197 lines
6.6 KiB
ArmAsm
197 lines
6.6 KiB
ArmAsm
/*
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* Copyright 2010 Tilera Corporation. All Rights Reserved.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation, version 2.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
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* NON INFRINGEMENT. See the GNU General Public License for
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* more details.
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*
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* Support routines for atomic operations. Each function takes:
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*
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* r0: address to manipulate
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* r1: pointer to atomic lock guarding this operation (for FUTEX_LOCK_REG)
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* r2: new value to write, or for cmpxchg/add_unless, value to compare against
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* r3: (cmpxchg/xchg_add_unless) new value to write or add;
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* (atomic64 ops) high word of value to write
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* r4/r5: (cmpxchg64/add_unless64) new value to write or add
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*
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* The 32-bit routines return a "struct __get_user" so that the futex code
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* has an opportunity to return -EFAULT to the user if needed.
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* The 64-bit routines just return a "long long" with the value,
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* since they are only used from kernel space and don't expect to fault.
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* Support for 16-bit ops is included in the framework but we don't provide
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* any (x86_64 has an atomic_inc_short(), so we might want to some day).
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*
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* Note that the caller is advised to issue a suitable L1 or L2
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* prefetch on the address being manipulated to avoid extra stalls.
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* In addition, the hot path is on two icache lines, and we start with
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* a jump to the second line to make sure they are both in cache so
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* that we never stall waiting on icache fill while holding the lock.
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* (This doesn't work out with most 64-bit ops, since they consume
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* too many bundles, so may take an extra i-cache stall.)
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*
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* These routines set the INTERRUPT_CRITICAL_SECTION bit, just
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* like sys_cmpxchg(), so that NMIs like PERF_COUNT will not interrupt
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* the code, just page faults.
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*
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* If the load or store faults in a way that can be directly fixed in
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* the do_page_fault_ics() handler (e.g. a vmalloc reference) we fix it
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* directly, return to the instruction that faulted, and retry it.
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*
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* If the load or store faults in a way that potentially requires us
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* to release the atomic lock, then retry (e.g. a migrating PTE), we
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* reset the PC in do_page_fault_ics() to the "tns" instruction so
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* that on return we will reacquire the lock and restart the op. We
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* are somewhat overloading the exception_table_entry notion by doing
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* this, since those entries are not normally used for migrating PTEs.
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*
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* If the main page fault handler discovers a bad address, it will see
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* the PC pointing to the "tns" instruction (due to the earlier
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* exception_table_entry processing in do_page_fault_ics), and
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* re-reset the PC to the fault handler, atomic_bad_address(), which
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* effectively takes over from the atomic op and can either return a
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* bad "struct __get_user" (for user addresses) or can just panic (for
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* bad kernel addresses).
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*
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* Note that if the value we would store is the same as what we
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* loaded, we bypass the load. Other platforms with true atomics can
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* make the guarantee that a non-atomic __clear_bit(), for example,
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* can safely race with an atomic test_and_set_bit(); this example is
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* from bit_spinlock.h in slub_lock() / slub_unlock(). We can't do
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* that on Tile since the "atomic" op is really just a
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* read/modify/write, and can race with the non-atomic
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* read/modify/write. However, if we can short-circuit the write when
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* it is not needed, in the atomic case, we avoid the race.
