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| en:multiasm:paarm:chapter_5_15 [2025/12/04 22:32] – [Speculative instruction execution] eriks.klavins | en:multiasm:paarm:chapter_5_15 [2025/12/04 23:12] (current) – [Power saving] eriks.klavins | ||
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| ===== Barriers(instruction synchronization / data memory / data synchronization / one way BARRIER) ===== | ===== Barriers(instruction synchronization / data memory / data synchronization / one way BARRIER) ===== | ||
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| + | Many processors today can execute instructions out of the programmer-defined order. This is done to improve performance, | ||
| + | |||
| + | The barrier instructions enforce instruction order between operations. This does not matter whether the processor has a single core or multiple cores; these barrier instructions ensure that the data is stored before the next operation with them, that the previous instruction result is stored before the next instruction is executed, and that the second core (if available) accesses the newest data. ARM has implemented special barrier instructions to do that: ''< | ||
| + | |||
| + | Since the instructions are prefetched and decoded ahead of time, those earlier fetched and executed instructions might not yet reflect the newest state. The ''< | ||
| + | ''< | ||
| + | ''< | ||
| + | ''< | ||
| + | ''< | ||
| + | |||
| + | The code example ensures that the control register settings are stored before the following instructions are fetched. The setting enables the Memory Management Unit and updates new address translation rules. The following instructions, | ||
| + | Sometimes the instructions may sequentially require access to memory, and situations where previous instructions store data in memory and the next instruction is intended to use the most recently stored data. Of course, this example is to explain the data synchronisation barrier. Such an example is unlikely to occur in real life. Let's imagine a situation where the instruction computes a result and must store it in memory. The following instruction must use the result stored in memory by the previous instruction. | ||
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| + | The first problem may occur when the processor uses special buffers to maximise data throughput to memory. This buffer collects multiple data chunks and, in a single cycle, writes them to memory. The buffer is not visible to the following instructions so that they may use the old data rather than the newest. The data synchronisation barrier instruction will solve this problem and ensure that the data are stored in memory rather than left hanging in the buffer.\\ | ||
| + | **Core 0:** \\ | ||
| + | ''< | ||
| + | ''< | ||
| + | ''< | ||
| + | |||
| + | **Core 1:**\\ | ||
| + | ''< | ||
| + | ''< | ||
| + | ''< | ||
| + | In the example, the data memory barrier will ensure that the data is stored before the function status flag. The DMB and DSB barriers are used to ensure that data are available in the cache memory. And the purpose of the shared domain is to specify the scope of cache consistency for all inner processor units that can access memory. These instructions and parameters are primarily used for cache maintenance and memory barrier operations. The argument that determines which memory accesses are ordered by the memory barrier and the Shareability domain over which the instruction must operate. This scope effectively defines which Observers the ordering imposed by the barriers extends to. | ||
| + | |||
| + | In this example, the barrier takes the parameter; the domain “ISH” (inner sharable) is used with barriers meant for inner sharable cores. That means the processors (in this area) can access these shared caches, but the hardware units in other areas of the system (such as DMA devices, GPUs, etc.) cannot. The “''< | ||
| + | These domain have their own options. These options are available in the table below. | ||
| + | <table tab_label> | ||
| + | < | ||
| + | ^ Option ^ Order access (before-after) ^ Shareability domain ^ | ||
| + | | OSH | Any-Any | Outer shareable | | ||
| + | | OSHLD | Load-Load, Load-Store | Outer shareable | | ||
| + | | OSHST | Store-Store | Outer shareable | | ||
| + | | NSH | Any-Any | Non-shareable | | ||
| + | | NSHLD | Load-Load, Load-Store | Non-shareable | | ||
| + | | NSHST | Store-Store | Non-shareable | | ||
| + | | ISH | Any-Any | Inner shareable | | ||
| + | | ISHLD | Load-Load, Load-Store | Inner shareable | | ||
| + | | ISHST | Store-Store | Inner shareable | | ||
| + | | SY | Any-Any | Full system | | ||
| + | | ST | Store-Store | Full system | | ||
| + | | LD | Load-Load, Load-Store | Full system | | ||
| + | </ | ||
| + | |||
