what and how to ensure the stores in the storeQueue between storeItWB and theload_inst->sqIt has finished address translation before the load?

[gxflying]

Hi sir, I am confused the assertion blow
when execute a load instruction the flow eventually goes into LSQUnit::read where handle the real read operations
the assertion means that the stores in the storeQueue between storeItWB and the load_inst->sqIt has finisded address translation,
BUT why ?
I think the stores do not have to be executed ahead of the load which is younger than the stores when the store have a long data dependency chain, take the blow riscv code for example
chances are that the load will be translated and be executed ahead of the store which is before it in program order
then the assertion will fail when there is nothing wrong !?

so what and where is the machanism in the gem5 code to ensure that the stores will always have finished translation when the load executes ?
please help to clarify this , thanks~

addi x1, x0, 0xef
multi  x2, x1, x1   # assumpting that this instr take a long time
store  x0,  0(x2)
load  x3, 0(x0)

gem5/src/cpu/o3/lsq_unit.cc

Line 1402 in 461af51

assert(store_it->instruction()->effAddrValid());

[gxflying]

hi @powerjg could you please help to clarify this ?
I think when a read is exectuted, it only needs to check the stores which has finished the address translation to check whether there is a address match and potentially perform a store-load data forwarding
a store may NOT been tanslated for any reason even if it is ordered (program order) before a certain load
so there is no guarantee for the assertion above...
thank you very much

[gxflying]

I found this assertion firstly be add here in 2007

31e78b0

[Harshil2107]

Hi,
I am not an expert on this. I tried a gem5 specific GPT model and asked it this question, this was the reply:

Your question touches on a complex aspect of the gem5 simulator related to the execution order and address translation in the load-store queue (LSQ) within an out-of-order (OoO) CPU model.

In gem5, particularly in the OoO CPU models like the DerivO3CPU, memory operations are handled in a somewhat speculative manner due to the out-of-order execution. The load-store queue (LSQ) plays a crucial role here. It's responsible for ensuring memory operation correctness, handling dependencies between loads and stores, and dealing with issues like memory reordering and address disambiguation.

The assertion assert(store_it->instruction()->effAddrValid()); you're referring to in lsq_unit.cc is essentially ensuring that by the time a load instruction is ready to access memory, all preceding store instructions (in program order) have their effective addresses resolved. This is crucial for maintaining memory consistency, as it ensures that the load is accessing the most recent data.

Now, addressing your specific concern:

1. Why Does the Assertion Exist?

   • The assertion is there to guarantee that when a load instruction is executed, all preceding stores (up to a certain point in the store queue) have had their addresses translated. This is vital for ensuring that the load gets the correct data, especially in a scenario where a preceding store might modify the same memory location.

2. Handling of Long Dependency Chains:

   • In your example, you're right in thinking that there could be long dependency chains, and a load might be ready to execute before a preceding store. However, gem5's OoO CPU models include mechanisms to handle such scenarios. These mechanisms ensure that loads are either delayed until the addresses of preceding stores are resolved or speculative execution is handled in such a way that consistency is maintained.

3. Mechanisms in gem5 for Address Translation Order:

   • In gem5, several mechanisms work together to ensure memory operation correctness:
     • Dependency Tracking: The simulator tracks dependencies between instructions, including memory dependencies. This ensures that a load will wait for the address of a preceding store to be resolved if they are dependent.
     • Memory Disambiguation: Techniques like store-to-load forwarding and memory dependency prediction are used to handle cases where the address of a store is not yet known.
     • Speculative Execution Handling: The simulator can handle speculative execution by either delaying the execution of speculative loads or by re-executing them if a preceding store updates the memory location they accessed.

4. Addressing Your RISC-V Code Example:

   • In the code snippet you provided, if the multi instruction takes a long time to execute and resolve the address for the store, the load might indeed be ready to execute first. However, gem5's OoO CPU model should handle this by either delaying the load until the store's address is resolved or by speculatively executing the load and then re-executing it if necessary.

In summary, while your concern about the assertion is valid in the context of long data dependency chains, gem5's out-of-order execution model includes several mechanisms to ensure memory operation correctness and consistency. The assertion is a part of these mechanisms, serving as a check to ensure that the memory model's assumptions about order and dependency resolution are being met.

[gxflying]

Thanks for your reply, it's almost clear, and it seems that GPT is realy helpful !
and there is still a a key point need to clarify.
when it says " gem5's OoO CPU model should handle this by either delaying the load until the store's address is resolved or by speculatively executing the load and then re-executing it if necessary."
we need to know the specfic way which the gem5 uses : by delaying or by speculatively executing ?
if the " by speculatively executing" is used, the aforementioned assertion "assert(store_it->instruction()->effAddrValid());" will fail !
because the store before it in program order has not finished address translation and the load is speculatively executed first!
and I don't find any hint that the gem5 delays the load in scenario we talk about above,
if so, chances are that the gem5 will speculatively execte the load, and the assertion maybe fail! and it will be a potential bug

[Harshil2107]

Here is GPT's answer to you clarification:
Your follow-up question delves deeper into the specifics of how gem5's out-of-order (OoO) CPU models handle the timing and execution order of load and store instructions, particularly in situations involving speculative execution and delayed address translation.

