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 an allyesconfig, with GCC 13.2.1, it saves roughly 8 KB.

  $ size --format=GNU vmlinux.before vmlinux.after
        text       data        bss      total filename
    60446534   70972412    2289596  133708542 vmlinux.before
    60429746   70978876    2291676  133700298 vmlinux.after

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

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
 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 ae0457d582b8..3f89b9dccc33 100644
--- a/arch/m68k/include/asm/bitops.h
+++ b/arch/m68k/include/asm/bitops.h
@@ -470,6 +470,9 @@ 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;
@@ -491,6 +494,9 @@ 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));
@@ -523,6 +529,9 @@ 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));
@@ -541,6 +550,9 @@ 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));
-- 
2.25.1

On Sun, Dec 17, 2023 at 8:13 AM Vincent Mailhol
<mailhol.vincent@wanadoo.fr> wrote:
>
> 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 an allyesconfig, with GCC 13.2.1, it saves roughly 8 KB.
>
>   $ size --format=GNU vmlinux.before vmlinux.after
>         text       data        bss      total filename
>     60446534   70972412    2289596  133708542 vmlinux.before
>     60429746   70978876    2291676  133700298 vmlinux.after

Still a win with gcc 9.5.0-1ubuntu1~22.04:

allyesconfig:

      text       data        bss      total filename
  58882250   72419706    2284004  133585960
vmlinux-v6.7-rc8-1-m68k-bitops-force-inlining
  58863570   72419546    2285860  133568976
vmlinux-v6.7-rc8-2-m68k-bitops-use-__builtin_clz,ctzl,ffs

So an even bigger win...

atari_defconfig:

      text       data        bss      total filename
   4117008    1579350     151680    5848038
vmlinux-v6.7-rc8-1-m68k-bitops-force-inlining
   4116940    1579534     151712    5848186
vmlinux-v6.7-rc8-2-m68k-bitops-use-__builtin_clz,ctzl,ffs

... but a small size increase here.


>
> Reference: commit fdb6649ab7c1 ("x86/asm/bitops: Use __builtin_ctzl()
> to evaluate constant expressions")

Please don't split lines containing tags.

> Link: https://git.kernel.org/torvalds/c/fdb6649ab7c1
>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds