Agner`s CPU blog

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Things from MIPS (and novel things)
Author: Agner Date: 2016-07-28 12:16
Anonymous wrote:
What do you think of "branch delay slots" ?
I would categorize this under the label of explicit parallelism. You are explicitly specifying what the processor should do while fetching a jump target. I think there is a problem with letting the compiler decide what to do in parallel rather than leaving this decision to the CPU. The compiler will have to know which things the processor can or cannot do in parallel and how many instructions it can execute in the time it takes to fetch the jump target. That will lock you into a particular hardware implementation. If you later develop a new processor with different timings or a different degree of parallelism then you cannot run optimally on the new processor unless you compile again for this particular processor.

We have seen this problem with the Itanium where instructions were put together in blocks of three. The decision of which three instuctions to execute in parallel was left to the compiler rather than the CPU. It was very difficult to make such a compiler, and you were pretty much locked into a fixed degree of parallelism. I think it is better to have a genuine superscalar design with full out-of-order capabilities. Then you can run the same binary code on different CPUs with different amounts of parallelism, and the compiler doesn't need to know the details of the CPU.

One thing software developers know is which procedure will be called on a function pointer (because of polymorphism, indirect branches have become quite popular), but the hardware couldn't possible know it.
In a typical application, the value of the function pointer will be available long before the values of the function parameters so that the call instruction can be executed out of order. The CPU will execute the call instruction and fetch and decode the instructions inside the function while it is waiting for the function parameters.
Cache miss is also a big topic. Suppose I have five functions (functions in the mathematical sense), A, B, C, D, E, which can be fully executed in parallel. The calls instructions are laid in the following order:

CALL A; CALL B; CALL C; CALL D; CALL E;

But only the data for the E function is ready (is on the cache). Would the processor stall, while waiting for the data of A, B, C, D arrive? And, even worse, wait for the completion of A, B, C and D?

No. Assuming that there are no code cache misses, but only data cache misses, a superscalar CPU will fecth and decode everything in advance, if the out-of-order buffer is big enough, while waiting for data. If the data for E are available before the data for the preceding functions, then E will be executed first.
I like that vectors are present on this new instruction set and are an important part of it. Many algorithms use matrix multiplication, which can greatly benefit from instruction-level parallelism, but the current instructions sets are simply disappointing.
Matrix multiplication often involves a lot of data permutation and horizontal addition which takes extra time.
 
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