[PATCH v4 2/5] m68k/bitops: use __builtin_{clz,ctzl,ffs} to evaluate constant expressions

[Vincent Mailhol]

The compiler is not able to do constant folding on "asm volatile" code.

Evaluate whether or not the function argument is a constant expression
and if this is the case, return an equivalent builtin expression.

On linux 6.7 with an allyesconfig and GCC 13.2.1, it saves roughly 11 KB.

  $ size --format=GNU vmlinux.before vmlinux.after
    text       data        bss      total filename
    60457964   70953697    2288644  133700305 vmlinux.before
    60441196   70957057    2290724  133688977 vmlinux.after

Reference: commit fdb6649ab7c1 ("x86/asm/bitops: Use __builtin_ctzl() to evaluate constant expressions")
Link: https://git.kernel.org/torvalds/c/fdb6649ab7c1

Reviewed-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
 arch/m68k/include/asm/bitops.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/m68k/include/asm/bitops.h b/arch/m68k/include/asm/bitops.h
index a8b23f897f24..02ec8a193b96 100644
--- a/arch/m68k/include/asm/bitops.h
+++ b/arch/m68k/include/asm/bitops.h
@@ -469,6 +469,9 @@ static __always_inline unsigned long ffz(unsigned long word)
 {
 	int res;
 
+	if (__builtin_constant_p(word))
+		return __builtin_ctzl(~word);
+
 	__asm__ __volatile__ ("bfffo %1{#0,#0},%0"
 			      : "=d" (res) : "d" (~word & -~word));
 	return res ^ 31;
@@ -490,6 +493,9 @@ static __always_inline unsigned long ffz(unsigned long word)
 	!defined(CONFIG_M68000)
 static __always_inline unsigned long __ffs(unsigned long x)
 {
+	if (__builtin_constant_p(x))
+		return __builtin_ctzl(x);
+
 	__asm__ __volatile__ ("bitrev %0; ff1 %0"
 		: "=d" (x)
 		: "0" (x));
@@ -522,6 +528,9 @@ static __always_inline int ffs(int x)
 {
 	int cnt;
 
+	if (__builtin_constant_p(x))
+		return __builtin_ffs(x);
+
 	__asm__ ("bfffo %1{#0:#0},%0"
 		: "=d" (cnt)
 		: "dm" (x & -x));
@@ -540,6 +549,9 @@ static __always_inline int fls(unsigned int x)
 {
 	int cnt;
 
+	if (__builtin_constant_p(x))
+		return x ? BITS_PER_TYPE(x) - __builtin_clz(x) : 0;
+
 	__asm__ ("bfffo %1{#0,#0},%0"
 		: "=d" (cnt)
 		: "dm" (x));
-- 
2.43.0