| + | Order access specifies which classes of accesses the barrier operates on. A " | ||
| + | |||
| + | All these barriers are practical with high-level programming languages, where unsafe optimisation and specific memory ordering may occur. In most scenarios, there is no need to pay special attention to memory barriers in single-processor systems. Although the CPU supports out-of-order and predictive execution, in general, it ensures that the final execution result meets the programmer' | ||
| + | * Share data between multiple CPU cores. Under the weak consistency memory model, a CPU's disordered memory access order may cause contention for access. | ||
| + | * Perform operations related to peripherals, | ||
| + | * Modifying the memory management strategy, such as context switching, requesting page faults, and modifying page tables. | ||
| + | In short, the purpose of using memory barrier instructions is to ensure the CPU executes the program' | ||
| ===== Conditional instructions ===== | ===== Conditional instructions ===== | ||
| + | Meanwhile, speculative instruction execution consumes power. If the speculation gave the correct result, the power wasn’t wasted; otherwise, it was. Power consumption must be taken into account when designing the program code. Not only is power consumption essential, but so is data safety. The Cortex-A76 on the Raspberry Pi 5 has a very advanced branch predictor, but mispredictions still cause wasted instructions, | ||
| + | |||
| + | The use of the conditional select (''< | ||
| + | ''< | ||
| + | ''< | ||
| + | |||
| + | This conditional instruction writes the value of the first source register to the destination register if the condition is TRUE. If the condition is FALSE, it writes the value of the second source register to the destination register. So, if the ''< | ||
| + | |||
| + | Other conditional instructions can be used similarly: | ||
| + | |||
| + | {{: | ||
| + | |||
| + | These conditional instructions are helpful in branchless conditional checks. Taking into account that these instructions can also be executed speculatively, | ||
| ===== Power saving ===== | ===== Power saving ===== | ||
| + | |||
| + | Some special instructions are meant to put the processor into sleep modes and wait for an event to occur. The processor can be woken up by an interrupt or by an event. In these modes, the code may be explicitly created to initialise interrupts and events, and to handle them. After that, the processor may be put into sleep mode and remain asleep unless an event or interrupt occurs. The following code example can be used only in bare-metal mode – without an OS. | ||
| + | < | ||
| + | < | ||
| + | < | ||
| + | .global idle_loop | ||
| + | idle_loop: | ||
| + | 1: WFI @ Wait For Interrupt, core goes to low-power | ||
| + | B | ||
| + | </ | ||
| + | </ | ||
| + | < | ||
| + | The example only waits for interrupts to occur. To wait for events and interrupts, the ''< | ||
| + | |||
| + | On a Raspberry Pi 5 running Linux, it is not observable whether the CPU enters these modes, because the OS generates many events between CPU cores and also handles many interrupts from communication interfaces and other Raspberry Pi components. | ||
| + | Another way to save more energy while running the OS on the Raspberry Pi is to reduce the CPU clock frequency. There is a scheme called dynamic voltage and frequency scaling (DVFS), the same technique used in laptops, that reduces power consumption and thereby increases battery life. On the internet, there is a paper named “Cooling a Raspberry Pi Device ”. The paper includes one chapter explaining how to reduce the CPU clock frequency. The Linux OS exposes CPU frequency scaling through sysfs, e.g.: | ||
| + | * ”/ | ||
| + | * “/ | ||
| + | |||
| + | It is possible to use syscalls in assembler to open and write specific values into them. | ||
| + | < | ||
| + | < | ||
| + | < | ||
| + | .global _start | ||
| + | .section .text | ||
| + | _start: | ||
| + | @ openat(AT_FDCWD, | ||
| + | mov x0, #-100 @ AT_FDCWD | ||
| + | ldr x1, =gov_path | ||
| + | mov x2, #1 @ O_WRONLY | ||
| + | mov x3, #0 @ mode (unused) | ||
| + | mov x8, #56 @ sys_openat | ||
| + | svc #0 | ||
| + | mov x19, x0 @ save fd | ||
| + | |||
| + | @ write(fd, " | ||
| + | mov x0, x19 | ||
| + | ldr x1, =gov_value | ||
| + | mov x2, #10 @ length of " | ||
| + | mov x8, #64 @ sys_write | ||
| + | svc #0 | ||
| + | |||
| + | @ close(fd) | ||
| + | mov x0, x19 | ||
| + | mov x8, #57 @ sys_close | ||
| + | svc #0 | ||
| + | |||
| + | @ exit(0) | ||
| + | mov x0, #0 | ||
| + | mov x8, #93 @ sys_exit | ||
| + | svc #0 | ||
| + | |||
| + | .section .rodata | ||
| + | gov_path: | ||
| + | .asciz "/ | ||
| + | gov_value: | ||
| + | .asciz " | ||
| + | </ | ||
| + | </ | ||
| + | Similar things can be done with CPU frequencies, | ||
| + | |||
