[PATCH v6 3/8] arm64/runtime-const: Introduce runtime_const_mask_32()

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. Since all the current use-cases
are of the form GENMASK(n, 0), with n > 0, a single:

  ubfx  w0, w0, #0, #widthm1     // w0 = w0 [widthm1:0]

instruction is used for amd64 to improve instruction dinsity and
performance.

"Arm A-profile A64 Instruction Set Architecture" manual, Sec.
"A64 -- Base Instructions" [1] for UBFX instruction highlights the
immediate "width" is encoded as width minus 1 in imms (Bits [15:10])
which is patched by __runtime_fixup_mask() once the mask is known.

If a future use case arises that needs to tackle arbitrary mask,
consider using:

  movz  w1, #lo16, lsl #0
  movk  w1, #hi16, lsl #16

to patch the 32-bit mask in the asm block and return "__ret & (val)"
from runtime_const_mask_32() which allows compiler to further optimize
the logical and operation. __runtime_fixup_ptr() already patches a
"movz, + movk lsl #16" sequence which can be reused when the need
arises.

A possible implementation for this alternate scheme can be found at [2].

Assisted-by: Claude:claude-sonnet-4-6
Suggested-by: Samuel Holland <[email protected]>
Suggested-by: Charlie Jenkins <[email protected]>
Link: https://developer.arm.com/documentation/ddi0602/2026-03/Base-Instructions/ [1]
Link: https://lore.kernel.org/lkml/[email protected]/ [2]
Reviewed-by: Charlie Jenkins <[email protected]>
Tested-by: Charlie Jenkins <[email protected]>
Signed-off-by: K Prateek Nayak <[email protected]>
---
changelog v5..v6:

o Picked up tags from Charlie. (Thanks a ton!)

o Check the val before using __fls(). Flagged by Sashiko for potential
  compiler optimizations that can consider val !=0 after an __fls() call
  and then silently pass the BUG_ON().
---
 arch/arm64/include/asm/runtime-const.h | 46 ++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/arch/arm64/include/asm/runtime-const.h b/arch/arm64/include/asm/runtime-const.h
index 838145bc289d..e221a868c48e 100644
--- a/arch/arm64/include/asm/runtime-const.h
+++ b/arch/arm64/include/asm/runtime-const.h
@@ -36,6 +36,17 @@
 		:"r" (0u+(val)));				\
 	__ret; })
 
+#define runtime_const_mask_32(val, sym) ({			\
+	unsigned long __ret;					\
+	asm_inline("1:\t"					\
+		"ubfx %w0, %w1, #0, #32\n\t"			\
+		".pushsection runtime_mask_" #sym ",\"a\"\n\t"	\
+		".long 1b - .\n\t"				\
+		".popsection"					\
+		:"=r" (__ret)					\
+		:"r" (0u+(val)));				\
+	__ret; })
+
 #define runtime_const_init(type, sym) do {		\
 	extern s32 __start_runtime_##type##_##sym[];	\
 	extern s32 __stop_runtime_##type##_##sym[];	\
@@ -73,6 +84,41 @@ static inline void __runtime_fixup_shift(void *where, unsigned long val)
 	aarch64_insn_patch_text_nosync(p, insn);
 }
 
+static inline void __runtime_fixup_mask(void *where, unsigned long val)
+{
+	unsigned int width = (val) ? __fls(val) + 1 : 0;
+	__le32 *p = where;
+	u32 insn;
+
+	/*
+	 * XXX: Current implementation only supports patching masks of
+	 * form GENMASK(n, 0) (n >= 0) using a single UBFX instruction
+	 * to improve performance, density, and covers all the current
+	 * use-cases.
+	 *
+	 * When the need arises to support any generic mask, and this
+	 * BUG_ON() is tripped, consider using a:
+	 *
+	 *   movz %w0, #imm16
+	 *   movk %w0, #imm16, lsl #16
+	 *
+	 * sequence to load the 32bit const mask, and perform a logical
+	 * and outside the asm block before returning the result. Fixup
+	 * can simply reuse the existing __runtime_fixup_16() to patch
+	 * the individual mov instructions.
+	 */
+	BUG_ON(!val || width > 32 || (GENMASK(width - 1, 0) != val));
+
+	/*
+	 * The width of the mask is encoded as (width - 1) in imms
+	 * which is 6 bits starting at bit #10.
+	 */
+	insn = le32_to_cpu(*p);
+	insn &= 0xffff03ff;
+	insn |= ((width - 1) & 0x1f) << 10;
+	aarch64_insn_patch_text_nosync(p, insn);
+}
+
 static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
 	unsigned long val, s32 *start, s32 *end)
 {
-- 
2.34.1