[PATCH v5 1/4] vdso/math64: Use OPTIMIZER_HIDE_VAR() in __iter_div_u64_rem()

[Zhan Xusheng]

__iter_div_u64_rem() divides by repeated subtraction, because its callers
only ever produce a small quotient.  A barrier inside the subtraction loop
keeps the compiler from replacing the loop with a division:

	asm("" : "+rm"(dividend));

The "rm" constraint permits a memory operand.  clang picks it and spills
the dividend inside the loop, at 32-bit and 64-bit alike.  The helper sits
on the clock_gettime() fast path through vdso_set_timespec(), so the spill
is not free: with clang 18 the x86 vDSO is 64 bytes larger in vdso64 text
and 112 bytes larger in vdso32 text than with a register-only barrier.

Use OPTIMIZER_HIDE_VAR(), which is the register-only form of the same
barrier and the form the rest of the kernel uses.  gcc 13 generates
identical code either way, and neither compiler turns the loop into a
division at either width.

Put the function signature on one line while touching it, and reformat the
comment, which described the asm() that is now gone.

Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
---
 include/vdso/math64.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/vdso/math64.h b/include/vdso/math64.h
index 22ae212f8b28..c628d6cf447c 100644
--- a/include/vdso/math64.h
+++ b/include/vdso/math64.h
@@ -2,15 +2,16 @@
 #ifndef __VDSO_MATH64_H
 #define __VDSO_MATH64_H
 
-static __always_inline u32
-__iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
+static __always_inline u32 __iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
 {
 	u32 ret = 0;
 
 	while (dividend >= divisor) {
-		/* The following asm() prevents the compiler from
-		   optimising this loop into a modulo operation.  */
-		asm("" : "+rm"(dividend));
+		/*
+		 * Prevent the compiler from optimising this loop into a
+		 * modulo operation.
+		 */
+		OPTIMIZER_HIDE_VAR(dividend);
 
 		dividend -= divisor;
 		ret++;
-- 
2.43.0