llvm dev - Apr 2017 - Question about Hexagon VLIWResourceModel::reserveResources

Hi All,

  I am reading Hexagon's VLIW scheduler source code, and have a
question about VLIWResourceModel::reserveResources.
>From what I understand, it checks if a SU can be put in current
Packet. Intuitively, if current Packet is full, isResourceAvailable
should return false immediately. However, I see the code below:

---

  // If packet is now full, reset the state so in the next cycle
  // we start fresh.
  if (Packet.size() >= SchedModel->getIssueWidth()) {
    ResourcesModel->clearResources();
    savePacket();
    Packet.clear();
    TotalPackets++;
    startNewCycle = true;
  }

---

  I don't see reserveResources check if the Packet is full already,
and it seems possible that we have Packet.size
>SchedModel->getIssueWidth().
Is the Packet valid?

  Thanks.

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj

I'd have to look at the code in more detail, but the packet checks are 
only approximate. There are several instructions that are not checked 
against resource availability, so adding them may indeed exceed the 
packet size limit.
The precise checks are a lot more complex. One thing to keep track of 
are "constant extenders", which are special instructions that only
hold
immediate values for operands that wouldn't otherwise fit in the 
encoding of the instruction that uses them.

-Krzysztof

On 4/21/2017 10:22 AM, 陳韋任 via llvm-dev wrote:
> Hi All,
> 
>    I am reading Hexagon's VLIW scheduler source code, and have a
> question about VLIWResourceModel::reserveResources.
>  From what I understand, it checks if a SU can be put in current
> Packet. Intuitively, if current Packet is full, isResourceAvailable
> should return false immediately. However, I see the code below:
> 
> ---
> 
>    // If packet is now full, reset the state so in the next cycle
>    // we start fresh.
>    if (Packet.size() >= SchedModel->getIssueWidth()) {
>      ResourcesModel->clearResources();
>      savePacket();
>      Packet.clear();
>      TotalPackets++;
>      startNewCycle = true;
>    }
> 
> ---
> 
>    I don't see reserveResources check if the Packet is full already,
> and it seems possible that we have Packet.size >>
SchedModel->getIssueWidth().
> Is the Packet valid?
> 
>    Thanks.
> 
> Regards,
> chenwj
> 
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