[PATCH] D150310: [TableGen][SubtargetEmitter] Add the StartAtCycles field in the WriteRes class.

[Michael Maitland via Phabricator via llvm-commits]

michaelmaitland added inline comments.


================
Comment at: llvm/utils/TableGen/SubtargetEmitter.cpp:1198
+          if (StartAtCycles[PRIdx] > Cycles[PRIdx]) {
+            PrintFatalError(WriteRes->getLoc(),
+                            Twine("Inconsistent resource cycles: StartAtCycles "
----------------
Do we need this check?

I am trying to model the following case:

I have two resources, `ResA` and `ResB`. I'd like it to be the case that `ResA` models the first of a two stage resource where each stage takes one cycle. When the first stage is not being used, another instruction can enter into the first stage. We can model this by letting `ResA` represent the first stage, and setting `ResourceCycles=1` for `ResA` and `Latency=2+LatencyFromResB` (The 2 represents 1 cycle from the first stage and 1 cycle from the second stage).

I'd like to model that `ResB` is used for one cycle, only after both stages of the resource have finished. In other words, this is two cycles from the start of issuing the instruction. Using `StartAtCycles` I write something like this:
```
let ResourceCycles = [1, 1], StartAtCycles =[0, 2] in
  WriteRes<ResourceName, [ResA, ResB]>;
```

Here, I run into this FatalError. Am I misunderstanding the semantics of StartAtCycle? Is there a better way to accomplish what I'm after?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D150310/new/

https://reviews.llvm.org/D150310