[PATCH v4 4/5] hexagon/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 clang 17.0.6, it saves roughly
4 KB.

  $ size --format=GNU vmlinux.before vmlinux.after
        text       data        bss      total filename
     4827900    1798340     364057    6990297 vmlinux.before
     4827072    1795060     364057    6986189 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@xxxxxxxxxx>
---
 arch/hexagon/include/asm/bitops.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/hexagon/include/asm/bitops.h b/arch/hexagon/include/asm/bitops.h
index e856d6dbfe16..c74a639c84f3 100644
--- a/arch/hexagon/include/asm/bitops.h
+++ b/arch/hexagon/include/asm/bitops.h
@@ -204,6 +204,9 @@ static __always_inline long ffz(int x)
 {
 	int r;
 
+	if (__builtin_constant_p(x))
+		return __builtin_ctzl(~x);
+
 	asm("%0 = ct1(%1);\n"
 		: "=&r" (r)
 		: "r" (x));
@@ -221,6 +224,9 @@ static __always_inline int fls(unsigned int x)
 {
 	int r;
 
+	if (__builtin_constant_p(x))
+		return x ? BITS_PER_TYPE(x) - __builtin_clz(x) : 0;
+
 	asm("{ %0 = cl0(%1);}\n"
 		"%0 = sub(#32,%0);\n"
 		: "=&r" (r)
@@ -242,6 +248,9 @@ static __always_inline int ffs(int x)
 {
 	int r;
 
+	if (__builtin_constant_p(x))
+		return __builtin_ffs(x);
+
 	asm("{ P0 = cmp.eq(%1,#0); %0 = ct0(%1);}\n"
 		"{ if (P0) %0 = #0; if (!P0) %0 = add(%0,#1);}\n"
 		: "=&r" (r)
@@ -264,6 +273,9 @@ static __always_inline unsigned long __ffs(unsigned long word)
 {
 	int num;
 
+	if (__builtin_constant_p(word))
+		return __builtin_ctzl(word);
+
 	asm("%0 = ct0(%1);\n"
 		: "=&r" (num)
 		: "r" (word));
@@ -282,6 +294,9 @@ static __always_inline unsigned long __fls(unsigned long word)
 {
 	int num;
 
+	if (__builtin_constant_p(word))
+		return BITS_PER_LONG - 1 - __builtin_clzl(word);
+
 	asm("%0 = cl0(%1);\n"
 		"%0 = sub(#31,%0);\n"
 		: "=&r" (num)
-- 
2.43.0