On Mon, 26 Jan 2026 at 12:16, David Laight [off-list ref] wrote:

On Mon, 26 Jan 2026 01:25:11 +0100
Marco Elver [off-list ref] wrote:

quoted

Rework arm64 LTO __READ_ONCE() to improve code generation as follows:

1. Replace the _Generic-based __unqual_scalar_typeof() with the builtin
   typeof_unqual(). This strips qualifiers from all types, not just
   integer types, which is required to be able to assign (must be
   non-const) to __u.__val in the non-atomic case (required for #2).

One subtle point here is that non-integer types of __val could be const
or volatile within the union with the old __unqual_scalar_typeof(), if
the passed variable is const or volatile. This would then result in a
forced load from the stack if __u.__val is volatile; in the case of
const, it does look odd if the underlying storage changes, but the
compiler is told said member is "const" -- it smells like UB.

2. Eliminate the atomic flag and ternary conditional expression. Move
   the fallback volatile load into the default case of the switch,
   ensuring __u is unconditionally initialized across all paths.
   The statement expression now unconditionally returns __u.__val.

Does it even need to be a union?
I think (eg):
        TYPEOF_UNQUAL(*__x) __val;      \
        ...
                : "=r" (*(__u32 *)&__val)       \
will have the same effect (might need an __force for sparse).

Unsure, but we might be treading on UB even with -fno-strict-aliasing
given all the inline asm around here.

Also is the 'default' branch even needed?
READ_ONCE() rejects sizes other than 1, 2, 4 and 8.
A quick search only found one oversize read - for 'struct vcpu_runstate_info'
in arch/x86/kvm/xen.c
Requiring that code use a different define might make sense.

I also did some x86-64 build timings with compiletime_assert_rwonce_type()
commented out.
Expanding and compiling that check seems to add just over 1% to the
build time.
So anything to shrink that define is likely to be noticeable.

The compiletime_assert_rwonce_type() is for the benefit of the
asm-generic variant, which is implemented like the 'default' case here
by default. This here is only the arm64 override of all that with LTO.