[llvm-dev] LLVM-MCA Processor Resources ID

[Andrea Di Biagio via llvm-dev]

Hi Pietro,

llvm-mca internally assigns each processor resource unit to a 64-bit mask
value often referred to as `processor resource mask`.
This convertion allows llvm-mca to speedup some set operations in the
ResourceManager using simple bit manipulation.

If I remember correctly, the setCustomStrategy works on processor resource
masks. So you need a way to map processor resource ID to mask values.
If you want to know how processor resource units are mapped to processor
resource masks, then you need to use function `computeProcResourceMasks`
(defined in llvm/MCA/Support.cpp).

```
void computeProcResourceMasks(const MCSchedModel &SM,

                                                  MutableArrayRef<uint64_t>
Masks)
```

Array Masks is populated with processor mask values (one per each processor
resource unit declared by the scheduling model).

You can visualise the mapping between resource IDs and resource masks using
a debug build of llvm-mca by simply passing flag `-debug-only=llvm-mca`.
Flag -debug` would work too. However `-debug` is much more verbose as it
doesn't filter out disassembler debug prints.

Example of processor masks computed by ` computeProcResourceMasks ` for
-mcpu=btver2:

Command: ` llvm-mca -debug-only=llvm-mca -mcpu=btver2 `

```
Processor resource masks:
[ 0]  - 0x00000000000000 - InvalidUnit
[ 1]  - 0x00000000000001 - JALU0
[ 2]  - 0x00000000000002 - JALU1
[ 3]  - 0x00000000004003 - JALU01
[ 4]  - 0x00000000000004 - JDiv
[ 5]  - 0x00000000000008 - JFPA
[ 6]  - 0x00000000000010 - JFPM
[ 7]  - 0x00000000000020 - JFPU0
[ 8]  - 0x00000000000040 - JFPU1
[ 9]  - 0x00000000008060 - JFPU01
[10]  - 0x00000000010018 - JFPX
[11]  - 0x00000000000080 - JLAGU
[12]  - 0x00000000020280 - JLSAGU
[13]  - 0x00000000000100 - JMul
[14]  - 0x00000000000200 - JSAGU
[15]  - 0x00000000000400 - JSTC
[16]  - 0x00000000041800 - JVALU
[17]  - 0x00000000000800 - JVALU0
[18]  - 0x00000000001000 - JVALU1
[19]  - 0x00000000002000 - JVIMUL
```

Here, each entry is a tuple {  ProcResourceIdx - Mask - ProcResourceName }.

A simple lookup of that array is enough to obtain the mask values for your
custom strategies.

I hope this helps.
-Andrea


On Sun, Dec 13, 2020 at 2:28 PM Pietro D'Ettole via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi everyone,
>
> I need to retrieve the ID of a particular ProcResource from the processor
> SchedModel but I'm having problems with it.
> In detail, I aim to attach a custom strategy to a particular resource and
> to do so I thought to use the ResourceManager::setCustomStrategy() method.
> I've already got to define my CustomStrategy but I can't retrieve the
> right ResourceID.
> I've thought to use the MCProcResourceDesc table inside MCSchedModel, but
> I'm not sure it'll work.
>
> Is that a good idea? Do you have in mind a better one?
>
> Thanks!
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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