[PATCH v7 1/5] vdso/math64: Use OPTIMIZER_HIDE_VAR() in __iter_div_u64_rem()

[Zhan Xusheng]

__iter_div_u64_rem() open-codes an empty asm() to keep the compiler from
turning the subtraction loop into a modulo operation.  That is what
OPTIMIZER_HIDE_VAR() is for, so use it and reflow the comment.

The macro lives in linux/compiler.h, which this header did not include.
vdso/math64.h is reached from linux/math64.h, whose includes are
linux/types.h, linux/math.h, asm/div64.h and vdso/math64.h; none of them
guarantees linux/compiler.h.  x86 happens to get it through
asm/div64.h -> linux/log2.h, m68k's asm/div64.h only pulls in
asm-generic/div64.h and linux/types.h, so the macro is undeclared there
and the build fails with -Werror=implicit-function-declaration.

Include linux/compiler.h so the header no longer depends on its includer.
This is a no-op for the vDSO, where lib/vdso/gettimeofday.c already gets
it through linux/build_bug.h, and vdso/unaligned.h sets the same
precedent by including linux/compiler_types.h.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202609300626.zwNjKaO6-lkp@xxxxxxxxx/
Closes: https://lore.kernel.org/oe-kbuild-all/202609301300.uAawkHuh-lkp@xxxxxxxxx/
Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
---
 include/vdso/math64.h | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/include/vdso/math64.h b/include/vdso/math64.h
index 22ae212f8b28..3c08909e742b 100644
--- a/include/vdso/math64.h
+++ b/include/vdso/math64.h
@@ -2,15 +2,18 @@
 #ifndef __VDSO_MATH64_H
 #define __VDSO_MATH64_H
 
-static __always_inline u32
-__iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
+#include <linux/compiler.h>
+
+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