为什么ARM PC寄存器指向下一条要执行的指令之后的指令? [英] Why does the ARM PC register point to the instruction after the next one to be executed?
问题描述
根据ARM IC.
ARM 状态下，PC 的值是当前指令的地址加上 8 个字节.
In ARM state, the value of the PC is the address of the current instruction plus 8 bytes.
在拇指状态:
- 对于 B、BL、CBNZ 和 CBZ 指令，PC 的值是当前指令的地址加上 4 个字节.
- 对于所有其他使用标签的指令，PC 的值是当前指令的地址加上 4 个字节，结果的 bit[1] 清零以使其字对齐.
简单的说就是PC寄存器的值指向下一条指令之后的指令.这是我不明白的事情.通常(特别是在 x86 上)程序计数器寄存器用于指向要执行的下一条指令的地址.
Simply saying, the value of the PC register points to the instruction after the next instruction. This is the thing I don't get. Usually (particularly on the x86) program counter register is used to point to the address of the next instruction to be executed.
那么，其背后的前提是什么?条件执行，也许?
So, what are the premises underlying that? Conditional execution, maybe?
推荐答案
这是一个令人讨厌的遗留抽象泄漏.
It's a nasty bit of legacy abstraction leakage.
最初的 ARM 设计有一个 3 级流水线(获取-解码-执行).为了简化设计，他们选择将 PC 读取为指令提取地址线上的当前值，而不是 2 个周期前当前正在执行的指令的值.由于大多数与 PC 相关的地址是在链接时计算的，因此让汇编器/链接器补偿该 2 条指令偏移比设计所有逻辑以纠正"PC 寄存器更容易.
The original ARM design had a 3-stage pipeline (fetch-decode-execute). To simplify the design they chose to have the PC read as the value currently on the instruction fetch address lines, rather than that of the currently executing instruction from 2 cycles ago. Since most PC-relative addresses are calculated at link time, it's easier to have the assembler/linker compensate for that 2-instruction offset than to design all the logic to 'correct' the PC register.
当然，这一切都在30 年前有意义的事情"这一堆上.现在想象一下，在当今 15 多个阶段、多问题、乱序管道中，要在该寄存器中保持一个有意义的值需要什么，您可能会理解为什么现在很难找到一个认为将 PC 暴露为寄存器是个好主意.
Of course, that's all firmly on the "things that made sense 30 years ago" pile. Now imagine what it takes to keep a meaningful value in that register on today's 15+ stage, multiple-issue, out-of-order pipelines, and you might appreciate why it's hard to find a CPU designer these days who thinks exposing the PC as a register is a good idea.
不过，从好的方面来说，至少它不像延迟槽那么可怕.相反，与您的假设相反，有条件地执行每条指令实际上只是围绕该预取偏移量的另一种优化.在围绕条件代码进行分支时(或者仍然像疯子一样执行管道中剩余的任何内容)，您不必总是必须采取管道刷新延迟，您可以完全避免非常短的分支；管道保持忙碌，当标志不匹配* 时，解码的指令可以作为 NOP 执行.再说一次，现在我们有了有效的分支预测器，但它最终成为一种障碍而不是帮助，但在 1985 年它很酷.
Still, on the upside, at least it's not quite as horrible as delay slots. Instead, contrary to what you suppose, having every instruction execute conditionally was really just another optimisation around that prefetch offset. Rather than always having to take pipeline flush delays when branching around conditional code (or still executing whatever's left in the pipe like a crazy person), you can avoid very short branches entirely; the pipeline stays busy, and the decoded instructions can just execute as NOPs when the flags don't match*. Again, these days we have effective branch predictors and it ends up being more of a hindrance than a help, but for 1985 it was cool.
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