[llvm] [MISched] Extend overridePostRASchedPolicy to support per-function scheduling direction (PR #149297)

[Harrison Hao via llvm-commits]

harrisonGPU wrote:

@wangpc-pp @mshockwave @jroelofs Thanks a lot for the guidance!

I've now updated both `overridePostRASchedPolicy` and `overrideSchedPolicy`. In the new version, I'm passing `MachineBasicBlock` instead of `MachineFunction`.

For graphics shader compilers, we have different shader stages (e.g., vertex shader, fragment shader), which means each stage corresponds to a separate function within the same pipeline. We want to control the post-RA scheduling direction per function using function attributes. Although a function may have multiple scheduling regions, we believe the post-RA scheduling direction should remain consistent across all regions within the same function.

In a previous patch, I implemented this using a target feature, but we think a function attribute makes more sense for finer-grained control. So, I propose to extend the `overridePostRASchedPolicy` interface by adding `MachineBasicBlock` as an argument:

https://github.com/llvm/llvm-project/pull/147708

With this change, the backend can access the function attribute and select the post-RA scheduling direction accordingly. Here's the updated hook:
```cpp
void GCNSubtarget::overridePostRASchedPolicy(MachineSchedPolicy &Policy,
                                             const MachineBasicBlock &MBB,
                                             unsigned NumRegionInstr) const {
  const Function &F = MBB.getParent()->getFunction();
  Attribute PostRADirectionAttr = F.getFnAttribute("amdgpu-post-ra-direction");
  if (!PostRADirectionAttr.isValid())
    return;

  StringRef PostRADirectionStr = PostRADirectionAttr.getValueAsString();
  if (PostRADirectionStr == "topdown") {
    Policy.OnlyTopDown = true;
    Policy.OnlyBottomUp = false;
  } else if (PostRADirectionStr == "bottomup") {
    Policy.OnlyTopDown = false;
    Policy.OnlyBottomUp = true;
  } else if (PostRADirectionStr == "bidirectional") {
    Policy.OnlyTopDown = false;
    Policy.OnlyBottomUp = false;
  } else {
    DiagnosticInfoOptimizationFailure Diag(
        F, F.getSubprogram(),
        Twine("invalid value for post-ra direction attribute: '") +
            PostRADirectionStr + "'");
    F.getContext().diagnose(Diag);
  }
}
```

https://github.com/llvm/llvm-project/pull/149297