Re: [PATCH 1/2] x86/segment: Introduce storesegment() helper to write segment selectors to memory

[Ingo Molnar]

* Uros Bizjak <ubizjak@xxxxxxxxx> wrote:

> On Tue, Mar 31, 2026 at 8:56 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> >
> > * Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >
> > > Introduce a new helper, storesegment(), that stores a segment selector
> > > directly into a u16 (or compatible) memory location without using an
> > > intermediate general-purpose register.
> > >
> > > To support this, split the existing SAVE_SEGMENT macro into two parts:
> > >
> > > SAVE_SEGMENT_VAR(): retains the current behavior of reading a segment
> > > register into an unsigned long via a register.
> > > SAVE_SEGMENT_PTR(): adds a new variant that writes the 16-bit selector
> > > directly to memory.
> > >
> > > The combined SAVE_SEGMENT() macro now generates both helpers for each
> > > segment register.
> > >
> > > The new storesegment() interface is preferred over savesegment() when
> > > the value only needs to be stored (e.g. into a struct field), avoiding
> > > an unnecessary register move and making the intent clearer.
> > >
> > > No functional change for existing users of savesegment().
> >
> > Why does the API have to be split into =r and =m variants?
> >
> > Coulnd't we use a more generic constraint and let the compiler
> > decide what the target is? Would that negatively impact
> > other aspects of code generation?
> 
> The "=r" variant actually outputs zero-extended value to the whole
> register width. So, the "=r" variant is used to eliminate
> zero-extensions when the value is used in the follow-up calculations,
> comparisons, or when the value is stored to a location that is more
> than 16-bits wide. Additionally, "r" variant always uses MOVL, where
> operand size prefix byte (0x66) is not needed.
> 
> The "=m" variant only outputs to a 16-bit location. Having "=rm" here
> would always emit a 0x66 operand size prefix when register is used as
> an output, and there would be many zero-extensions emitted, because
> the compiler needs to zero-extend the value from 'unsigned short' to
> anything wider.
> 
> Other than that, GCC (and Clang, too) has serious problems with "=rm"
> output constraints. Forward propagation (AKA combine pass) does not
> work reliably with assembly outputs (due to always present clobbers
> for assembly clauses), so there will be many cases of moves to a
> temporary register and even to a temporary stack location with this
> constraint. Having two separate functions (with clear and
> informational function comment) leaves the decision to the programmer,
> which function is the most optimal.

Yeah, so I still have a problem with the split API, 'savesegment()'
is very similar to 'storesegment()' and there's no real way to tell
them apart in practice.

Since the main difference is that the =m variant outputs to an u16, I'd
suggest naming the two APIs according to the type they handle, not some
random word that nobody remembers:

	savesegment()
	savesegment_u16()

... or so?

Thanks,

	Ingo