[PATCH] m68k: implement runtime consts

Daniel Palmer
2025-11-27T00:05:23Z

This implements runtime consts for m68k which hopefully improves
performance a tiny bit, maybe, hopefully... going downhill with
the wind behind us.

Constant pointers are just a register load with an immediate that is
fixed up.

Constant shifts are a very small register load with an immediate that
is fixed up and then the val is shifted by the register. Putting the
shift amount in a register is needed as the shift instruction can only
encode a maximum of shift by 8 with an immediate.

Before:

0013c6ea <__d_lookup_rcu_op_compare>:
...
13c6fc: 2003 movel %d3,%d0
13c6fe: 2239 0069 a980 movel 69a980 <d_hash_shift>,%d1
13c704: e2a8 lsrl %d1,%d0
13c706: e588 lsll #2,%d0
13c708: d0b9 0069 a97c addl 69a97c <dentry_hashtable>,%d0
...

After:

0013c6c0 <__d_lookup_rcu_op_compare>:
...
13c6d2: 207c cafe f00d moveal #-889262067,%a0
13c6d8: 2003 movel %d3,%d0
13c6da: 720c moveq #12,%d1
13c6dc: e2a8 lsrl %d1,%d0
13c6de: e588 lsll #2,%d0
13c6e0: d1c0 addal %d0,%a0
...

Signed-off-by: Daniel Palmer <daniel@thingy.jp>
---
arch/m68k/include/asm/runtime-const.h | 76 +++++++++++++++++++++++++++
1 file changed, 76 insertions(+)
create mode 100644 arch/m68k/include/asm/runtime-const.h

diff --git a/arch/m68k/include/asm/runtime-const.h b/arch/m68k/include/asm/runtime-const.h
new file mode 100644
index 000000000000..576031c5b808
--- /dev/null
+++ b/arch/m68k/include/asm/runtime-const.h
@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * runtime const implementation for m68k
+ * Copyright (C) 2025 Daniel Palmer<daniel@thingy.jp>
+ *
+ * Based on arm64 version.
+ */
+#ifndef _ASM_RUNTIME_CONST_H
+#define _ASM_RUNTIME_CONST_H
+
+#include <asm/cacheflush.h>
+#include <linux/bitfield.h>
+
+#define runtime_const_ptr(sym) ({ \
+ typeof(sym) __ret; \
+ asm_inline("1:\t" \
+ "mov.l #0xcafef00d,%0\n\t" \
+ ".pushsection runtime_ptr_" #sym ",\"a\"\n\t" \
+ ".long 1b - .\n\t" \
+ ".popsection" \
+ : "=a" (__ret)); \
+ __ret; })
+
+static inline void __runtime_fixup_ptr(void *where, unsigned long val)
+{
+ u32 *value = where + 2;
+ const unsigned long start = ((unsigned long) where) + 2;
+ const unsigned long end = start + sizeof(*value);
+
+ *value = val;
+
+ flush_icache_range(start, end);
+}
+
+#define runtime_const_shift_right_32(val, sym) ({ \
+ u32 __tmp; \
+ asm_inline("1:\t" \
+ "moveq #12, %0\n\t" \
+ "lsr.l %0, %1\n\t" \
+ ".pushsection runtime_shift_" #sym ",\"a\"\n\t" \
+ ".long 1b - .\n\t" \
+ ".popsection" \
+ : "=&d" (__tmp), "+d" (val)); \
+ val; })
+
+#define MOVEQ_DATA GENMASK(7, 0)
+static inline void __runtime_fixup_shift(void *where, unsigned long val)
+{
+ u16 *insn = where;
+ const unsigned long start = (unsigned long) where;
+ const unsigned long end = start + sizeof(*insn);
+
+ FIELD_MODIFY(MOVEQ_DATA, insn, val);
+
+ flush_icache_range(start, end);
+}
+
+#define runtime_const_init(type, sym) do { \
+ extern s32 __start_runtime_##type##_##sym[]; \
+ extern s32 __stop_runtime_##type##_##sym[]; \
+ runtime_const_fixup(__runtime_fixup_##type, \
+ (unsigned long)(sym), \
+ __start_runtime_##type##_##sym, \
+ __stop_runtime_##type##_##sym); \
+} while (0)
+
+static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
+ unsigned long val, s32 *start, s32 *end)
+{
+ while (start < end) {
+ fn(*start + (void *)start, val);
+ start++;
+ }
+}
+
+#endif
--
2.51.0