

Linus Torvalds torvalds at linux-foundation.org
Thu Oct 25 18:12:46 PDT 2018


On Wed, Oct 24, 2018 at 1:42 PM Palmer Dabbelt <palmer at sifive.com> wrote:
>
> I'm pretty sure this is our largest patch set since the original kernel
> contribution, and it's certainly the one with the most contributors.
> While I don't have anything else I know I'm going to submit for the
> merge window, I would be somewhat surprised if I didn't screw anything
> up.

So I'm not entirely sure this is a screw-up, but it does have some
signs of being one:

>       lib: Add umoddi3 and udivmoddi4 of GCC library routines

We have *actively* tried to avoid needing umoddi3 and friends, and
have so far succceeded in doing that across many different
architectures, and many many years.

And when we _have_ added it, we've added it as architecture-specific
optimized routines when people really need it.

The reason to avoid a full 64-by-64 divide is that

 (a) it is really expensive

 (b) code that needs it is almost always actually wrong

Almost always, the 64-bit divide turns out to be entirely pointless,
and the code can be written as a (much cheaper) 64-by-32 divide
instead using the functions in <asm/div.h> instead.

Does RISC-V _really_ need that disgusting full 64-bit divide, or was
the addition of that simply a "we didn't know better, and didn't
realize just how expensive and wrong it is"?

I've pulled this, but I really _really_ hope that I will get a second
pull request that gets rid of this again, and fixes whatever RISC-V
code that triggered this to just do not do an insane full 64-bit
divide on a 32-bit architecture.

                Linus



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