[PATCH v3 4/7] riscv/runtime-const: Introduce runtime_const_mask_32()

[K Prateek Nayak <kprateek.nayak@amd.com>]

Futex hash computation requires a mask operation with read-only after
init data that will be converted to a runtime constant in the subsequent
commit.

Introduce runtime_const_mask_32 to further optimize the mask operation
in the futex hash computation hot path. GCC generates a:

  lui   a0, 0x12346       # upper; +0x800 then >>12 for correct rounding
  addi  a0, a0, 0x678     # lower 12 bits
  and   a1, a1, a0        # a1 = a1 & a0

pattern to tackle arbitrary 32-bit masks and the same was also suggested
by Claude which is implemented here. The (__mask & val) operation is
intentionally placed outside of asm block to allow compilers to further
optimize it if possible.

__runtime_fixup_ptr() already patches a "lui + addi" sequence which has
been reused to patch the same sequence for __runtime_fixup_mask().

Assisted-by: Claude:claude-sonnet-4-5
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
---
Changelog v2..v3:

o Moved the "&" operation outside the inline asm block to allow for
  compilers to further optimize it if possible. (Based on David's
  comment on ARM64 bits).
---
 arch/riscv/include/asm/runtime-const.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
index d766e2b9e6df..85efba8ecf12 100644
--- a/arch/riscv/include/asm/runtime-const.h
+++ b/arch/riscv/include/asm/runtime-const.h
@@ -153,6 +153,22 @@
 	__ret;							\
 })
 
+#define runtime_const_mask_32(val, sym)				\
+({								\
+	u32 __mask;						\
+	asm_inline(".option push\n\t"				\
+		".option norvc\n\t"				\
+		"1:\t"						\
+		"lui	%[__mask],0x89abd\n\t"			\
+		"addi	%[__mask],%[__mask],-0x211\n\t"		\
+		".option pop\n\t"				\
+		".pushsection runtime_mask_" #sym ",\"a\"\n\t"	\
+		".long 1b - .\n\t"				\
+		".popsection"					\
+		: [__mask] "=r" (__mask));			\
+	(__mask & val);						\
+})
+
 #define runtime_const_init(type, sym) do {			\
 	extern s32 __start_runtime_##type##_##sym[];		\
 	extern s32 __stop_runtime_##type##_##sym[];		\
@@ -256,6 +272,12 @@ static inline void __runtime_fixup_shift(void *where, unsigned long val)
 	mutex_unlock(&text_mutex);
 }
 
+static inline void __runtime_fixup_mask(void *where, unsigned long val)
+{
+	__runtime_fixup_32(where, where + 4, val);
+	__runtime_fixup_caches(where, 2);
+}
+
 static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
 				       unsigned long val, s32 *start, s32 *end)
 {
-- 
2.34.1