By: Adrian (a.delete@this.acm.org), August 4, 2022 4:56 am
Room: Moderated Discussions
NoSpammer (no.delete@this.spam.com) on August 4, 2022 3:18 am wrote:
>
> I guess, apart from stack housekeeping, the primary reason is many instructions have high or variable
> latency, which means that you will not be able to reuse the top of stack immediately. So you either
> need to shuffle or you continue with dependent instruction anyways and let OOO handle that. But still,
> for optimal execution it's more optimal to put instructions closer to the order of execution, to release
> resources earlier. Even if you use stacks the optimal number of addressable sources will be close to
> what the register requirements studies have found, so you will have to address about 16-32 somehow,
> if not directly then you will be shuffling like x87. So I see no advantage for stacks.
What you say is correct when there is a single operand stack, like in Intel 8087.
In an ISA with multiple operand stacks, the reason of having more of them is to dedicate each stack to a distinct chain of *dependent* instructions.
So you always want to reuse immediately the top-of-the-stack and all instructions that target the same stack will be executed at the pace determined by their latencies.
The latencies are hidden by not using the same stack in instructions that are adjacent in the instruction stream.
Of course an OoO implementation can relax the requirement of always alternating between stacks, because it can reorder the instructions itself, but you still need to have multiple chains of dependent instructions anyway, otherwise even OoO cannot do miracles, and when there is a single chain it would still be limited by latencies.
Even when the compiler does not have to be careful with the scheduling, the correct code generation for such an ISA would be to always use a stack per chain of dependent instructions, as that could enable hardware optimizations for the placement of intermediate results.
With multiple stacks, it is likely that many cases when shuffling was required in a single stack can better be done with copying/moving the top of one stack to another stack, at appropriate times, instead of using shuffling.
>
> I guess, apart from stack housekeeping, the primary reason is many instructions have high or variable
> latency, which means that you will not be able to reuse the top of stack immediately. So you either
> need to shuffle or you continue with dependent instruction anyways and let OOO handle that. But still,
> for optimal execution it's more optimal to put instructions closer to the order of execution, to release
> resources earlier. Even if you use stacks the optimal number of addressable sources will be close to
> what the register requirements studies have found, so you will have to address about 16-32 somehow,
> if not directly then you will be shuffling like x87. So I see no advantage for stacks.
What you say is correct when there is a single operand stack, like in Intel 8087.
In an ISA with multiple operand stacks, the reason of having more of them is to dedicate each stack to a distinct chain of *dependent* instructions.
So you always want to reuse immediately the top-of-the-stack and all instructions that target the same stack will be executed at the pace determined by their latencies.
The latencies are hidden by not using the same stack in instructions that are adjacent in the instruction stream.
Of course an OoO implementation can relax the requirement of always alternating between stacks, because it can reorder the instructions itself, but you still need to have multiple chains of dependent instructions anyway, otherwise even OoO cannot do miracles, and when there is a single chain it would still be limited by latencies.
Even when the compiler does not have to be careful with the scheduling, the correct code generation for such an ISA would be to always use a stack per chain of dependent instructions, as that could enable hardware optimizations for the placement of intermediate results.
With multiple stacks, it is likely that many cases when shuffling was required in a single stack can better be done with copying/moving the top of one stack to another stack, at appropriate times, instead of using shuffling.
