The LWX and SWX instructions are used to implement common semaphore operations, including test and set, compare and swap, exchange memory, and fetch and add. They are also used to implement spinlocks.
These instructions are typically used by system programs and are called by application programs as needed.
Generally, a program uses LWX to load a semaphore from memory, causing the reservation to be set (the processor maintains the reservation internally). The program can compute a result based on the semaphore value and conditionally store the result back to the same memory location using the SWX instruction. The conditional store is performed based on the existence of the reservation established by the preceding LWX instruction. If the reservation exists when the store is executed, the store is performed and MSR[C] is cleared to 0. If the reservation does not exist when the store is executed, the target memory location is not modified and MSR[C] is set to 1.
If the store is successful, the sequence of instructions from the semaphore load to the semaphore store appear to be executed atomically—no other device modified the semaphore location between the read and the update. Other devices can read from the semaphore location during the operation.
For a semaphore operation to work properly, the LWX instruction must be paired with an SWX instruction, and both must specify identical addresses.
The reservation granularity in MicroBlaze is a word. For both instructions, the address must be word aligned. No unaligned exceptions are generated for these instructions.
The conditional store is always attempted when a reservation exists, even if the store address does not match the load address that set the reservation.
Only one reservation can be maintained at a time. The address associated with the reservation can be changed by executing a subsequent LWX instruction.
The conditional store is performed based upon the reservation established by the last LWX instruction executed. Executing an SWX instruction always clears a reservation held by the processor, whether the address matches that established by the LWX or not.
Reset, interrupts, exceptions, and breaks (including the BRK and BRKI instructions) all clear the reservation.
The following provides general guidelines for using the LWX and SWX instructions:
- The LWX and SWX instructions should be paired and use the same address.
- An unpaired SWX instruction to an arbitrary address can be used to clear any reservation held by the processor.
- A conditional sequence begins with an LWX instruction. It can be followed by memory accesses and/or computations on the loaded value. The sequence ends with an SWX instruction. In most cases, failure of the SWX instruction should cause a branch back to the LWX for a repeated attempt.
- An LWX instruction can be left unpaired when executing certain
synchronization primitives if the value loaded by the LWX is not zero. An
implementation of Test and Set exemplifies
this:
loop: lwx r5,r3,r0 ; load and reserve bnei r5,next ; branch if not equal to zero addik r5,r5,1 ; increment value swx r5,r3,r0 ; try to store non-zero value addic r5,r0,0 ; check reservation bnei r5,loop ; loop if reservation lost next: - Performance can be improved by minimizing looping on an LWX instruction that
fails to return a desired value. Performance can also be improved by using an
ordinary load instruction to do the initial value check. An implementation of a
spinlock exemplifies
this:
loop: lw r5,r3,r0 ; load the word bnei r5,loop ; loop back if word not equal to 0 lwx r5,r3,r0 ; try reserving again bnei r5,loop ; likely that no branch is needed addik r5,r5,1 ; increment value swx r5,r3,r0 ; try to store non-zero value addic r5,r0,0 ; check reservation bnei r5,loop ; loop if reservation lost - Minimizing the looping on an LWX/SWX instruction pair increases the likelihood that forward progress is made. The old value should be tested before attempting the store. If the order is reversed (store before load), more SWX instructions are executed and reservations are more likely to be lost between the LWX and SWX instructions.