In gem5, the handling of loads and stores in the presence of dependencies and address translation delays is a nuanced issue. The key point is understanding how gem5 decides between delaying a load or executing it speculatively, and how this decision affects the assertion you mentioned.

1. Delaying Loads vs. Speculative Execution:

   • Delaying Loads: If a load instruction depends on the address of a previous store instruction that has not yet been resolved, gem5's default behavior is to delay the load. This ensures memory consistency and adheres to the program order as closely as possible.
   • Speculative Execution: gem5 can also speculatively execute loads before the address of a preceding store is known. However, this is typically managed in a way that maintains consistency. For instance, if a speculative load is executed and later it's found that a preceding store modifies the same memory location, the load may need to be re-executed.

2. Understanding the Assertion assert(store_it->instruction()->effAddrValid());

   • This assertion checks whether the effective address of all preceding stores (in program order) up to the current load instruction has been resolved. If gem5 is speculatively executing a load, this assertion should not fail under normal circumstances. This is because speculative execution in gem5 is designed to account for unresolved addresses and reorder or re-execute instructions as necessary to maintain consistency.

3. Potential Scenarios and the Assertion:

   • If the assertion fails, it could indicate a scenario where gem5's speculative execution or dependency tracking mechanisms are not handling a particular case as expected. It might be a rare edge case or a situation not adequately covered by the current implementation.
   • However, it's important to note that speculative execution in gem5, especially in complex OoO models, is handled with several layers of checks and balances. These are designed to prevent exactly the kind of inconsistency your concern raises.

4. Investigating Further in gem5:

   • To fully understand how gem5 handles your specific scenario, you would need to delve into the source code, particularly the parts dealing with the OoO CPU model, LSQ, and memory dependency resolution.
   • You could also experiment by setting up a test scenario in gem5 that matches your example and observe how the simulator handles the load and store instructions, particularly with respect to their address translations.

In conclusion, the behavior of gem5 in the scenario you described (whether it delays the load or executes it speculatively) depends on how the simulator's OoO model is implemented and the specific conditions of the simulation. The assertion in question is part of the mechanisms ensuring memory consistency, and its failure in a normal simulation scenario would indeed be indicative of an issue needing further investigation. However, gem5's speculative execution and dependency handling are designed to prevent such inconsistencies under normal operation.

[gxflying]

maybe I should refer to the source code for this,
the GPT answer seems to be general and sort of common sense.
thank you very much for your time~

[muke101]

@gxflying did you ever figure this out? I've been confused about the same thing for a while! It seems for sure that stores are inserted into the SQ on dispatch - i.e. before their operands may be ready - and so I've no idea how this assertion can make sense or allow speculative execution.

[muke101]

Hey, managed to figure this out.

From the start of the loop to search the SQ for forwarding opportunities, you can see the size of the store's memory access is taken. In the if condition just before the assertion we're confused about, the size is guarded against being 0. It turns out this is effectively checking that the store has a valid address, as both the store's address and access size aren't assigned to the instruction until the memory access is initiated in iew::executeInsts. Before this point, they're both always 0.

    // Check the SQ for any previous stores that might lead to forwarding
    auto store_it = load_inst->sqIt;
    assert (store_it >= storeWBIt);
    // End once we've reached the top of the LSQ
    while (store_it != storeWBIt && !load_inst->isDataPrefetch()) {
        // Move the index to one younger
        store_it--;
        assert(store_it->valid());
        assert(store_it->instruction()->seqNum < load_inst->seqNum);
        int store_size = store_it->size();

        // Cache maintenance instructions go down via the store
        // path but they carry no data and they shouldn't be
        // considered for forwarding
        if (store_size != 0 && !store_it->instruction()->strictlyOrdered() &&
            !(store_it->request()->mainReq() &&
              store_it->request()->mainReq()->isCacheMaintenance())) {
            assert(store_it->instruction()->effAddrValid());

This means then only stores with valid addresses make it past this if condition, but all older stores are iterated over. I think the problem is that this code just isn't especially clear - I personally had glossed over the size check in the past assuming it was checking for some corner case similar to cache maintenance operations. Maybe it could stand to be more explicit in what it's checking :)

[gxflying]

Year, make sense， the store_size >0 is true only when the stie has finised the address translation ,so the effective address (physical addr) is true definitley