[PATCH] csky: bitops: Use the compiler builtin for __fls()

[Tzu-Ching Yang <shingekinocore@gmail.com>]

The csky __fls() is a C wrapper, fls(x) - 1, around the ff1 asm
volatile in fls(), so the compiler can neither fold it for a constant
argument nor merge repeated calls. GCC implements __builtin_clzl()
with the same ff1 instruction for CK807, CK810 and CK860; CK610
(abiv1) cannot be built with upstream GCC.

Use the asm-generic builtin __fls() instead. ffs(), __ffs() and fls()
keep their asm: GCC has no ctz pattern for csky, so builtin ffs() and
__ffs() would become calls to __ffssi2() and __ctzsi2(), and a builtin
fls() needs a test for zero that ff1 makes unnecessary by returning 32
for 0.

__fls() now returns unsigned int instead of unsigned long, like the
asm-generic version; both are 32 bits wide on csky. __fls(0) remains
undefined.

With GCC 9 to 16, the text of every defconfig-based vmlinux tested
shrinks by 128 to 320 bytes.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/all/affe7da1-41aa-4710-bee0-5e3c2b8a8dfc@app.fastmail.com/
Signed-off-by: Tzu-Ching Yang <shingekinocore@gmail.com>
---
In a test function, __fls(x) takes 4 instructions (return included)
before and after with each GCC and CPU tested, at -O2 and -Os; with a
constant argument, it now folds.

CK610 needs an abiv1 GCC; the only one I could find is the vendor's 6.3,
older than the 8.1 minimum and without a clz pattern for abiv1. There,
__fls() would become a call to __clzsi2(), which lib/clz_ctz.c
implements with the fls() asm, so it would still work; a builtin fls()
would make __clzsi2() call itself.

Testing: 35 defconfig-based kernels for CK807, CK810 and CK860, some
with KUnit or SMP, built before and after with W=1 by kernel.org GCC 9.5
to 16.2 (GCC 8 has no csky port, and GCC 9 to 11 reject -mcpu=ck860): no
new warnings and no calls to __ctzsi2(), __clzsi2() or __ffssi2().
allmodconfig with GCC 12 and 16 compiles the same objects with the same
warnings, but does not reach a vmlinux because of GCC internal compiler
errors, before and after. Under XuanTie's qemu-cskyv2 user mode, the old
and new helpers return the same values for every defined 32-bit input,
built with the kernel's flags by each GCC for each CPU at -O2 and -Os.
On qemu-system-cskyv2 -M virt, the ffs and bitops KUnit tests pass
before and after; test_bitmap passes with GCC 12 to 16 and has the same
one failure with GCC 9 to 11 in both, from a test bug fixed in
linux-next. No csky hardware was available.

Against the linux-next branch of c-sky/csky-linux.git (abb81e5ce7d9);
also applies to next-20261021 and to bitmap-for-next. Full build and
test data on request.

 arch/csky/include/asm/bitops.h | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/arch/csky/include/asm/bitops.h b/arch/csky/include/asm/bitops.h
index 80d67eee6e86..118fb0df5267 100644
--- a/arch/csky/include/asm/bitops.h
+++ b/arch/csky/include/asm/bitops.h
@@ -49,14 +49,7 @@ static __always_inline __attribute_const__ int fls(unsigned int x)
 	return (32 - x);
 }
 
-/*
- * asm-generic/bitops/__fls.h
- */
-static __always_inline __attribute_const__ unsigned long __fls(unsigned long x)
-{
-	return fls(x) - 1;
-}
-
+#include <asm-generic/bitops/builtin-__fls.h>
 #include <asm-generic/bitops/ffz.h>
 #include <asm-generic/bitops/fls64.h>
 

base-commit: abb81e5ce7d995baa41556b8125fa59e28ba3be8
-- 
2.34.1