On Tue, Jan 20, 2026, at 16:10, Lorenzo Stoakes wrote:
> On Tue, Jan 20, 2026 at 09:36:19AM -0400, Jason Gunthorpe wrote:
>
> I am not sure about this 'idiomatic kernel style' thing either, it feels
> rather
> conjured. Yes you wouldn't ordinarily pass something larger than a register
> size
> by-value, but here the intent is for it to be inlined anyway right?
>
> It strikes me that the key optimisation here is the inlining, now if the issue
> is that ye olde compiler might choose not to inline very small functions
> (seems
> unlikely) we could always throw in an __always_inline?
I can think of three specific things going wrong with structures passed
by value:
- functions that cannot be inlined are bound by the ELF ABI, and
several of them require structs to be passed on the stack regardless
of the size. Most of the popular architectures seem fine here, but
mips and powerpc look like they are affected.
- The larger the struct is, the more architectures are affected.
Parts of the amdgpu driver and the bcachefs file system ran into this
with 64-bit structures passed by value on 32-bit architectures
causing horrible codegen even with inlining. I think it's
usually fine up to a single register size.
- clang's inlining algorithm works the other way round from gcc's:
inlining into the root caller first and sometimes leaving tiny
leaf function out of line unless you add __always_inline.
Arnd