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*/
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#include <linux/linkage.h>
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#include <asm/atomic.h>
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#include <asm/page.h>
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#include <asm/processor.h>
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.section .text.atomic,"ax"
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ENTRY(__start_atomic_asm_code)
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.macro atomic_op, name, bitwidth, body
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.align 64
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STD_ENTRY_SECTION(__atomic\name, .text.atomic)
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{
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movei r24, 1
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j 4f /* branch to second cache line */
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}
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1: {
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.ifc \bitwidth,16
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lh r22, r0
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.else
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lw r22, r0
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addi r28, r0, 4
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.endif
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}
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.ifc \bitwidth,64
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lw r23, r28
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.endif
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\body /* set r24, and r25 if 64-bit */
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{
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seq r26, r22, r24
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seq r27, r23, r25
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}
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.ifc \bitwidth,64
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bbnst r27, 2f
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.endif
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bbs r26, 3f /* skip write-back if it's the same value */
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2: {
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.ifc \bitwidth,16
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sh r0, r24
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.else
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sw r0, r24
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.endif
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}
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.ifc \bitwidth,64
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sw r28, r25
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.endif
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mf
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3: {
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move r0, r22
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.ifc \bitwidth,64
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move r1, r23
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.else
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move r1, zero
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.endif
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sw ATOMIC_LOCK_REG_NAME, zero
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}
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mtspr INTERRUPT_CRITICAL_SECTION, zero
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jrp lr
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4: {
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move ATOMIC_LOCK_REG_NAME, r1
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mtspr INTERRUPT_CRITICAL_SECTION, r24
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}
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#ifndef CONFIG_SMP
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j 1b /* no atomic locks */
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#else
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{
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tns r21, ATOMIC_LOCK_REG_NAME
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moveli r23, 2048 /* maximum backoff time in cycles */
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}
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{
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bzt r21, 1b /* branch if lock acquired */
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moveli r25, 32 /* starting backoff time in cycles */
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}
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5: mtspr INTERRUPT_CRITICAL_SECTION, zero
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mfspr r26, CYCLE_LOW /* get start point for this backoff */
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6: mfspr r22, CYCLE_LOW /* test to see if we've backed off enough */
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sub r22, r22, r26
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slt r22, r22, r25
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bbst r22, 6b
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{
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mtspr INTERRUPT_CRITICAL_SECTION, r24
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shli r25, r25, 1 /* double the backoff; retry the tns */
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}
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{
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tns r21, ATOMIC_LOCK_REG_NAME
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slt r26, r23, r25 /* is the proposed backoff too big? */
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}
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{
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bzt r21, 1b /* branch if lock acquired */
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mvnz r25, r26, r23
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}
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j 5b
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#endif
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STD_ENDPROC(__atomic\name)
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.ifc \bitwidth,32
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.pushsection __ex_table,"a"
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.word 1b, __atomic\name
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.word 2b, __atomic\name
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.word __atomic\name, __atomic_bad_address
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.popsection
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.endif
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.endm
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atomic_op _cmpxchg, 32, "seq r26, r22, r2; { bbns r26, 3f; move r24, r3 }"
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atomic_op _xchg, 32, "move r24, r2"
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atomic_op _xchg_add, 32, "add r24, r22, r2"
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atomic_op _xchg_add_unless, 32, \
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"sne r26, r22, r2; { bbns r26, 3f; add r24, r22, r3 }"
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atomic_op _or, 32, "or r24, r22, r2"
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atomic_op _andn, 32, "nor r2, r2, zero; and r24, r22, r2"
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atomic_op _xor, 32, "xor r24, r22, r2"
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atomic_op 64_cmpxchg, 64, "{ seq r26, r22, r2; seq r27, r23, r3 }; \
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{ bbns r26, 3f; move r24, r4 }; { bbns r27, 3f; move r25, r5 }"
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atomic_op 64_xchg, 64, "{ move r24, r2; move r25, r3 }"
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atomic_op 64_xchg_add, 64, "{ add r24, r22, r2; add r25, r23, r3 }; \
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slt_u r26, r24, r22; add r25, r25, r26"
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atomic_op 64_xchg_add_unless, 64, \
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"{ sne r26, r22, r2; sne r27, r23, r3 }; \
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{ bbns r26, 3f; add r24, r22, r4 }; \
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{ bbns r27, 3f; add r25, r23, r5 }; \
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slt_u r26, r24, r22; add r25, r25, r26"
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jrp lr /* happy backtracer */
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ENTRY(__end_atomic_asm_code)
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