search

t-crest / patmos-simulator

Host the Patmos ISA simulator
2 stars 2 forks source link

Added --permissive-dual-issue flag #27

Closed Emoun closed 1 year ago

Emoun commented 1 year ago

'--permissive-dual-issue' flag is meant to test the effect of allowing some instructions of the instructions currently disallowed in the second issue slot.

Currently this enables loads, stores, and branches in the second issue slot with the following requirement: Only 1 instructions of each kind may be enabled in a bundle, i.e., if there are two loads in a bundle an error will be thrown if they are both enabled. This "check" is done in the simulator at runtime and is thus a guarantee the programmer/compiler provides. The idea is the hardware can treat a violation as undefined behavior (i.e., hardware can do whatever it wants).

I have chosen to allow a store and a load to be enabled in the same bundle. This is assuming the load and store hardware is separate and may be able to execute a load and a store simultaneously , which does seem to be common but I haven't checked whether Patmos can easily implement this.

Also fixed #17

schoeberl commented 1 year ago

OK, good idea. However, I would not allow a load and store both enabled. This is hard to support in hardware.

Emoun commented 1 year ago

Ok, I'll disallow store and load together.

Emoun commented 1 year ago

I have added 2 more cases to the bundling:

1) Disallow 2 main-memory targeting instructions. E.g., cannot do a data-cache load and a call the call must fetch instruction and the cache might miss. 2) Disallow 2 stack operations like in #25 (this is a temporary solution until that issue is decided)

Question: Does it make sense to allow a stack load/store with, e.g., a call (even if the call will need to load memory)? This is currently allowed

schoeberl commented 1 year ago

Probably not good to mix a load with a call.Cheers,MartinOn 7 Sep 2023, at 12.16, Emad Jacob Maroun @.***> wrote: I have added 3 more cases to the bundling:

Disallow 2 main-memory targeting instructions. E.g., cannot do a data-cache load and a call the call must fetch instruction and the cache might miss. Disallow 2 stack operations like in #25 (this is a temporary solution until that issue is decided)

Question: Does it make sense to allow a stack load/store with, e.g., a call (even if the call will need to load memory)? This is currently allowed

—Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you commented.Message ID: @.***>

Emoun commented 1 year ago

What do you mean by "not good"? Are there any hardware limitations you can think of? The compiler could take advantage of allowing it.

schoeberl commented 1 year ago

As you correctly stated that a call needs to access memory (for the function size) and a load don’t work together.

Martin

On 10.09.2023, at 12:20, Emad Jacob Maroun @.***> wrote:

What do you mean by "not good"? Are there any hardware limitations you can think of? The compiler could take advantage of allowing it.

— Reply to this email directly, view it on GitHub https://github.com/t-crest/patmos-simulator/pull/27#issuecomment-1712774737, or unsubscribe https://github.com/notifications/unsubscribe-auth/AAE63GHX4TDOAH2GM4QJQV3XZWH5DANCNFSM6AAAAAA4KOEYNE. You are receiving this because you commented.

Emoun commented 1 year ago

But here we are talking about only stack load/stores, which don't touch main memory, so no contention there.

schoeberl commented 1 year ago

You are right.

Emoun commented 1 year ago

Ok, I will keep the rules and requirements as is right now.

Also, commented out code as specified in #28 .