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

1. Replace _Generic-based __unqual_scalar_typeof() with more complete
   __rwonce_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).

Once our minimum compiler versions are bumped, this just becomes
TYPEOF_UNQUAL() (or typeof_unqual() should we decide to adopt C23
naming).  Sadly the fallback version of __rwonce_typeof_unqual() cannot
be used as a general TYPEOF_UNQUAL() fallback (see code comments).

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.

This refactoring appears to help the compiler improve (or fix) code
generation.

With a defconfig + LTO + debug options builds, we observe different
codegen for the following functions:

	btrfs_reclaim_sweep (708 -> 1032 bytes)
	btrfs_sinfo_bg_reclaim_threshold_store (200 -> 204 bytes)
	check_mem_access (3652 -> 3692 bytes) [inlined bpf_map_is_rdonly]
	console_flush_all (1268 -> 1264 bytes)
	console_lock_spinning_disable_and_check (180 -> 176 bytes)
	igb_add_filter (640 -> 636 bytes)
	igb_config_tx_modes (2404 -> 2400 bytes)
	kvm_vcpu_on_spin (480 -> 476 bytes)
	map_freeze (376 -> 380 bytes)
	netlink_bind (1664 -> 1656 bytes)
	nmi_cpu_backtrace (404 -> 400 bytes)
	set_rps_cpu (516 -> 520 bytes)
	swap_cluster_readahead (944 -> 932 bytes)
	tcp_accecn_third_ack (328 -> 336 bytes)
	tcp_create_openreq_child (1764 -> 1772 bytes)
	tcp_data_queue (5784 -> 5892 bytes)
	tcp_ecn_rcv_synack (620 -> 628 bytes)
	xen_manage_runstate_time (944 -> 896 bytes)
	xen_steal_clock (340 -> 296 bytes)

Increase of some functions are due to more aggressive inlining due to
better codegen (in this build, e.g. bpf_map_is_rdonly is no longer
present due to being inlined completely).

Signed-off-by: Marco Elver <elver@google.com>
---
v3:
* Comment.

v2:
* Add __rwonce_typeof_unqual() as fallback for old compilers.
---
 arch/arm64/include/asm/rwonce.h | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
index fc0fb42b0b64..42c9e8429274 100644
--- a/arch/arm64/include/asm/rwonce.h
+++ b/arch/arm64/include/asm/rwonce.h

@@ -19,6 +19,20 @@
 		"ldapr"	#sfx "\t" #regs,				\
 	ARM64_HAS_LDAPR)
 
+#ifdef USE_TYPEOF_UNQUAL
+#define __rwonce_typeof_unqual(x) TYPEOF_UNQUAL(x)
+#else
+/*
+ * Fallback for older compilers (Clang < 19).
+ *
+ * Uses the fact that, for all supported Clang versions, 'auto' correctly drops
+ * qualifiers. Unlike typeof_unqual(), the type must be completely defined, i.e.
+ * no forward-declared struct pointer dereferences.  The array-to-pointer decay
+ * case does not matter for usage in READ_ONCE() either.
+ */
+#define __rwonce_typeof_unqual(x) typeof(({ auto ____t = (x); ____t; }))
+#endif

 /*
  * When building with LTO, there is an increased risk of the compiler
  * converting an address dependency headed by a READ_ONCE() invocation

@@ -32,8 +46,7 @@
 #define __READ_ONCE(x)							\
 ({									\
 	typeof(&(x)) __x = &(x);					\
-	int atomic = 1;							\
-	union { __unqual_scalar_typeof(*__x) __val; char __c[1]; } __u;	\
+	union { __rwonce_typeof_unqual(*__x) __val; char __c[1]; } __u;	\
 	switch (sizeof(x)) {						\
 	case 1:								\
 		asm volatile(__LOAD_RCPC(b, %w0, %1)			\

@@ -56,9 +69,9 @@
 			: "Q" (*__x) : "memory");			\
 		break;							\
 	default:							\
-		atomic = 0;						\
+		__u.__val = *(volatile typeof(*__x) *)__x;		\