Subject: Re: [PATCH V13 5/7] rust: Make __udivdi3() and __umoddi3() panic

From: Segher Boessenkool <[email-masked]>
Date: Fri, 24 Apr 2026 06:54:05 -0500

Hi!

On Fri, Apr 24, 2026 at 11:17:40AM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:
> The core crate currently depends on these two functions for i64/u64/
> i128/u128/core::time::Duration formatting, but we shouldn’t use that in
> the kernel so let’s panic if they are ever called.
> 
> This doesn’t yet fix drm_panic_qr.rs, which also uses __udivdi3 when
> CONFIG_CC_OPTIMIZE_FOR_SIZE=y, but at least makes the rest of the kernel
> build on PPC32.

GCC uses calls to functions like even __addsi3 whenever the (sub-)target
does not implement some RTL, doesn't have a define_insn and the like for
it.  When you write a new port you only *have* to implement a very few
things, the rest is done in libgcc (you might still have to write some
of that for your target, no free lunch etc.)

32-bit PowerPC has no instructions for 64-bit divisions, nor
instructions that help implementing it in software.  It still very often
helps to hand-write machine code for it, it very easily can usually be
more than twice as fast for example (for example if the divisor is less
than half a word big, the common case -- it can be made tens of times
faster then).

There can be many reasons why a GCC backend decides to call a libgcc
routine.  For __udivdi3 on -m32 rs6000 you'll be good AFAICS :-)
(but poisoning functions like you do is a terrible idea in general!)

Segher